Beaks, Bones and Bird Songs: How the Struggle for Survival Has Shaped Birds and Their Behavior - Roger Lederer (2016)
BIRDS, BEAKS, AND BELLIES
The Whys and Wherefores of Foraging
In birds the mouth consists of what is called the beak, which in them is a substitute for lips and teeth. This beak presents variations in harmony with the functions and protective purposes which it serves.
—ARISTOTLE, On the Parts of Animals
Ever watch birds jockeying for position to snatch the best morsels of seed at a bird feeder? The same phenomenon occurs in the woods, but is not as easily observed as it happens in a much larger venue with a wider variety of birds and food items. Foraging, from the Old French fourrage (to forage, pillage, or plunder), refers to the ways birds find food. This activity, followed by feeding, consumes much of a bird’s day. And for good reason: efficient foraging is indispensable for survival. Birds have many other challenges (weather, predators, competitors, migratory journeys) on the way to their fundamental goal of reproduction, but these only add to the burden of finding food.
The feeding behavior of birds has been studied for many years, but in the 1960s ornithologists recognized that birds that maximize their energy intake per unit time spent foraging produce the most offspring. Those birds that ate foodstuffs with less nutritional value or spent too much—or not enough—time seeking food are no longer with us. As a result of this revelation, the importance of foraging in avian ecology has been reflected in a large percentage of field studies ever since.
Successful foraging is the result of beak (or bill, the terms are interchangeable) shape, which in turn determines much of a bird’s lifestyle. The beak, with few exceptions, is the bird’s only tool. Birds use their beak not only to forage and feed, but also to preen and oil feathers, defend territories, attack predators, build nests, and aid in courtship displays. So if it is an all-around tool, why aren’t all bird bills the same—some perfect all-purpose beak created by evolution? Because natural selection, in its wise ways, reduced competition by making different bills for different foods.
THE EVOLUTION FROM TEETH TO BEAK, THE ULTIMATE TOOL
“Rare as hen’s teeth” has come to mean rarity itself. Of all the vertebrate groups (fish, amphibians, reptiles, birds, and mammals) only birds lack teeth as we know them: enamel outcroppings fixed in jaws. About 150 million years ago in the Jurassic Period there lived an almost perfect example of the transition between reptiles and birds—an animal with rooted teeth, a long bony tail, and the abdominal ribs of reptiles, but also feathers. This animal, Archeopteryx (ancient wing), is certainly among the most important fossils ever found and for many years was considered to be the first bird. However, since its discovery in 1861, many other fossils of potential bird ancestors have been uncovered, such as the Chinese fossil Xiaotingia zhengi, discovered in 2011.
Many early birds shared major skeletal characteristics with coelurosaurian (hollow-tailed lizard) dinosaurs: forwardly located pelvic (hip) bones, large bony eye sockets, light, hollow bones, reduced tail vertebrae, elongated arms and hands, and fused clavicles (making the wishbone). They also shared a similar egg structure and some had feathers. As birds evolved, improvements in flying ability required morphological changes, such as the need to weigh less. Teeth became smaller and reduced in number until they finally disappeared. Lighter bills, both hooked and serrated, replaced heavier teeth. Along with bills, birds developed other ways to grasp prey such as tongues and feet with spines or prickles. Whereas toothed mammals chew food to begin digestion, birds simply snatch and quickly swallow food like insects, nectar, fruit, worms, and seeds. To accommodate these changes, the feeding mechanisms of birds became specialized not only in the form of beaks but further down, bellies (crops and gizzards).
Archeopteryx, the “first bird.”
A bird’s beak is composed of an upper and lower bony jaw covered by a thick layer of keratin, a structural protein, the same substance that forms skin, feathers, scales, fingernails, and turtle shells. This is the rhamphotheca (Greek for beak case or sheath), which grows throughout the year, sometimes changing color seasonally as it does in the European Starling whose wintery black bill becomes yellow in the spring.
Beaks, which evolved for the purpose of food getting, are as varied in shape and size as ice cream flavors, reflecting the diversity of food and its source. Beaks range in size from the African Shoebill’s enormous wooden shoe–shaped beak to the miniscule beaks of small finches. Hooked, long, thick, wide, pointed, blunt, up- or downturned, bent, crossed, swollen, or serrated, beaks tear, probe, suck, filter, chip, crack, tweeze, chisel, crush, strain, spear, or seize food items.
Along with differing shapes, styles, and lengths of beaks came different feeding styles. Heavy conical bills handle seeds well, whereas flat triangular bills catch flies easily. Crossbills use their overlapping mandibles for opening pine cones; long-billed shorebirds probe deeply into mudflats locating their quarry without seeing, touching, or smelling it; and skimmers slice through the top of the oceans’ waters, snatching food items near the surface. Every bird has its own beak-defined niche; a flycatcher could not survive by feeding on mudflats any more than a sandpiper could live in thick woods.
Bills often serve as sexual signals as well. Male Zebra Finches with bright bills attract females as do puffins with their multicolored beaks. Bird bills also help to regulate body temperature by radiating heat, partially compensating for birds’ lack of sweat glands. So sometimes there is a bit of a compromise among the functions of the bill, but eating is the highest priority.
Consider the finches of the Galapagos Islands and their bill shapes. An ancestral finch or two landed on the Galapagos from South or Central America. The birds increased in number and spread to different islands until eventually 14 different species with beak variations came to be. Three species of finches eat seeds off the ground, three more live on cactuses and eat mainly fruit and insects, and one seedeater lives in trees. The rest are arboreal insect eaters, including the tool-using Woodpecker Finch that extracts insect larvae from tree branches with the use of a cactus spine. Evolving a different beak meant exploiting a new food source and sharing the food supply, which benefited everyone.
