Beaks, Bones and Bird Songs: How the Struggle for Survival Has Shaped Birds and Their Behavior - Roger Lederer (2016)
How Birds Live Together
Birds … are sensitive indicators of the environment, a sort of “ecological litmus paper,” … . The observation and recording of bird populations over time lead inevitably to environmental awareness and can signal impending changes.
—ROGER TORY PETERSON, Peterson Field Guide to Birds of North America
Like many kids, I grew up fascinated by animals—live, stuffed, or fossilized. Catching snakes, frogs, and lightning bugs was part of it. At Chicago’s Field Museum of Natural History, I loved seeing the dioramas of stuffed gorillas and ostriches with faux backgrounds of rocks and plants. And of course Brontosaurus, emerging from a swamp with a mouthful of soggy plants. I didn’t really notice the plants—the trees, the ferns, the grasses—they were just there for decoration. Even in graduate school I would facetiously observe that the function of plants was just to give birds somewhere to perch. Slowly I realized that I was missing a great deal, as green plants are the basis of any ecosystem. Later, in my professorial role, I taught field biology to budding elementary school teachers. I encouraged students to look at a forest or grassland or lake and see not just a field of green or brown or an expanse of water, but a superorganism of intertwined life forms, all striving to survive in an environment of continual challenges. Birds are part of this large and intricate biological system, the framework of which is vegetation.
Birds live in a habitat of complex physical features and share space with many other organisms, but that’s often difficult to picture based upon popular literature. Folktales and legends often describe one kind of bird, arising from the ashes, turning into a beautiful swan, or learning to sing. Kids’ books include Flora the Flamingo, Hoot Owl, and Redbird. With a focus on the life of one bird or species, often missing is the larger context in which birds live and confront the challenges of survival. Birds do not live alone, ignore birds of different species, or avoid interacting with other animals. They don’t go wherever they want and do whatever they wish. They are part of amazingly vibrant communities, which constantly pose opportunities and threats.
For many years, ornithological observation meant sitting and watching a bird and describing its behavior. Arthur Cleveland Bent, a businessman who became interested in birds as a child, spent nearly 50 years compiling 21 volumes of Life Histories of North American Birds. A typical excerpt reads: “When we think of the kingbird, even if it be winter here in the north, and he is for the time thousands of miles away in the Tropics, we picture him as we see him in summer, perched on the topmost limb of an apple tree, erect in his full-dress suit—white tie, shirt-front, and waistcoat.” No criticism of Bent—his works contained enormous amounts of credible information—but a flowery description of one bird tells us little about its community. It wasn’t until the middle of the 20th century that ornithologists began looking at assemblages of birds and seeing how their survival depended on interactions with their neighborhood and neighbors.
Every geographic area, large or small, is defined by a unique assortment of organisms surrounded and constrained by a distinctive set of physical elements: soil, weather, water, topography, and geology. This is an ecosystem—a collection of organisms interacting with each other and the physical environment—each one possessing its own physical, physiological, and behavioral attributes. Alexander von Humboldt, Prussian naturalist and explorer, wrote at the turn of the 18th century of the extravagant sounds and sights of the tropical forest and speculated that it was so dense “that there was simply no room to add another plant.” Early naturalists wrote about flashing specks of light filtering through the thick canopy onto the forest floor, oppressive humidity, hordes of undulating army ants marching over thin soil, magnificent butterflies, colossal buttress roots supporting trees reaching to the sky, palm trees with thorny trunks, and masses of intertwined vines shooting upward and drilling downward. They wrote poetically about birds of every color, shape, and voice. The tropical forest that awed them is filled with squawking parrots in the canopy, antbirds shuffling the litter, brown creepers inching up tree trunks, toucan bills jutting from tree cavities, and hummingbirds flitting around dazzling flowers. Natural selection made these birds part of the tropical forest; few of them could survive elsewhere. The tropical forest and every other habitat have their own defined set of birds—the avifauna. The individual members of the avifauna do not act alone and unfettered; they are evolutionarily obliged to confine themselves to certain roles to survive.
The Arctic tundra, the Namib Desert, Lake Baikal, Central Park, and your backyard have much in common. They contain a set of plants and animals living in a framework of physical factors. A group of living organisms in any area constitutes a community; it may be an insect community, a plant community, or a community of birds. Communities are not random assemblages—they evolved over long periods of time as each species in the community fit itself into the mosaic. In a human context, a community is a group of people living in a particular location, conveying the various roles that individuals play such as shopkeepers, teachers, and doctors. In the beginning a community might be rural with few people and the only doctor a general practitioner. As the community grows into a town and becomes more complex there may be pediatricians, surgeons, ophthalmologists, and family doctors. Avian communities and ecosystems develop similarly.
SUCCESSION TO EQUILIBRIUM: ARRIVE, THRIVE, AND DISAPPEAR
The earth came into being about 4.6 billion years ago and the first organisms appeared a billion years later. Birds didn’t arrive on the scene until much later, more than 150 million years ago. Along the way, the earth underwent massive alterations in its geology, hydrology, and atmosphere. Organisms were simultaneously evolving and making their own contribution to the composition of the earth. Bird species transformed over time, existing ones continually replaced by new ones with improved adaptations for survival in the changing environment. There were once 200 pound penguins in Australia and 1000 pound Elephant Birds in Madagascar that stood 10 feet tall. These birds disappeared and others came to be. The survival time on earth of any species is ephemeral when measured in geologic time; the average life span of a bird species is about 125,000 years from appearance to extinction. Perhaps 160,000 species of birds have at some time lived on the earth, 16 times as many as the 10,000 species that exist today. So bird communities continually change, as some members are added and others drop out.
Geological time is a long perspective, but ecosystems develop on a much shorter scale. As a kid in Chicago, I watched the empty lot next to our house, denuded by construction activities, transform into an ecosystem. This abandoned plot of dirt was invaded by plants we call weeds. Grasses arrived, multiplied, and were eventually replaced by shrubs and trees. Insects appeared, followed by rodents, snakes, and birds. Had it not been again denuded by new construction it would have become a deciduous forest. This procession happens everywhere all the time. Fires, hurricanes, floods, earthquakes, and human disturbances yield a blank canvas and raw materials for new ecosystems. Lakes fill in to become marshes, grasslands, and perhaps forests. Islands arise from the sea, hot, rough, and naked, but are ultimately covered by plants and soil and inhabited by animals. These are examples of succession, the gradual and predictable development of communities. The process works the same way in every climate. In northern temperate zones, hardy low-growing grasses and forbs begin to colonize the area. Eventually, hardy shrubs take hold and ultimately loom over the grasses and forbs, shading and usurping their sunlight and nutrients. Tree seeds carried by the wind, water, or birds arrive, germinate, and become saplings. The trees in turn shade and outcompete the shrubs and form a young forest. The saplings become tall trees, living for many years, and the forest’s composition and structure remain essentially the same for centuries.
Destruction or severe disturbance of a forested area, partially or completely denuding it because of fire, flood, biohazard, or other human disturbance, leads to eventual recovery of the habitat by successional stages.
Changes in bird species composition parallel the changes in plant species. Grassland birds are replaced by birds of the shrubs, followed by birds of the forest. The more complex the plant community, the greater the number of different bird species and the more individuals of each species. Just as we can predict the end result of plant community succession in any specific geographic location, we can predict the succession and ultimate constitution of avian communities.
Succession never ends but it slows down considerably as an ecosystem can hold just so many species of birds (or plants or insects), and every new species has the potential to cause the extinction of an existing one. New species arrive at all stages, although not all of them survive. In fact, most don’t. A well-studied example is Krakatoa, a volcanic island in Indonesia lying between Sumatra and Java. Krakatoa erupted with explosive force in 1883, killing 36,000 people and destroying two-thirds of the land. The eruption was so powerful that tsunami waves rocked ships off the coast of South Africa. Although a tragedy for the local populace, it afforded opportunities for gathering firsthand knowledge of the development of an ecosystem and its avifauna. In 1889, six years after the eruption, seeds had blown or washed in and vegetation started to recover, but there were no resident birds. By 1908, more plants had appeared and 13 species of birds had taken up tenancy. By 1924 tropical forest plants were abundant and 28 bird species were breeding there, although two previous occupants had disappeared. In 1934, 171 plant species were identified along with 29 bird species, but three earlier bird species were gone. In 1952 there were 33 bird species, but three former species had disappeared. And in 1984-1986, there were 36 resident bird species, but four previous occupant species were absent. Today, 38 species survive. Just like the plants, the bird species arrive, thrive, and disappear as succession occurs until equilibrium is reached.
This 1888 lithograph shows the Island of Krakatoa as it might have appeared when it erupted in 1883.
Recovery from geologic events can differ greatly. The volcanic island of Surtsey, 20 miles south of Iceland, arose anew from the Atlantic Ocean during the years 1963-1967; today only 12 species of birds, mostly seabirds, survive on the isolated island. Gulls are the most abundant birds on the island and have significantly influenced the growth of plants on the island because they fertilize the soil with guano. Eventually the avifauna will expand as the vegetation flourishes. The eruption of Mount St. Helens in Washington State in 1980 covered the surrounding countryside in ash, devastating the entire avifauna of the enormous blast area, but four days after the eruption birds were seen flying over the site. Since then more than 80 bird species have colonized the mountain from the surrounding ecosystems. Details of succession vary, but the underlying concept is that ecosystems develop in an orderly and predictable way and at maturity are dynamic but stable. And in every ecosystem each bird species occupies a particular niche.
NICHES AND HABITATS: MAY THE BEST BIRD WIN
Each Nutch in a Nitch knows that some other Nutch Would like to move into his Nitch very much.
—DR. SEUSS, On Beyond Zebra
Each bird species in an ecosystem occupies an ecological niche, defined as its relationships with the living and non-living portions of its environment. The niche includes all the variables a bird has to deal with to survive such as climate, food, competitors, predators, and vegetation structure. The niche can also be described as the bird’s role in its community, or its job, and the habitat (the physical place it occupies) as the bird’s place of employment. Some birds like jays and House Sparrows have wide-reaching niches because they are omnivorous and nest most anywhere. Other birds like hummingbirds, Ospreys, shorebirds, and pelicans have specific needs and thus narrow niches.
Pioneering ecologist and zoologist Joseph Grinnell was a potent influence in moving ornithology from a collecting and cataloging venture to one that examined the lifestyles and habitats of birds. In 1904 he accurately stated that the range of the Chestnut-backed Chickadee of the Pacific Northwest is due to “atmospheric humidity, with associated floral conditions.” In a coniferous forest, warblers prefer tree branches while thrushes ply the ground. Spotted Sandpipers strut on the edge of a creek along the forest’s edge while kingfishers perch furtively overhead. The narrow-winged Sharp-shinned Hawk nimbly flies through the forest while the broad-winged Red-tailed Hawk soars overhead. In the ocean, various seabirds forage at different depths and different distances from shore and nest on cliffs, on the ground, in burrows, or in trees.
Narrowly circumscribed niches allow multiple species to coexist in the same physical location by sharing resources. Consider a human community with retail businesses like a hardware store adjacent to a candy shop. They coexist because they are in separate niches and use different resources (customers with different needs). So woodpeckers, warblers, and sparrows coexist as they have fairly different niches, but what happens if more birds move in? Add more woodpeckers and sparrows and new birds like creepers and nuthatches and vireos: how will things work now?
Birds of one species may have the potential to utilize an assortment of foraging sites, roosting spots, and nesting locations, but with many bird species in an ecosystem, there is going to be some overlap in their requirements, and sharing can only go so far. The Russian biologist G. F. Gause developed the competitive exclusion principle which simply states that no two organisms can occupy the same niche if they have exactly the same requirements. How different they have to be depends on the environment and its resources. As we shall see, even seemingly subtle differences can allow coexistence.
In his 1859 Origin of Species, Darwin said, “We have reason to believe that species in a state of nature are limited in their ranges by the competition of other organic beings quite as much as, or more than by adaptation to particular climates.” This competition for limited resources results in what Herbert Spencer called “survival of the fittest” after reading Darwin’s original work. Darwin used the phrase later along with “natural selection,” but they are not the same. The best fit birds are not just the ones who survive; they are the ones that go on to have the most young, perpetuating their genes. They are the ones who compete the best.