Every bird has its own technique of seeking out and acquiring sustenance with beak shape defining a range of behaviors and food items. Since we are most familiar with the birds that visit our feeders, let’s start with the seedeaters.
Several Galapagos (Darwin’s) Finches showing a diversity of bill shapes. 1. Geospiza magnirostris 2. Geospiza fortis 3. Geospiza parvula 4. Certhidea olivacea
SEEDEATERS OR GRANIVORES
Granivory (seed and grain eating) evolved in tandem with birds developing the ability to fly as seeds provide accessible sources of concentrated energy. The lower jaws of granivores are solidly muscularized, enabling the jaw to push the seed upward into the almost immoveable upper jaw. The hard palate of the upper jaw is heavily keratinized and has ridges, bumps, and spine-like projections that serve to husk the seed and direct the digestible part backward.
Seeds are not easy to digest, so after the seeds are swallowed they move to the crop (from the Old English cropp, meaning craw), an expanded part of the esophagus. Most bird species have a crop, but some, like owls and geese, do not. The crop may have evolved as part of the active lifestyle of some birds. Fossil evidence from China indicates that perhaps as far back as 140 million years ago some birds had crops for temporary food storage as did some herbivorous dinosaurs. Rather than eating food slowly and digesting it before moving on, birds with a crop fill it to almost bursting. Digestion then begins as the food slowly makes its way downward. I once found a dead grouse with a fist-sized crop full of juniper needles. Veterinarian Thomas Caceci dissected a Wood Duck and found 10 decent-sized acorns in its crop; that would be like a human throat filled with 10 golf balls.
From the crop the food goes to the small intestine and then the glandular part of the stomach that secretes digestive enzymes, the proventriculus (before the small belly). The second part of the stomach is the ventriculus (small belly) or gizzard, from the Old French gésier, chicken entrails. The gizzard is muscular and, substituting for teeth, mechanically grinds food; it often contains sand grains or small rocks to help in the process. Apparently this organ has fascinated people for years. Spallanzani, an Italian Catholic priest of the 18th century, claimed that he fed turkeys scalpel blades, which the gizzard ground to pieces. Some say that if you hold a live chicken up to your ear you can hear gizzard stones grinding.
From the basic conical shape, seedeater bills vary to match the contours of seeds and their different sizes, shapes, and hardness. Deeper bills have more musculature and exert greater forces, so in general, the deeper the bill, the larger the seed that can be handled. Evening Grosbeaks—so named by French explorers who thought the birds only fed in the evening—have a large bill that can exert enough force to crack open cherry pits. Following robins and other cherry eaters that digest the fruit and regurgitate the seeds, Evening Grosbeaks feast on the pits. The European Hawfinch can crack open olive and plum pits as well and feeds its young by regurgitating partially digested seeds.
Birds typically choose the seeds that are most available and easily handled. At a bird feeder you might find White-crowned Sparrows gingerly manipulating seeds to husk them, Eurasian Collared Doves gulping larger seeds, and tiny goldfinches picking through the seed pile for smaller, softer seeds. Finches place the seed laterally on the edge of their lower jaw and slide their jaw slightly forward and back to crack it; to husk it, the bird moves the seed to the middle of the palate and moves its jaws laterally until the shell comes off. Watch the dining habits of these birds at your feeder closely and you’ll discover lots of different styles, like your relatives at Thanksgiving dinner.
The Red Crossbill is a specialist. Its crossed mandibles and strong jaws enable the bird to pry open the scales of pine cones and extract the small seeds—the bird holds a cone with one foot while extracting a seed. (Interestingly, individuals with the lower mandible crossed to the right hold the cone with their right foot, and left-crossed birds use their left foot.) This adaptation gives crossbills almost exclusive access to this particular seed source, but specialization has its downside. As seed removal takes time, crossbills need seed abundances two to three times greater than other bird species to fill their daily energy requirements as they have less time to forage. The production of seeds in coniferous forests goes down every three to five years, at which time the crossbills are at a disadvantage competing for other kinds of seeds that they can’t handle as well.
Male Red Crossbill; the females are greenish in color. The lower mandible crosses either to the left or right, half of the population being lefties and the other half righties.
Acorn Woodpeckers of the western United States and Mexico store acorns in “granary” trees and defend them aggressively. They wedge the acorns into holes in trees or wooden telephone poles so tightly that crows, squirrels, and rats can’t raid their supply. To remove an acorn, a woodpecker hammers it with its bill to crack the shell and extract the meat. Clark’s Nutcrackers, capable of carrying more than 90 pine seeds at a time in a pouch under their tongue, store many of them in caches, even under the snow. They cache two to three times what they need for the winter and eventually find half or more of their seed caches later. Not only do the birds recall the site of these caches for up to nine months, they also remember the relative number of seeds and the size of the seeds in each cache. Florida Scrub Jays cache food by burying one acorn at a time; if they observe another jay, a potential cache robber, watching them, they will return later to move the acorn. But they will only do this if they themselves were cache robbers in the past. Seems that honest jays trust the other ones and thieves do not.