The requirements of two different bird species may overlap just a little or not at all, like a vulture and cormorant, but when the niches of two species are so similar that they compete, we have interspecific (between species) competition. In the winter, Blue and Great Tits in Europe roost in tree cavities or nest boxes. Belgian researchers set up nest boxes with large entry holes that allowed both species of tits to enter and shelter during cold winter nights. But they did not provide enough boxes for all the birds. The larger Great Tits physically prevented the smaller Blue Tits from utilizing the boxes, resulting in a higher survival rate of Great Tits. The following spring the expected population increase occurred in the Great Tit but not in the Blue Tit population, showing that the size and aggressiveness of the former enhanced their survival.
In the forests of Finland we find the Willow Tit, Great Tit, and Crested Tit (“tit” comes from a Norwegian dialect word titta, meaning small). These small, mainly insectivorous birds supplement their diet with berries and seeds in the winter. In the woods, the Willow and Great Tit forage in the same tree: the Willow Tit uses the upper and outer branches and the Great Tit feeds in the lower and inner parts. But if the Crested Tit arrives, the Willow Tit moves to the lower and inner branches and the Great Tit moves away to trees on the forest edge. By altering their foraging behaviors to reduce competition, they all get food and survive the winter.
The classic example of similar birds sharing the habitat is MacArthur’s warblers. Robert MacArthur was an influential ecologist and a founder of the field of evolutionary ecology. For his PhD thesis at Yale, he studied five species of warblers in the coniferous forests of the northeastern United States. The breeding ranges of the Cape May, Blackburnian, Bay-breasted, Yellow-rumped, and Black-throated Green Warblers overlap. Until MacArthur’s study, ornithologists assumed that these birds looked and behaved so much alike that they might be exceptions to the competitive exclusion principle and actually share the same niche. MacArthur figuratively divided the trees into 16 different vertical and horizontal zones. He observed the foraging habits of the birds closely and found that if different species fed on the same coniferous tree, they foraged in different parts. This study of niche segregation is often quoted as a model example of how birds with similar niches can coexist.
The Great Tit, a woodland bird, has easily adapted to human habitation.
More common than interspecific competition is intraspecific (within species) competition because birds of the same species have exactly the same needs. A study of Northern Gannet nesting colonies in the United Kingdom discovered that bigger colonies grew more slowly than smaller ones. Gannets from larger colonies faced greater competition for food and were obliged to fly further out to sea to forage than birds from smaller colonies. Longer foraging forays meant fewer trips, less provisioning for the young, and lower chick survival. Similarly, the population growth of Little Penguins on small islands off of eastern Australia slows as colonies get bigger since the adult birds have a short foraging range and feed at the surface or moderate depths, effectively limiting their food supply. As the colony grows and competition for food increases, the adult penguins spend more time foraging for scarcer food and the young get less food, resulting in an inverse relationship between the weight of the nestling penguins and the size of the colony.
The Black-bellied Seedcracker, a finch of the equatorial rainforest areas of Africa, feeds on two sedge plant species, one with hard seeds and one with soft. This bird species is divided into two morphs (morphological types), distinguished by width of their lower bill; one morph’s bill is wide and the other’s narrow. When both sedge seeds are abundant, the overlap in the morphs’ diet of seeds is considerable. When seeds are scarce at the end of the dry season, the wide-billed morphs primarily choose the hard seeds and the narrow-billed birds eat the soft sedge seeds as well as other foods. The difference in bill width is genetic and the mating of a wide-billed bird and a narrow-billed one results in offspring that are either wide- or narrow-billed. Recently, a third morph with an even wider bill that fed on even harder seeds was discovered.
MacArthur’s warblers. Shaded portions of trees reflect those areas mostly used by the species indicated.
In rocky habitats of Eurasia we find the Western Rock Nuthatch of Croatia, Greece, and Turkey, and the Eastern Rock Nuthatch of eastern Asia. In the eastern and western extremes of their ranges, the species look very much alike and eat similar foods, but where the populations overlap, in Iran, the Asian bird has a larger bill than the European bird. When they share the same space, not only do they eat different-sized foods, but their eyestripes are different. Dissimilar bill sizes reduce competition for food; dissimilar eyestripes enable them to recognize individuals of both species, lessening confusion in figuring out who is who, reducing time and effort in territorial or courtship display.
Where the ranges of Western and Eastern Rock Nuthatch overlap, the birds display differences in their eyestripe and bill size.
The One-Third Difference Phenomenon
Pairs of different species sometimes look very similar. These “sibling species” are presumed to have been split off from one former species. The geographical ranges of Lesser and Greater Yellowlegs, North American shorebirds, overlap considerably. The birds make their living by probing the muck of wetlands for invertebrates. The Lesser Yellowlegs bill is about as long as its head while the similar but taller Greater Yellowlegs has a bill at least one-third longer. The Greater Yellowlegs also eats frogs and crayfish and skims the water’s surface in search of fish, which the Lesser Yellowlegs, eating smaller items, never does. Another example is the Cooper’s and Sharp-shinned Hawks. The two birds share their looks and geography but the Cooper’s Hawk is almost one-third larger. Cooper’s Hawks feed mostly on medium-sized birds like robins and starlings, meaning that their average prey is more than double in size that of Sharp-shinned Hawks, which feed on smaller songbirds. In eastern India, four species of kingfishers all live in the same mangrove habitat, but differ in food preferences and behavior—the height of their feeding perch, distance covered on foraging forays, and the size of prey, all reflective of their different body sizes. The larger the bird, the bigger and higher the perch, the farther it flies to feed, and the larger the food items.