THE GRAZERS: BIG, SMALL, FLYING, AND FLIGHTLESS
Lots of birds are grazers, and some of them are considered crop pests—blackbirds in North America and Java Sparrows in Indonesia eat rice crops, while parrots damage almond crops in Australia. The Red-billed Quelea of Africa may be one of the worst pests, because, some say, it is the most abundant bird in the world; super-colonies of an estimated 30 million birds have been observed. Flocks are so large that when they land in trees they break large limbs off. A large flock of quelea can eat 50 tons of grain a day and since quelea are kept as pets in Australia, Queensland Biosecurity is concerned about their possible escape and damage to corn, wheat, and cereal crops. One quelea adaptation for survival is the behavior of breaking up into small search parties to hunt for food and then returning to the colony to transmit information about the new food source. Studies indicate, perhaps not surprisingly, that the number of birds and variety of bird species that feed on food crops is much higher in organically grown fields than in non-organic ones. The numbers of insects, as well as weedy plants, are also double or triple in organic crop fields because of the absence of pesticides.
While fishing on my favorite lake, I admired the numerous Canada (not Canadian) Geese overhead, on the water, and in the shoreline grass. Once in serious decline in the early 20th century because of overhunting and habitat destruction, their current North American population may be nearly six million. The birds graze on grass blades, stems, and seeds, grasping the plants with the lamellae (sharp ridges) of their bills. Not possessing a crop, they eat constantly and the not-so-nutritious food, with a large amount of minimally digestible cellulose, moves quickly through the digestive system. This results in a lot of bird pooping, about every 20 minutes, a big reason the birds are considered pests on school grounds, parks, and golf courses. The flightless Kakapo or owl parrot from New Zealand is also a dedicated vegetarian, regurgitating indigestible fiber. It has a small gizzard for a plant eater, probably because its jaw, tongue, and beak structure allow it to grind up plant matter before swallowing. Unusual among land birds, the Kakapo can also store a large amount of body fat, making it the world’s heaviest parrot. It is also the world’s rarest parrot and perhaps the longest-lived bird at an estimated 90 years.
Grouse and ptarmigan digest about a fifth of the fiber they eat. If they had evolved a fermentation chamber as part of their digestive system, they would be able to process more, but that would add weight to these birds, which are already weak fliers. Adding a fermentation chamber allowed the evolution of flightless birds such as the Ostrich, Emu, and rheas. These large ground dwellers graze on green plants and seeds, digesting much of the cellulose they ingest by fermenting it in their caecum, comparable to our appendix. The efficient gut of the Emu has a muscular gizzard with a strong grinding ability, aided by grit (in addition to small stones in the gizzard, pieces of glass, wood, and metal are occasionally found) and an acid environment. These birds prefer high-energy foods such as fruit and seeds and are able to extract enough energy from plant stems to support up to two-thirds of their daily energy needs. Ostriches, whose gizzards might contain three pounds of material, have been known to eat rings, bottle caps, spark plugs, bicycle valves, and even pieces of baling wire. However, they do not eat tin cans or hide their heads in the sand.
GLEANERS, HAWKERS, AND PROBERS: THREE WAYS TO EAT BUGS
Another major food source is bugs, really arthropods, of every stripe—spiders, flies, millipedes, ants, beetles, and relatives. Birds catch these nutritious creatures in three ways: gleaning, hawking, or probing.
Gleaning is a type of foraging strategy in which birds pick bugs off the ground, leaves, rocks, or tree trunks. Many gleaning birds, such as tits and kinglets, flutter, hop, hang, or hover glean in the foliage, as they pluck at their prey. Warblers make their way through a tree, nipping bugs off leaves, while thrushes and towhees move along the ground scratching for prey items among the litter. Verdins of the southwestern United States and Mexico use their strong feet to pull leaves toward them or hang upside down to inspect the undersides for tiny prey. They will also eat a larger food item such as a caterpillar by grasping it under one foot and consuming it piece by piece. Black-capped Chickadees don’t just move around randomly hoping to find insects. They find caterpillar prey by searching for leaves damaged by caterpillars. They look for dry curled leaves or tree branches and inspect them, distinguishing between caterpillars that have fed on trees with toxic or distasteful substances, such as tannins and glycosides, and palatable prey. After finding a caterpillar, Black-capped Chickadees will often hang upside down by one foot while pecking at the larva held in the other foot.
Although the gleaning group is generally composed of small insectivorous birds, many other birds glean—woodpeckers, quail, grackles, crows, robins, gulls, pigeons, and turkeys. Some have unusual food sources. Oxpeckers of Africa glean parasites like ticks off the back of various ungulates like giraffes and rhinos. The assumption for many years was that this was a case of mutualism—the birds got food and the hoofed animals were rid of skin parasites. However, a new study determined that only 15 percent of the time that birds spent on the backs of ungulates is devoted to devouring ticks; the rest of the time the birds fed on skin wounds, ear wax, and other goodies found by probing through the hair. Cattle Egrets follow in the footsteps of buffalo, wildebeest, zebra, or cattle, feeding on the invertebrates the mammals scare up. It is clearly profitable as the cattle-following egrets spend two-thirds the amount of energy and get up to three times as much food in a given amount of time as do egrets that forage in the absence of cattle. The birds tend to follow ungulates that walk at a moderate pace because a slower speed does not stir up enough insects and a faster pace hurries their feeding.
Other birds have more indirect methods of gleaning insects. House Sparrows have learned the trick of waiting in a highway diner’s parking lot to glean the freshly killed insects off the grills and radiators of cars. Boat-tailed Grackles have been observed doing the same thing in the parking lot of the Kennedy Space Center in Florida. Great-tailed Grackles survive in Death Valley, California, by picking insects off the license plates of tourists’ cars; the grills and radiators are presumably too hot.