G. Evelyn Hutchinson, often considered the father of modern ecology, put forward an explanation in his classic 1959 paper (“Homage to Santa Rosalia or Why Are There So Many Kinds of Animals?”) as to why such differences in size occur and why diversity is limited. Hutchinson proposed that there needs to be about a one-third difference in the sizes of animals with similar niches in order to coexist. This is sometimes called the Hutchinsonian ratio. Lesser and Greater Yellowlegs and the Sharp-shinned and Cooper’s Hawks show that difference while in the case of the Indian kingfishers, the largest is 1.25 times larger than the next largest bird which is 1.12 times larger than the third largest bird, which in turn is 1.5 times as large as the smallest bird. Other examples of similar species differing in size by one-third are the Lesser Spotted and Greater Spotted Woodpeckers, the Merlin and Peregrine Falcon, the Whimbrel and Eurasian Curlew, Snow and Ross’s Geese, and American and Fish Crows. Although it has became a sort of rule of thumb that birds that have similar niches can only coexist if they are about one-third different from each other, the idea is controversial. DNA studies of sibling species reveal that some pairs are closely related and that some have greatly diverged genetically.
Greater (left) and Lesser Yellowlegs(right).
Territories: Get Outta My Way
Anywhere a bird goes is its home range and within that may be a defended area: the territory. A territory serves to spread out competing individuals into their own spaces and reduce competition. Robert Ardrey in The Territorial Imperative, argues that territoriality is innate in all animals, including humans, and entitles his first chapter “Of Men and Mockingbirds,” appropriate since the Northern Mockingbird is known for its strongly territorial behavior. The defended space could be a foraging area, an area around the nest, the nest itself, a place for courtship, or a roosting area. Territories are generally held only during the breeding season but some birds hold winter territories to protect their foraging sites.
Food is often a major factor in determining whether or not territories will be held and because food supplies fluctuate, territorial behaviors sometimes change. Hummingbirds are good examples as the concentration of nectar available among flowers changes often. One day a botanist colleague of mine and a few of his students trucked 500 containers of flowering plants to Yosemite National Park. They placed the plants on the ground in a predetermined grid and out of sight of any other blooming flowers. Within five minutes, Anna’s Hummingbirds found the flowers and established territories around several of them. A few hours later the territorial boundaries changed as the nectar concentrations of the flowers changed.
Rufous and Calliope Hummingbirds in Nevada feed mainly on one species of flower. The Rufous Hummingbird tends to feed at heights of 8 inches or more above the ground and defends its food source vigorously. The smaller and faster-flying Calliope Hummingbirds do not hold territories. Instead, they simply raid the territories of the Rufous Hummingbirds by zooming in below the Rufous Hummingbird feeding zones, which change as flowers bloom and wilt. The smaller size and higher metabolic rate of the Calliopes make territoriality too energetically expensive so they simply robbed Rufous Hummingbird territories but avoid most confrontations by feeding closer to the ground.
Several species of closely related flycatchers in western North America—the Dusky, Gray, Willow, Alder, Pacific-Slope, Hammond’s, and a few more—are notoriously difficult to tell apart as they look and act very much alike. Like MacArthur’s warblers it appears that they would strongly compete for food and nest sites during the breeding season. But the birds, instead of feeding in different parts of a tree or squabbling over nest sites, hold both interspecific and intraspecific territories to spread themselves out and reduce competition.
Red-winged Blackbirds inhabit marshes, wetlands along roadsides, and golf course ponds where males establish territories, sitting on top of cattails, singing and flashing their red shoulders to attract females and defending against intruding males. (One experimenter captured territorial males, painted their red epaulets black, and released the birds. Almost immediately they were attacked by red-shouldered males and driven off.) After males fill the habitat with territories there are still floaters on the edges—males that were unable to establish a territory but are ready to fill the position of a male that leaves or dies. There seems to be little difference in the fitness of the floater males compared to the territory holders because once a floater is able to obtain his own territory, he does just fine in defending it. Territory holders appear to have more at stake and defend the territory vigorously, whereas a floater can abandon a challenge to a territory holder with little risk.
Red-winged Blackbird displaying epaulets in territorial defense.
FORAGING GUILDS AND DIVERSITY
Studying bird communities is a difficult business partly because some bird species are more numerous or obvious whereas others are harder to find. Bird community composition also changes with the seasons, further complicating any analysis. So we need to satisfy ourselves with a snapshot that reflects an avian community at one particular time.
Because bird bills define much of a bird’s niche, the study of foraging habits has become a major tool in the study of bird communities. One study of 22 species of insectivorous birds in a deciduous forest in New Hampshire—warblers, thrushes, vireos, chickadees, sapsuckers, and wrens, among others—classified their feeding maneuvers into 17 different categories. Researchers noted behaviors like hawking, probing, and sallying, the height at which these maneuvers occurred, and in which of eight species of trees they took place. Based upon feeding habits, the 22 bird species were grouped into guilds—assemblages of birds that feed in similar ways. (“Guild,” in Medieval times, meant a group of craftsmen, workers, or merchants who shared the same interests.) Avian guilds were defined in this case as ground foragers, tree trunk and branch foragers, canopy feeders, and those that feed in other parts of the vegetation. Within each guild the birds were subdivided by their differential use of foraging substrates (such as bottom or top of leaf), the use of different tree species (such as oak or maple) and foraging maneuvers (such as hovering or probing). This research, like many other similar studies, demonstrates that avian communities can be defined and studied by using categories of foraging styles. We can also define guilds by taxonomic relationships like woodpecker guilds or habitat relationships like shorebird guilds, or even guilds like seedeaters that include not only birds, but rodents, insects, and others.
Pileated Woodpecker (sometimes mistaken for the similar but extinct Ivory-billed) represents the woodpecker guild, with about 200 species nearly worldwide that make their living in very similar ways.
Foraging guilds give us both a flowchart and blueprint of an avian community, so let’s look at the guilds found in the typical ecosystem and how they fit into the avifaunal community. As we move along, consider what would happen if a guild of birds were to disappear.
DETRITIVORES OR DECOMPOSERS are the vultures, obligate scavengers that recycle dead bodies and limit the spread of diseases. They fly and soar slowly over wide areas, often in groups, to increase their chances of discovering food. They are big because the unpredictability of their food sources necessitates survival on their body reserves between feeding bouts. Condors can consume more than 4 pounds of carrion at one feeding and Turkey Vultures can put on enough fat to survive for two weeks without eating. Vultures don’t generally pose much of a competitive threat to other bird species as most vultures specialize in consuming carcasses. Some facultative scavengers like crows and ravens might share in the meal if they have the opportunity, and in dense environments like a tropical forest where it is difficult for vultures to maneuver, crows, ravens, and jays might become the major scavengers. You would think that putrefying corpses would be the major attraction, but vultures prefer fresh kills. A vulture’s major competitor is bacteria, which rapidly make the dead body unpalatable.