Mountain Chickadee demonstrating its agility by hanging on a sunflower head.
Hawking birds fly out from a perch, snatch a bug, and then return to the branch or post. Also called sallying or flycatching, this technique is a good way of making a living, as evidenced by hundreds of species of birds. The flycatcher family, Tyrannidae, is the most diverse family of birds with more than 400 species, including the smallest songbirds in the world (such as the pygmy-tyrants) and the species with the longest tail relative to body size of any bird (the Fork-tailed Flycatcher). The Black Phoebe, a common flycatcher resident of the southwestern United States and Central America, sits on a low branch near a creek or pond and waits for a flying insect such as a bee, wasp, or beetle to approach. After flying out and snapping its hooked-tip bill down on the bug, the bird will eat it in mid-air, or, if the bug is big, take it back to the perch and whack it on the branch a few times to kill it. Flycatchers, like many birds in temperate regions, choose their prey by size: the largest flycatchers eat the largest insects, the smallest flycatcher the smallest, and the medium-sized birds take intermediate-size prey.
My PhD thesis was a study of the foraging habits and diet of seven species of flycatchers in different habitats. After doing the requisite library research, I discovered that most ornithologists thought that flycatchers swept insects into their bills with their long rictal bristles, modified feathers on either side of the jaw. But because no evidence supported this idea, I captured flycatchers, put them in a large flight cage, and filmed the birds at 400 frames per second as they caught flies in mid-air. Viewed at normal speed, the films showed the birds snapping up flies in the tips of their rapidly closing bills, with the help of a small downward hook on the upper bill; rictal bristles played no direct role. Later anatomical studies indicated that these bristles have sensory connections to the brain that probably help the bird determine speed and orientation in flight.
Swifts are successful and abundant around the world, numbering about 100 species. With their thin, knife-like wings emanating from a cigar-shaped body and their rudimentary tails, swifts resemble a boomerang. They lead an aerial lifestyle, feeding, drinking, mating, and even sleeping on the wing. They rarely stop except to build a nest. Their feet are so small that they cannot perch but instead cling to vertical walls when they do rest (they were once lumped with the hummingbirds into the order Apodiformes; “apodi” means without feet). Normally feeding at moderate heights, swifts may go as high as 3000 feet to feed. While raising young, a pair of White-throated Swifts might bring as many as 5000 arthropods to their nestlings each day.
Flycatcher showing rictal bristles.
Swallows, which have somewhat wider wings and longer tails, are aerial feeders although they will occasionally swoop down to pluck insects off the ground or the surface of a lake. They tend to fly fairly close to the ground and choose larger insects than swifts but avoid stinging ones like wasps and bees. Swallows perch regularly on wires or tree branches to rest and occasionally land on the ground although they walk only awkwardly. In times of insect scarcity, swallows will partake of fruit. The Greater Striped Swallow of Africa even eats acacia seeds and feeds them to its nestlings.
The similarly shaped but larger nighthawks and relatives are crepuscular feeders, venturing out in the evening when flying insects are abundant. Their bills are small, barely noticeable, but their open jaws reveal a huge sticky mouth. Nighthawks and relatives are sometimes referred to as goatsuckers because their wide mouth was once thought to allow the birds to suckle on the teats of goats. Flying low and slow with seemingly erratic wingbeats, nighthawks scoop insects into their open jaws and rapidly send them down the gullet, catching a few thousand insects in an evening.
The honeyguides of Africa (family Indicatoridae) is an unusual group of insect eaters, although they are actually omnivorous. They guide humans, such as the Boran tribe of East Africa, to bee colonies. The Boran people whistle to the birds to attract them and the birds respond by signaling the humans with chattering sounds, flying a short distance, and calling again—until the hive is reached. The humans take the honey and the birds get the bee eggs, larvae, pupae, and beeswax. Everybody eats.
Tree Swallows—so named because they nest in tree cavities or nest boxes—winter farther north than any other swallow species.
Probers poke their beaks into large and small crevices to extract invertebrate morsels. The Brown Creeper of North America is a small bird with a sharp down-curved bill. Flying to the base of a tree, it works its way upward, spiraling around the trunk searching under and around the bark for insects, their larvae or eggs, and other small creatures. Its long claws and spiky tail feathers move the bird upward as it clings close to the tree appearing to be a piece of bark. Nuthatches operate similarly but with a sharp straight bill, a short tail, and a habit of starting at the top of the tree and working their way down. Although mainly insectivorous, nuthatches get their name from “hatching”—cracking—nuts and seeds open with their sharp bill. Like a few other birds, nuthatches will cache food, both insects and seeds, but typically with only one type of food item per cache.
Woodpeckers probe by sticking their bills deep into crevices, whether tree bark, roof shingles, or the ground. A most interesting arrangement allows the tongue to extend out perhaps four times the length of the beak. Some woodpecker tongues cover the hyoid apparatus, a series of muscle-covered bones in the shape of a Y with the horns of the Y originating forward of the nostrils or in the orbits of the eyes, looping over and down the skull, coming together as one bone in the throat, and extending out between the jaws as the tongue. Depending on the species, the tongue may be sticky, have barbs or spines, or be flattened at the end.
Woodpecker skull showing route of tongue.
Pecking and probing, woodpeckers also pound, and the perennial question is why they don’t get headaches. Three reasons: the skull is a spongelike matrix of bones that helps to absorb blows; the lower jaw bends a bit, lessening the shock; and the hyoid bone acts like a seatbelt, preventing excess movement of the head. New helmets for Army tank drivers and bicyclists are being designed based upon the ingenuity of this woodpecker trait.
Shorebirds have made probing a real science. Probing into the mud for invertebrates such as worms, insect larva, amphipods, crustaceans, and mollusks, their bills go shallow, deep, or somewhere in between, divvying up the food sources by depth. Short-billed plovers probe down to a depth of 2 inches; medium-billed stilts feed at intermediate depths; and the long-billed curlews feed from the surface down to 8 inches. How does the curlew find and snatch its prey? Think about how difficult it would be to completely open a long pair of forceps in thick muck; long-billed shorebirds have figured this out—only the tips of their bill open and close—sort of like those picker-upper gadgets one uses to collect litter from the curb. Avocets, with long up-curved bills, sweep their bills side to side through the water or silt in search of prey or swim to deeper water and dabble, head down, like ducks.
A variety of shorebird bills. 1. Spotted Sandpiper 2. Long-billed Curlew 3. Marbled Godwit 4. Stilt Sandpiper 5. Snowy Plover 6. Black-necked Stilt
But how do shorebirds find food in the first place, being that it might be buried in sand and muck? Recent research has shown that many shorebirds such as the Western and Least Sandpipers and the Red Knot can detect prey under the sand of the shoreline without seeing, smelling, or touching it. Herbst corpuscles, mechano-sensory organs just underneath the surface of the bill, are the answer. Sticking their bills less than a half-inch into the sand, the birds can detect the presence of a rock or a prey item because the corpuscles detect differences in the pressure gradient. We used to think that the nocturnal kiwi of New Zealand located its worm prey by smell since its nostrils are located near the tip of the bill and the olfactory lobes of the brain are large, but recent research indicates that kiwis have mechanoreceptors similar to that of shorebirds. Ibises also possess this adaptation and as the birds are not closely related, these mechanoreceptors must have evolved independently.
Oystercatchers live in almost all coastal areas of the world and feed on a wide variety of prey—on land they probe for worms and insects and in the shallows they poke about for marine worms and shelled invertebrates. But their long bills also serve as wedges to pry chitons and limpets off rocks or to crack their shells. Oystercatchers are named for their style of feeding on mussels and oysters by sticking their bills between the valves of a feeding mussel and cutting the posterior adductor muscle to open the shells. Young oystercatchers must learn this technique, often stealing food from adults before their education is complete.
Of all the probers, hummingbirds and sunbirds are probably the most recognized. Sticking their beak deeply into a flower, they extract nectar with their fringed and grooved tongues. There are 132 species of sunbirds and almost three times as many hummingbirds, so this lifestyle is obviously successful. Because birds and flowering plants came into being about the same time, during the Jurassic period about 150 million years ago, flowers and flower-probing birds have a close connection. Many flowers, actually their petals, have adapted a form and function to attract pollinators and reward them with nectar. Hummingbird-pollinated flowers tend to be yellow, orange, or red; have only a mild scent; are above average in nectar concentration and amount of pollen; hang horizontally or downward; and are tubular in shape. Hummingbirds hover and transfer pollen with their beaks; the Heliconia (false bird-of-paradise) flowers of Southern Hemisphere rainforests have sticky threads of pollen that adhere to hummingbird bills to make transfer even more efficient. Because hummingbirds feed while airborne, hummingbird-pollinated flowers can be located almost anywhere on the plant. But sunbirds sit while they feed, so sunbird flowers provide a perch. The rat’s tail (Babiana ringens), a flowering plant endemic to South Africa, grows a spike specifically as a bird perch. Some sunbird-pollinated flower species bloom close to the ground to make their flower parts more accessible. And since they perch while feeding, sunbirds often transfer the pollen to another plant by transporting it with their feet.
Green Violetear Hummingbird supping nectar from a Salvia (sage) flower.
FRUGIVORES: THE SEED DISPERSERS
Animal distribution of pollen is a much more efficient system than wind and exemplifies coevolution nicely. Pollination leads to fertilization and the development of seeds, which also need to be dispersed, so seeds are often encased in fruit to attract seed dispersers. Birds eat the fruits, and the hard-cased seeds exit the digestive tract or are regurgitated; the plant’s seeds get moved away from the parent plant and the bird gets nutrition. Everybody wins. Fruit eating is common among birds; in one study in Costa Rica, 70 bird species were seen feeding on the fruit of 170 species of plants. Birds choose fruit based upon its nutritional value, seed-to-fruit size ratio, taste, time of ripening, and color. Berries are an important, sometimes sole, source of food in the winter when insects are scarce; the Mistle Thrush of England will defend holly berry patches and the Townsend’s Solitaire of North America will defend stands of juniper trees. The Yellow-rumped Warbler is the only warbler able to digest waxy myrtle and bayberries, allowing the birds to survive the winter farther north than any other warbler species of North America.
Bird-dispersed fruit tends to be red but may also be blue or black, whereas mammal-dispersed fruit is generally yellow, orange, or brown. You would think that brightly colored red fruits would be most appealing to birds because they are to us, but experiments with Redwings in Finland indicate that blue fruits are attractive as well because birds see ultraviolet, UV light being reflected by some blue berries. In an area with few fruit-eating birds, fruits tend to be multicolored to enhance their attractiveness.