GRANIVORES account for about 15 percent of the avifauna, mainly finches and sparrows, in a deciduous forest; in a coniferous forest they comprise about 35 percent of the bird species; in grasslands 60 percent; and in grain fields up to 90 percent. Granivores may consume a large portion of the seed production of an ecosystem, up to 20 percent of the annual seed production of a coniferous forest. Seeds have to survive on their built-in resources until they germinate, so they are packed with nutrients—up to 65 percent carbohydrates, some fiber, some protein, and a bit of fat. Granivores cannot survive on seeds alone, however, and need to supplement their diet with insects for more protein.
HERBIVORES eat approximately 10 percent of the plants (roots, shoots, or leaves) in any ecosystem, but only about 3 percent of bird species use vegetation as a major source of food as plant parts are fibrous, only 20 percent digestible and contain less than 20 percent protein. To make the most of plant eating, herbivorous birds strategically select plant parts high in protein and low in fiber. The Vegetarian Finch of the Galapagos Islands feeds primarily on buds, leaves, flowers, fruit, and soft bark under twigs; the bird has a parrot-like bill, a large gizzard, and an exceptionally long intestine to digest plant materials. Like granivores, most herbivorous birds have to supplement their diet with insects for protein. One exception is the Plantcutters, small South American birds with beaks, jaws, and palates modified to macerate plant material, a muscular gizzard, and highly folded intestines that allow them to digest plants so thoroughly they have no need for animal protein.
FRUGIVORES comprise about 12 percent of all bird species. Half are songbirds, but a variety of other birds also consume fruit. High in carbohydrates and low in protein, fruit has a fat content varying from 1 to 67 percent. Many fruits have indigestible parts such as skin, seeds, or a hard seed coat, or contain distasteful or toxic compounds. Frugivores are mostly residents of the Neotropics and are major plant dispersers; many plants evolved fruits with characteristics specifically to attract dispersers. Seed-dispersing frugivores are especially important in succession—revegetating damaged ecosystems or developing habitats on new islands. In supplying this service to plants, frugivores ensure their own survival by providing a continual supply of food. There is little competition or specialization among frugivores because fruits tend to be superabundant when they are ripe and fruit eaters need to be able to handle whatever is available at the time. The dominant frugivores in tropical America are the various species of toucans, as caricatured by Toucan Sam, the mascot of Froot Loops cereal (which contains no fruit, just fruit flavoring).
INSECTIVORES, birds that survive on arthropods, account for approximately 60 percent of the world’s birds. Insects are mainly protein and very digestible, so it is no wonder that more than 7400 species of birds feed on invertebrates, including 44 mostly insectivorous hawks. There is such a variety of exploitable arthropods that birds employ a wide array of foraging behaviors like hawking, sallying, gleaning, or probing. The Blue-tailed Bee Eater is a dedicated insectivore specializing in bees and wasps, which it beats on a branch to kill and soften for easier swallowing. The tropics offer bugs all year but winters in temperate zones make arthropods scarce so permanent residents have to be flexible and find dormant insects, larvae, or eggs, switch to another food source, or leave. Downy Woodpeckers probe the crevices of tree trunks, galls, the stems of weeds for arthropod larvae, as well as seeds and berries. Chickadees will eat the seeds of coniferous trees, berries, and small nuts. They have even been observed picking the fat off of dead squirrels. Beechnuts are an important winter food for the Great Tit. Both chickadees and tits also cache considerable amounts of food, as much as 100,000 items in a season.
Flycatchers, warblers, swallows, and swifts are dependent on active insects so in the winter they migrate to the tropics where they have access to the creatures. What happens to the resident birds when all these insect-eating migrants arrive, sometimes doubling the bird population? Insectivorous birds that are permanent residents in the tropics tend to be specialists, surviving in narrow foraging niches. For example, 11 percent of insectivorous birds in the upper Amazon basin feed solely by acrobatically gleaning insects off aerial leaf litter (dead leaves hanging from understory plants), as some antthrushes and ovenbirds do. Although dead leaves are less numerous than live ones, dead leaves hold more arthropods and thus offer a higher energy yield. Migratory birds arriving in the tropics are opportunists and survive by feeding wherever and however they can. An exception is the Worm-eating Warbler from the eastern United States. In the spring it spends about 75 percent of its time searching live leaves, but when wintering in Central America it forages 75 percent of the time on dead leaves, like tropical ovenbirds and antthrushes.
Blue-tailed Bee Eater.
Insectivorous birds are important for keeping insect levels under control in forests and reducing plant damage. Researchers in southern Sweden used nets to exclude birds from tree trunks and branches. After four weeks, plant-eating arthropods on the covered tree parts increased by 20 percent. A similar study in a Jamaica coffee plantation resulted in a 60-70 percent increase in arthropod populations on the trees. In 1918 the State of Michigan estimated the worth of insectivorous birds to farmers at $10,000,000; pretty amazing considering that the state at the same time considered the total worth of all its game mammals, birds, and fish at $500,000. Insectivores do not have as great an influence on insect control in temperate ecosystems as they do in tropical ones because cold winters keep insect populations dampened.
NECTARIVORES are small but important pollinators such as hummingbirds, sunbirds, honeycreepers, and honeyeaters. More than 900 birds feed on nectar and serve as pollinators to more than 500 plant species, pollinating as much as 10 percent of wild plants in the tropics and perhaps 6 percent of agricultural crops such as bananas and papayas. Pollinators are especially important for isolated populations of plants because wind pollination is unreliable. As long as the avian population stays healthy, bird pollination works well, but consider the New Zealand gloxinia, a flowering shrub that is largely dependent on the Bellbird and Stitchbird for pollination. When the birds went extinct on New Zealand’s North Island in the 1870s, the plants became far less productive than they once were.