Birds that usually eat smaller fruits whole defecate the smaller seeds away from the tree, whereas birds that tend to eat the large seeds of large fruits at or near the tree drop them underneath rather than dispersing them. But since large seeds are more successful at germinating, and it takes fewer seeds to establish a new plant, the strategies are more or less equivalent. Most birds pass the seeds a few hours after ingestion, so the seeds are not deposited a great distance from the parent plant. However, two shorebirds—the Killdeer and Least Sandpiper—may retain seeds of bindweed, mallow, and sumac in their guts for 6–14 days. As long-distance migrants, these birds may deposit seeds thousands of miles from their origin.
Cedar Waxwings live mainly on fruit and are so efficient at digesting that they can pass mistletoe and other berries through their digestive tract in 16 minutes. They will gorge themselves, filling their throats and crops with berries until they move down the digestive tract. Sometimes the birds are so satiated that they will pick a berry and pass it down a line of birds until one decides to eat it. The story is often told about waxwings gorging on fermented berries and getting drunk. It happens, not only to waxwings but blackbirds and robins as well; the ethanol in fermenting berries causes the inebriated birds to fall from trees or fly into objects, often causing their demise.
Although Cedar Waxwings devour insects for protein during breeding season, the vast majority of their diet consists of berries.
The fruits of some plants have evolved to be bigger, tastier, more nutritious, and generally more attractive to birds for dispersal, but other plants have toxic seeds to deter birds from digesting or damaging their seeds. Nandina, an ornamental plant commonly called heavenly bamboo, which is native to Asia but found in some parts of Europe and the United States, is poisonous to birds as parts of the plant, including the berries, contain cyanide. Parrots have enormously strong bills and can decimate and digest seeds, even those that contain toxins such as alkaloids (cacao), cyanide (apple), and persin (a fungicidal toxin in avocados.) Why aren’t the birds affected? Parrots eat clay along South American river banks and one explanation is that the clay binds with the toxins in the digestive tract and renders the poison ineffective.
Coevolution is a dynamic process in all biological communities. A study in the Brazilian forest, dwindling in size because of logging, found that the size of the seeds of several species of palm trees has been shrinking. With the reduction in the number of large birds such as toucans because of habitat loss, palm trees are producing smaller seeds to attract smaller bird dispersers. But smaller seeds could mean bad news for these tree species as the seeds contain less nutrition and are less tolerant of drought.
Only 3 percent of birds eat leaves (folivory) with any regularity. The leaf-eating Hoatzin of South America is an especially unusual bird. Its diet is primarily leaves, which it ferments, as in a cow stomach, in its crop. The crop is so large and well developed that the bones and muscles of the chest have been compromised, making the bird a poor flier. The young are fed with partly digested regurgitated leaves, and if that isn’t unusual enough, the newly hatched birds possess two claws on each hand with which they can clamber through the vegetation.
The Hoatzin is the only member of the family Opisthocomidae—the family name is Greek for long hair in back, which refers to the bird’s crest.
CARNIVORES OR RAPTORS
Perhaps the most impressive of all birds are the large carnivorous ones—hawks and eagles, falcons and kestrels, hobbies and kites. From the Latin rapere for thief or plunderer, all these birds, which hunt and eat other animals, are called raptors. Raptors themselves have few enemies and tend to survive for many years, but like most birds, the first year of life poses the most challenges and fewer than half of their young survive to their first birthday. Raptors have numerous adaptations for a predatory life: strong legs and feet, sharp talons, and exceptional hearing and eyesight. Excellent vision is especially important when flying in pursuit of a meal. You would think a predatory bird would focus on and head straight for its prey. But observations of many hawks and falcons indicate that at some distance from their quarry, the birds fly with their head at a 45-degree angle while focusing on the prey. This requires that they fly in a spiral pattern to approach the victim; when less than 26 feet away, they straighten their head and fly directly at the quarry. Even though flying in a spiral takes longer and requires slower flight, Vance Tucker, a pioneer of bird flight studies, found that focusing on the prey at an angle insures a greater success rate than had the bird approached its meal straight on from a distance.
Hawks and eagles are essentially the same although the larger ones tend to be called eagles. There are about 60 species of eagles, mostly distributed in Eurasia and North America. They feed on live prey—mice, rabbits, skunks, gophers, groundhogs, snakes, other birds, and large insects. The Golden Eagle tends to hunt near the ground and swoops down upon its prey with talons forward and wings back, snatching a poor bunny off the ground and shredding it into edible pieces elsewhere with its sharp beak. The long claws and muscular legs of the Golden Eagle have allowed it at times to carry off foxes and coyotes and even lift a small deer off the ground. In Alaska eagles occasionally prey upon young Dall sheep and caribou and young domestic sheep from ranches. The Snake-eating and Serpent-eating Eagles of Africa prey mainly on snakes and don’t avoid venomous ones even though the birds are not immune to the venom. The Rufous Crab-hawk of South America eats only crabs, using its long legs to snag them. The Bald Eagle is more of a fish catcher, but it might make five or six attempts before successfully nabbing one, sometimes taking a swim in the process. Bald Eagles will also eat mammals and birds and won’t hesitate to steal a fish from the smaller Osprey. They will eat carrion and are often seen in garbage dumps in Alaska, where there are no vultures. Symbol of the United States since 1782 (winning the vote over Ben Franklin’s choice of the turkey), the Bald Eagle, not bald at all, gets its name from the patch of white on its head and tail, a description known as piebald, meaning large spots or patches.