CARNIVORES, which include the hawks, eagles, owls, falcons, caracaras, and relatives, are at the top of the food chain. The vast majority (90 percent) of all raptors live either exclusively or mainly in the tropics, a reflection of the productivity of the tropical ecosystem. Some are specialists like the Osprey, the only diurnal bird of prey that feeds exclusively on fish, the Black-chested Snake Eagle of Africa, Pel’s Fishing Owl, and the bird-eating Peregrine Falcon. But most raptors are generalists and opportunists and have few predators so they are mainly limited by competition for food. Of the nearly 500 raptorial birds, about one-third are nocturnal, lessening the competition for prey.
Bird Species Diversity
Within an ecosystem, avian communities of various foraging guilds all make a living—survive—by competing for food while simultaneously avoiding being food. Although all ecosystems and avian communities work in the same general way, each one is unique. So how can we compare them? One of the most important goals in describing an avian community is to establish a baseline against which any future changes can be measured. What are the possible impacts of projects like wind turbines, dam construction, traffic, or increased noise, for example? If we know how the bird community works in a particular system, we can measure the potential effect of these projects and suggest possible mitigation measures. So how do we assess the avian environment? One of the most potent ways to evaluate a bird community and its changes over time is to measure diversity.
Ecologists have for years argued about what diversity means and how important it is. A workable definition is that diversity is a combination of species richness and equitability, a combination of the number of bird species and the numbers of individuals in each species. It’s obvious that a community of three species with 10 individuals in each is more diverse than a three-species community with 26 individuals in one species and two in each of the others. But intuition goes just so far.
It is important to have some quantitative measure of bird diversity in a habitat that we can use for comparison from year to year. If the numbers change—fewer birds of one species surviving, perhaps—something is happening that needs to be investigated. Perhaps more importantly, the measurement of bird populations provides an accurate reflection of the health of the entire ecosystem.
A classic paper by Robert and John MacArthur proffers an elegant solution to comparing the diversities of birds in a habitat. After several weeks in the field, we can get a good handle on the number of bird species and the number of individuals of each species in a particular area. Using the MacArthurs’ simple mathematical formula we can compare the bird diversity of different habitats. We know that birds segregate themselves at least partially by their foraging locations, so it’s essential to measure the physical structure of the habitat—the vegetation. We do this by determining the density of the vegetation at different heights. Think about walking through the woods, having to step over the underbrush and walk around trees and shrubs, encountering open spaces and impenetrable brambles. Shrubs at 2 feet have a different configuration than a tall coniferous tree at 10 feet and both differ from a dead tree. Clearly, the structure and abundance of the plants are different in each habitat and the birds react to those. The idea here is that there is a close relationship between the plant structure of a habitat and the bird species diversity. Complex vegetative structure means more niches, although scientists have argued the nuances of this concept ad infinitum.
Is diversity important? Ecologists and conservationists have contended for years that diversity means stability, and that complex ecosystems are more likely to resist and recover from perturbations than simple ones. Not everyone agrees, but the idea does make some sense. Consider a mechanical pocket watch. The watch has a bunch of parts, some more essential than others. Remove a few parts like the crystal, second hand, and numbers on the watch face, and the instrument will still work. But if you continue removing parts at some point the watch will malfunction. Do the same with an ecosystem. Remove one of the frugivores and other frugivores will likely fill in and provide seed dispersal. Eliminate a nectar-supping pollinator and another one will probably take its place. Kill off one species of hawk and the forest will probably continue to function more or less as before. But continue to simplify the system by removing more birds and the ecosystem will eventually suffer deleterious effects.
If a guild of birds disappeared there would be major changes in both plant and animal communities, or even geological effects. The Bay of Fundy, touching New Brunswick, Nova Scotia, and the state of Maine, is frequented by several species of migratory shorebirds. The shorebirds feed on amphipods (small shrimp-like creatures); one sandpiper might eat as many as 10,000 per day. The amphipods feed on diatoms—tiny, mostly one-celled algae. Although miniscule, the diatoms are extremely abundant and play a significant role in the tidal ecosystem, producing adhesive chemicals that help stabilize shoreline sediments. If the shorebirds were to disappear, amphipods would increase, diatom numbers would decrease, and the shoreline would erode. That’s a pretty significant outcome. Another example: the Eurasian Jay is a major disperser of many European oak species. The jays pick acorns from trees and bury them in abandoned croplands, pastures, and openings in the forest, and retrieve them in the fall when food is scarce. A pair of jays can scatter and hoard thousands of acorns in a season. Although the birds retrieve a good percentage of them, many acorns go uneaten and become oaks. Any decline in the jay’s population would reduce the distribution and survival of many oak species.
Predation can affect a bird community directly by reducing prey numbers or indirectly by causing birds to change their behavior. Some researchers argue that the non-lethal effects of predation (not being eaten) are at least as strong, if not stronger, than the lethal (being eaten) effects. If birds change their behavior to avoid predation and spend less time foraging, they or their nestling young may starve. The threat of predation may keep birds from defending their territory or attracting a mate. Raptors are a major force in creating and maintaining the structure of the avian community by keeping the numbers of some birds down and sometimes reducing the overexploitation of a resource.
Hawks and owls, for example, eat small birds, but also cause those same birds to forage less frequently for fear of being eaten. One study in Finland measured the distances of songbird nests from a nesting European Kestrel; as expected, there were fewer nests of songbirds near the kestrel nest than there were farther away. Kestrels prefer open habitats and prey species tend to space themselves a good distance from the predator’s nest as they are more likely to see the approach of a predator and have the opportunity to escape. A similar study on the European Sparrowhawk, a forest-dwelling raptor, showed a minimal effect on the nesting distance of nearby songbirds because in this study there were more locations (bushes, shrubs, trees) to build nests in and more places for prey to conceal themselves nearby.
The mixed deciduous forest Wytham Woods at Oxford University has provided a venue for considerable bird research for many years. Researchers have studied the Blue and Great Tit populations there since 1947 and virtually every individual bird of these two species has been banded. Approximately 1000 nest boxes have been provided for the birds and careful observation has provided good data on all aspects of their lives, including predation pressure. Sparrowhawks take 20-25 percent of the tit population each year, but the tit breeding population remains steady. Predation makes room for immigrant tits, mostly young birds looking for an opportunity to mate and a place to nest. There’s an ecological balance here—the predators eat some tits, which are then replaced by others. The number of available tits limits the predator population.
Intimidation by hawks can be put to practical use. In order to control Rock Doves in Trafalgar Square, the City of London paid a falconer to utilize trained Harris’s Hawks to scare the birds. The pigeon population in 2005 was reduced from about 4000 to a few hundred. Hawks, falcons, and eagles are also used at airports around the world to scare birds such as gulls and blackbirds to avoid collisions with aircraft. In the United States alone more than 6000 aircraft-bird strikes are reported annually. Of course, raptors control mammals as well. One study estimated that a Barn Owl, in its lifetime, could eat 11,000 mice that would otherwise have consumed 13 tons of crops. The Lesser Kestrel prefers large insects like locusts that are often serious crop pests. Kestrel populations are declining across Europe, a study in Spain indicating the cause as the disappearance of grasslands and their replacement by sunflower fields (which makes hunting by the hovering kestrel more difficult).
Lesser Kestrel (ranges from the Mediterranean to Asia) is experiencing population decline in Europe.
Birds don’t just casually sit around and wait to get picked off or cower in the presence of a predator. Prey birds employ numerous anti-predator mechanisms to survive—escaping, hiding, possessing cryptic coloration, and feeding in flocks (more eyes mean a better warning system). They have also evolved specific behaviors like the broken wing act of the Killdeer, a distraction display to lure potential predators away from the nest. The bird fakes an injury to its wing by flopping and dragging it along the ground and the hungry predator follows. At a safe distance from the nest, the wing miraculously heals and the bird flies off. Other birds employ this ruse as well, such as the Mourning Dove and various Lapwings. (The name “lapwing” did not derive from this behavior but comes from the erratic flapping flight of the birds.)
Like zebras and tigers, many shorebirds have bands or stripes on their abdomen to break up their outline, making them harder to spot. Nighthawks, owls, and lots of sparrows and finches are various colors of brown and black to make them inconspicuous. The American Bittern’s bold brown vertical neck stripes help it blend in with its marsh habitat of reeds and cattails. To narrow its body profile and remain hidden from its prey and predators, the bird points its bill skyward and slicks its feathers. On windy days, the bird sways slowly back and forth, like a bunch of reeds moving in the wind.
Sunbitterns of South and Central America inhabit open edges of rivers in tropical forests and are susceptible to predation by raptors. Not being fast fliers they prefer to walk unless crossing a river. When threatened, they spread their wings, tilt them vertically, and raise their tail to fill the space between. Their rufous-, gold-, and black-patterned wings resemble the menacing eyes of a large animal. Males, females, and young birds all show this pattern, with no intermediate plumages, indicating that the display is a defensive mechanism and not one of courtship.
One behavior that you are likely to have observed as it happens in the open and engaged in by many species is “mobbing”—ganging up on a perceived predator. When small birds detect a predator, they issue alarm calls and start to fly toward the predator in the hopes of chasing it off or at least annoying it. A few birds begin mobbing but eventually a number of them become involved. Gulls and terns are well known for their mobbing behavior but many songbirds use this tactic too. A predator sitting on a branch or limb or fence post, be it a cat, crow, hawk, owl, or some other creature, depends upon stealth to catch its prey and mobbing exposes them. If the predator is near a bird’s nest, the mobbing might be particularly ferocious. Small birds have an advantage if they become aware of the threat early and become the aggressor themselves. Crows and soaring hawks are often chased by mobs of smaller birds; being less agile than the small pursuers the predators simply fly off. But agile fliers that prey on small birds such as Cooper’s Hawks or Peregrine Falcons are sometimes mobbed as well. Turkey Vultures, Osprey, and even Great Blue Herons get mobbed even though they are little threat to songbirds simply because mobbing a big bird is an innate survival behavior. Birds most at risk, such as gulls and terns that nest in the open, are more likely to participate in mobbing whereas those birds that nest on ledges inaccessible to predators do not mob. In addition to mobbing the predator and sounding alarm calls, some birds defecate or regurgitate on the predator with amazing accuracy.
Some predators are greater threats than others so small birds sometimes use different calls to distinguish between the severity of the threats. Communally nesting Arabian Babblers issue two kinds of calls when they detect a predator. The oldest male babbler sits on the farthest outside edge of the colony, presumably because he is the most experienced at detecting threats. One call is a short, metallic-sounding “tzwick,” his other a long trill. A cat, a snake, or other non-flying predator elicits trills, while a hawk generates mainly tzwicks with a more urgent meaning. Colonial nesting effects higher rates of survival and probably has evolved as a result of predation pressure since nesting in groups provides more eyes to detect and ward off danger.
This 17th-century painting illustrates an owl being mobbed by a variety of songbirds.
EXOTIC SPECIES: INVADERS OR IMMIGRANTS?
I have visited nearly 100 countries and one of the first things I do in an exotic venue after deplaning is look for birds. Typically, the first bird I see is a non-native: it made a lasting impression on me when I visited South Africa for the first time, landing in Johannesburg, and there, on the tarmac at the foot of the airplane stairs, was a House Sparrow.
Exotic species have been around for some time. Until the introduction of the House Sparrow into the United States in 1861, the most common bird of farms, villages, and cities was the Chipping Sparrow, about which Audubon wrote: “Few birds are more common throughout the United States than this gentle and harmless little Bunting.” In 1890 Eugene Schieffelin released some 60 European Starlings into New York City’s Central Park. He was chairman of the American Acclimatization Society, a group of New Yorkers who were dedicated to introducing plants and animals from Europe. This was the time of the “melting pot” in the United States when immigration from Europe was in full swing and people wanted reminders of their old country. Schieffelin, a fan of Shakespeare, decided that every bird mentioned in Shakespeare’s plays ought to be imported into the United States, such as the starling from Henry IV and the skylark from Romeo and Juliet. Several other species were introduced such as the Java Sparrow, Chaffinch, and European Robin, but unsuccessfully. The feral Rock Dove has been in North America since 1606, imported by the French at Port Royal, Nova Scotia, perhaps as food.