Hawks, from the Old High German Habischt (seizer), eat all kinds of animal prey. Searching for prey usually requires looking downward, and as we all know, bright sunlight can interfere with vision. So hawks have a bony ridge above their eye, located behind where an eyebrow would be, that shades their eyes like a built-in baseball cap—and also gives them an ominous look. Broad-winged soaring hawks are also known as buteos, after their genus name, and the accipiters (usually called sparrowhawks or goshawks) are the narrow-winged, fast-flying hawks. Buteos such as the Red-tailed Hawk of North America and the Common Buzzard of Europe ply the skies using updrafts as much as possible, flapping little, and looking for prey. The Red-shouldered Hawk leans toward lizards and snakes while the Red-tailed Hawk prefers skinless small mammals—the birds strip mice and gophers naked before swallowing them. The accipiters have long tails and zoom adroitly through the woods. Cooper’s Hawks and their little brother look-alikes, the Sharp-shinned Hawks, are active fliers and hunt both from perches and on the wing. They are faster than the soaring hawks, feeding on both rodents and birds, but are most admired for their agility in pursuing songbirds through the brush. Autopsies of Cooper’s Hawks indicate that a third of them had broken one or both clavicles at some time, most likely by flying into a branch in pursuit of avian prey.
The Sharp-shinned Hawk is named for its sharply keeled lower legs.
Owls, nocturnal carnivores, share many similarities with hawks. They survive by catching and decimating small- to medium-sized prey with their muscular legs, strong feet and claws, tearing beaks, and good eyesight and hearing. Eating whole birds, mice, and gophers means the bird swallows hard-to-digest parts. About 10 hours after their meal, owls (as well as hawks, grebes, loons, cormorants, and others) regurgitate a round or oval pellet of indigestible items such as bones, feathers, fur, plant parts, and even bird bands from an unfortunate chickadee or sparrow that served as a raptor’s lunch. When regurgitating the pellet, the owl will make the sound and movement of retching and wear an understandably pained expression. Owl pellets contain fur and feathers because owls don’t pluck or skin their prey like hawks do—they just gulp the creature down. Owl pellet examination to determine an owl’s diet is so common in schools and nature centers that you can buy pellets commercially, selecting for size and content (choose mole, bird, or pocket gopher). Barn Owl pellets are the best as their stomach juices are less acidic than other owls and so the pellets contain the finer bones.
Most owls have a varied diet, but some are specialists. The biggest owl in the world, Blakiston’s Fish Owl of far eastern Asia, stands on the edge of a hole in the ice and waits for potential piscine prey. Pel’s Fishing Owl of Africa is well equipped for dipping its talons into a river after prey as it can swing one of its three front toes to the back so that two sharp claws are positioned in front and back in order to grasp a fish. Most owls have extensions of feather barbs on their flight feathers to allow for silent flight, but fishing owls do not because they don’t need to be quiet to hunt for fish. Pygmy owls and owlets are avid bird-eaters; Pygmy owls are known to take hummingbirds while the little guys are at feeders. Tawny, Long-eared, and Barn Owls account for 11 percent of the mortality of bats in the United Kingdom. Smaller owls of the tropics prey most on insects, especially moths, beetles, and crickets. Several kinds of owls eat snakes at times, but the Western Screech Owl of the southwestern United States occasionally brings a Texas Blind Snake to its nest, alive. The snake, which normally feeds on ant and termite larvae, will eat insect pests in the nest such as maggots (fly larvae). Young owls in nests with snakes grew faster and had a higher survival rate than snake-free nests.
Falcons, with narrow and tapered wings, are the fastest raptorial birds and include the kestrels, hobbies, and the well-known Peregrine Falcon, which is reputed to be able to exceed 200 mph in a dive. The Peregrine is found virtually everywhere in the world except Antarctica. Its diet is almost exclusively small birds. I once observed a Peregrine with a live bluebird in its talons; the falcon plucked the songbird totally naked, ate its head, and delivered the body to two young falcons in the nest. Because of their speed, agility, and beauty, Peregrine Falcons are popular with falconers. The species name peregrinus (Latin for wanderer or traveler) refers to the habit of falconers in the Middle Ages catching adolescent birds en route to their breeding grounds. The genus Falco (from the Latin falx) refers to the sickle-shaped wings (or talons or beak).
PISCIVORES: DIVERS, SKIMMERS, AND MORE
Piscivores, fish eaters, include loons, grebes, penguins, murres, guillemots, puffins, terns, cormorants, anhingas, gannets, and Ospreys. The Northern Gannet is a world-class fish catcher, diving straight down into the sea as deep as 75 feet. Well-adapted for diving, they have no external nostrils, a strong sternum, and special air sacs, not unlike bubble wrap, that act as air bags to cushion the shock of hitting the water. Unlike the diving gannet, the Osprey (from the French ossifrage, bone breaker) can catch fish only near the water’s surface. A bird of worldwide distribution, the Osprey catches fish like Pel’s Fishing Owl by swiveling a toe around from a three in front and one in back perch position to a two and two hunting arrangement. Hunting on the wing, the bird flies at different heights to locate prey and then dives rapidly with its wings held back and talons forward. Ospreys are successful about one-third of the time, and their sharply curved talons and sharp spines on the bottom of the toes assure that the slippery prey does not get away.
Especially odd birds are the skimmers, a small family that includes only three species. The Black Skimmer opens its mouth, lowers its longer lower mandible into the water, and skims the surface for fish and squid along ocean shores. When it detects an edible item, it rapidly and reflexively grabs the prey in its bill and tosses it up for swallowing. A fast-growing rhamphotheca compensates for the wear and tear on the bill. With this seemingly energy-intensive feeding strategy, their day-to-day survival is assisted by their tendency to fly at a moderate speed, taking advantage of the uplifting aerodynamics of the water’s surface, and feeding at night when more fish and squid come close to the surface.