About the same time the European Starling arrived in New York, the Crested Mynah, native to Indonesia and China, was brought to Vancouver, British Columbia, and by 1920 an estimated 20,000 birds were living in the area. The Crested Mynah population stabilized as the birds never left the environs of the city because they couldn’t tolerate the cold of the mountains. In 1950 the European Starling, spreading westward, reached Vancouver. The starling and the mynah had similar niches—they ate similar foods and both preferred to nest under the eaves of buildings. However, mynahs evolved in a warm location and starlings in a temperate one. Equipped with better insulation than the mynahs, the starlings could more successfully survive lower temperatures. In addition, even though the clutch sizes of the mynahs and starlings are both four to six eggs, because the mynahs come from a semitropical environment, their natural habit was to incubate their eggs only about half the day while starlings incubated all day and were more successful in raising young. So as the starling population increased, the mynah population decreased and by 2003 mynahs had disappeared from Vancouver.
An estimated 200 million starlings inhabit North America today. So what allows an exotic bird to survive in a new ecosystem? There have been thousands of other introductions of hundreds of species of birds across the world. Most don’t make it or exist only in small numbers, but occasionally some flourish. A determining factor in the survival of a new arrival is the native range of that bird species: the greater the range, the more likely the species has a broad tolerance of climatic conditions and food choices. The European Starling’s natural environment encompassed Europe and western Asia while that of the House Sparrow was even wider, spreading across eastern Asia, India, and northern Africa. Species with large ranges also tend to be reproductively prolific and spread quickly. Birds that are most likely to invade a new habitat are generalists and are likely to find the resources they need in a novel environment and are behaviorally flexible so they can adjust to a new niche. Diverse and undisturbed ecosystems are less prone to allow invading birds to establish a population in any numbers, but disturbed habitats leave open niches that may be accessible to exotics.
European Starlings and House Sparrows survived their importation quite well and spread across North America because they came from a similar temperate environment and invaded cities and agricultural areas inhabited by few other bird species. They are also comparatively bold. In the United States today 90-some species of free-living non-native birds subsist. Most survive in small, isolated populations, but others (besides the starling, pigeon and sparrow) such as the Ring-necked Pheasant and Mute Swan, are so abundant that most people do not realize they are imports. Most imports were deliberate, but about a third were accidental. In New Zealand, 130 non-native bird species have been introduced and 41 species have established populations at various degrees of survival. The European Blackbird and House Sparrow, introduced in the late 1800s to the islands by settlers to remind them of their former English home, were quite successful while the Cirl Bunting remains rare and the nightingale never made it.
When an invader arrives, it may quickly disappear, establish a small population, become peaceably incorporated into the ecosystem, become an agricultural or cultural pest, and/or overwhelm native populations of birds. There are 35 endangered native bird species on Hawaii, partly because of habitat destruction and partly thanks to alien bird species (as well as rats and mongooses). The islands have been colonized by 58 exotic bird species and another 82 introductions failed. The Japanese White-eye was introduced to the islands in 1929 and successfully invaded old-growth forests, usurping the niches of eight native bird species. The white-eyes impacted the food supplies of native birds, causing them to become stunted and more susceptible to disease. In a 19-year study on the island of Hawaii, researchers measured the body weights and bill and tarsi sizes of seven species of native birds and found that the body mass of the native birds decreased and their bills and tarsi became shorter, resulting in a lowered survival rate. The population of the Akepa, an endangered Hawaiian Honeycreeper, declined and finally crashed, parallel to the increase in the white-eye population.
The Eurasian Collared Dove is another “success” story for exotic species. Its original range in the 19th century was the warm temperate and subtropical zones of Asia and southern Turkey. In the early 1920s it began its spread across Europe. In 1970 they were brought to the Bahamas as pets, escapees reached Florida in 1982, and now they are found throughout much of North America, Europe, Asia, and even Iceland and the Arctic Circle. They tolerate a wide temperature regime and tend to nest around human habitation. Part of the reason for their quick spread is their reproductive capacity; even though they lay only two eggs, they have three to six broods a year. Eurasian Collared Doves are so prolific that in Texas they are increasing at 15 percent a year; there, as in other states, there is an open hunting season on them.
Recent studies in New Zealand and Australia strongly indicate that exotic and native species distribute themselves along a gradient of habitat from natural or undisturbed to disturbed environments. Exotic and native birds separate themselves along a continuum ranging from plantation forests of exotic trees to undisturbed native forests, from thin and open vegetation to thick foliage, and shorter to taller vegetation structure. The more disturbed the environment, the more likely exotic species will establish a foothold. Generally, natural ecosystems are resistant to invasion by exotic birds, but when the ecosystem is disturbed, or carved into smaller parcels, invaders exploit the new conditions as new habitats provide open niches.
Native to Asia, the Ring-necked Pheasant was introduced into the states of Washington and Oregon in 1881 and 1882 and proliferated. Ten years later, the first pheasant-hunting season in Oregon yielded 50,000 birds. Croplands and adjacent brush are excellent habitat for pheasants and the birds quickly spread across the United States. Adaptable birds, they even populate tropical Hawaii. But today the pheasant population is declining in many areas. Clean farming, the use of pesticides, the disappearance of grasslands, grain fields converted to root crops, and changing weather patterns share the blame.
To sum up, avian communities are complex assemblages of interacting species. All these interactions are modified and affected by the structure of the vegetation and a myriad of environmental factors. Avian communities are organized to the extent that they are able to partition resources among the members so that they all survive to reproduce enough that they at least replace themselves. Birds have a variety of roles from scavenger to predator and nectarivore to granivore, and fit themselves into a niche, physically and biologically. Community organization is complex and it is often difficult to show cause and effect between one factor and another, but all the players interact at some level. Communities, like the one you live in, are in a constant state of flux, from hour to hour and decade to decade. All the organisms in the avian community have evolved survival skills to deal with all that turmoil. These assemblages of birds are the result of relentless competition over evolutionary time. As human society and our infrastructure grow we will continue to impact the natural communities of birds. Can they adapt? Perhaps. That is what the next chapter is about.