At least as strange is the anhinga, sometimes called snakebird because it often appears with only its head and skinny neck above water. Although the anhinga has a functional uropygial gland—the oil gland on the rump of birds that produces the substance used to waterproof feathers—the anhinga’s feathers are three times as wettable as other aquatic birds. They are able to swim underwater easily and, with their very flexible neck, can spear fish, toss them upward, and swallow them intact.
The Black Skimmer at work.
Flamingoes remind me of the scene in Alice in Wonderland when the Queen of Hearts uses a flamingo as a croquet mallet. These unusual birds utilize rows of horny plates in their jaws to filter out microorganisms from water—sometimes so filthy looking that it’s hard to imagine it supports any life. Flamingos feed with their head upside down, the lower beak pushing up against the upper. The thick tongue serves as a pump, moving back and forth as frequently as four times a second, pushing microorganisms against the filter mechanism while the bird sweeps its head through the water, the filtered water gushing out the corners of the bird’s mouth. The pink color of the leg, face, and feathers comes from their diet of carotenoids—pigments in crustaceans and some microorganisms. The carotenoids are deposited in the integument (such as skin or feathers) of the bird and when they dissolve in fat molecules, the reddish color is exposed. This is the same thing that happens when you boil a lobster.
Dabbling ducks, those that tip down and stick their butt up while floating, use the lamellae (ridges) on their bill edges to filter out seeds and small organisms from the bottom of a lake or pond. The Northern Shoveler has especially well developed lamellae that extend into fine hairs for exceptional filter feeding ability. Unlike other dabblers such as the Mallard and American Widgeon, the Northern Shoveler can also sieve organisms from the water’s surface. The spoon-shaped bill of the shoveler has earned the bird its colloquial names of Hollywood Mallard, Smiling Mallard, Daffy Duck, Daisy Duck, and Spoonie.
VULTURES AND RELATIVES: THE FULL-TIME SCAVENGERS
Many birds are occasional scavengers like the Bald Eagle, gulls, pigeons, and members of the crow family. But the birds that eat dead creatures as a living are the most fascinating, although most birdwatchers would probably not count them as their favorites. Many people think that vultures circling overhead means that something is dead below them. But in the event of a deceased animal on the ground, the vultures would be there, feasting, not soaring above and drooling. The birds are circling because the wind conditions are such that it’s easy to stay aloft and search for corpses without too much wing work. Impress your friends the next time you see vultures circling by calling the formation by its proper term “kettle,” as if the birds were being stirred.
The New World vultures are in the family Cathartidae, from the Greek skathairein, to purge or cleanse. The word could refer to the fact that vultures rid the environment of dead animals, or to their habit of regurgitating stomach contents upon the approach of a predator to lighten the load for takeoff. In any case, they eat dead things, which are undoubtedly full of bacteria. To counter potential infection vultures have strong stomach acids to kill pathogens and they defecate on their feet both as an antiseptic wash and, on hot days, to help cool them off. They have naked heads as a feathered head would be impossible to clean; a naked head also allows sunlight to disinfect the skin. Most vultures of the New World have an excellent sense of smell and detect ethyl mercaptan, a gas produced by decaying bodies. That’s a handy survival mechanism, but what about Black Vultures whose sense of smell is not nearly as well developed? They fly above Turkey Vultures and follow them to a meal of carrion; they will also prey on young or injured mammals.
Although they generally look and act alike, New World vultures (from the Americas) are not closely related to the vultures of the Old World (Europe, Africa, Asia). One big difference is that Old World vultures do not have a good sense of smell. Since they need to find their food by sight, African vultures fly considerable distances each day, often following ungulate herds. More than 80 percent of the time they find food by joining a group of vultures that has already begun feeding on a large carcass. As they converge upon a carcass, which I’m sure you have seen in nature films, they will squabble, but these groups have developed ritualized displays to establish a dominance hierarchy to minimize aggression. Many of the carcasses of large ungulates that vultures feed on died of disease, starvation, or both; although vultures might spread some bacteria via their feathers, they do a pretty thorough job of ridding the environment of diseased carcasses.
Vultures and condors are at risk worldwide because of the contamination of their food sources. Lead shot left by hunters and poisons used to kill predators or pests are often found in the carcasses of animals that the vultures consume. A spectacularly sad case is that of the White-rumped Vulture in India whose population numbered in the tens of millions in the 1980s; today only a few thousand survive. The culprit is Diclofenac, an anti-inflammatory drug given to cattle that is poisonous to vultures. Cattle are mainly used for milk and rarely eaten in India, so millions of carcasses are available to vultures, India’s primary animal disposal system. Even with their tough and resilient lifestyle, vultures cannot survive this drug that shuts down the kidneys.
The Griffon Vulture is an Old World vulture found across the Mediterranean into the Middle East and Asia.
When you see a bird in the wild, it is usually searching for, manipulating, eating, or carrying some sort of food item. Although birds may not always be initially successful in establishing or defending a territory, finding a mate, building a nest, or raising young, they will often get another chance. But if they are not successful in foraging, they have no chance at reproduction and their genes will be lost from the gene pool. The everyday habits of birds, circumscribed by the shape of their bill and their foraging behavior, provide essential information in the study of avian species. Watch your bird feeders and marvel at how evolution has shaped what you observe, because the beak and feeding habits you see garner the fuel for all the other activities birds need to survive.