Beaks, Bones and Bird Songs: How the Struggle for Survival Has Shaped Birds and Their Behavior - Roger Lederer (2016)
WEATHER SURVIVAL STRATEGIES
Enduring Heat, Cold, Wind, and Rain
October extinguished itself in a rush of howling winds and driving rain and November arrived, cold as frozen iron, with hard frosts every morning and icy drafts that bit at exposed hands and faces.
—J. K. ROWLING, Harry Potter and the Order of the Phoenix
Birds are at the mercy of the meteorological environment. Avian anatomy and physiology including body size, wing and leg length, beak shape, fat deposition, feather color, salt retention, respiration rates, and red blood cell count are partly the evolutionary results of climatic settings. Birds lose water from their skin and respiratory system, pant, shiver, flutter their throats or tongues, dilate or constrict capillaries, seek shade, lift a leg, spread their wings, fluff up, or otherwise respond to an environment that is at the edge of their physical comfort zone. People act similarly, but can generally seek relief offered by modern conveniences like air conditioning and furnaces. A bird’s physical environment undergoes only gradual changes unless some major disruption occurs. The composition of the air, soil, and underlying geologic substrate are stable and the photoperiod changes exactly the same way every year. The only significant variable is weather. Although birds have evolved to cope with the unpredictability of the weather to some degree, extreme events can seriously challenge their survival. The year 2012 saw an arduous breeding season for 23 out of 24 of the most common birds in Britain. Heavy rains in May and June washed away nests and made insects hard to come by, according to the Royal Society for the Protection of Birds. Only the European Blackbird saw an increase in its population, probably because it feeds on worms and other critters that normally inhabit the ground and are still accessible after a deluge. That same year, the California Audubon Society noted that after three years of a severe drought, hawks in the Central Valley showed a 90–95 percent nest failure rate because the plants that produce food and provide cover for rodents had become scarce. Penguin chicks in Argentina have suffered from multiple effects of climate change as heavy rains, strong storms, and exceptional heat take their toll. A colony of 3500 Magellanic Penguins was studied from 1983 to 2010 in Punta Tombo, Argentina. This region’s climate is historically mild and dry, but 13 years of the study had an exceptional amount of rainfall as well as unusually cold weather. The soft down of penguin chicks that keeps them warm in dry weather is not waterproof and when the cold rains fell, up to 50 percent of the chicks died.
Studying birds in their natural environment is the only way to really understand how they cope with what nature throws at them. In Northern California for several winters I collected data on the territorial behavior of Townsend’s Solitaires, thrushes that spend November to March in a cold juniper and sagebrush habitat at an elevation of 5200 feet. Eating juniper tree berries and an occasional larval insect, Townsend’s Solitaires survive the winter by safeguarding their berry supply against intruders—other solitaires and American Robins. A solitaire would sit on a treetop whipped by the wind and call to announce its presence. Then it would drop down to the lower branches of the junipers or the ground to feed and quickly return back to the apex of a tree. After four winters of taking notes, I went to my office and crunched numbers—and this is where science pays off: the analysis of data.
The data showed that in years when the juniper berries were in short supply the birds defended the resource, so both solitaires and robins established territories, sang, called, and chased intruders. In years of abundant berries, the Townsend’s Solitaires paid little attention to each other or to their robin relatives. Both species just wandered around within a loosely defined area eating berries ad libitum. Energy expended in defending a territory when there is sufficient food only puts a bird at risk for accidents or predation. But when food is scarce—especially in the winter when any morsel of energy can mean the difference between surviving the night or not—it is worth protecting. These kinds of strategies and struggles go on all the time but weather overlays every challenge to survival.
A Townsend’s Solitaire surveying its territory.
REGULATING BODY TEMPERATURE
Humans are comfortable at certain temperatures, but other temperatures make us shiver or sweat. That range of ambient temperatures that don’t make us sweat or shiver is our thermoneutral zone (TNZ), in which our basal metabolic rate (BMR) is stable and biochemical processes are optimized. Above and below the TNZ, birds and mammals have to expend additional energy to keep cool or warm. We are able to acclimate physiologically (to some degree) to different ranges of temperatures, depending on where we live. If you live in North Dakota a temperature slightly above freezing might seem mild, but Californians experiencing that same temperature will be pulling out their down parkas and earmuffs. Birds too can acclimate somewhat, so their TNZ changes with the season, latitude, and local conditions. In general the TNZ spans a greater range of temperatures in colder environments than in warmer ones and in larger birds compared to smaller ones. For example, the TNZ of the tiny Green-backed Firecrown hummingbird of Chile and Argentina is 55–82°F versus 50–122°F for the Ostrich of mid- and south Africa, which weighs as much as two average humans. The TNZ of the 6 ounce desert-dwelling Gambel’s Quail of the southwestern United States is 86–111°F, and the Adélie Penguin of the Antarctic coast is 14–68°F.
We say that birds and mammals are warm-blooded, but that’s a misleading term. The desert iguana, sitting on a rock in the sunshine of a desert can absorb enough solar energy to bring its body temperature up to 115°F. Even though that would make the lizard warm-blooded, lizards are considered cold-blooded animals. The proper term for warm-bloodedness is “homeothermic,” meaning that the body temperature remains relatively constant regardless of the ambient temperature; this enables birds and mammals to exploit habitats with extremes of temperatures. Homeothermy requires a cornucopia of mechanisms to maintain the stability of the body’s internal environment. In the winter homeotherms need to produce more heat because more heat is lost to the environment in cold temperatures. To produce this heat, the BMR has to increase. Studies of small birds in winter environments showed that the BMR of Black-capped Chickadees increased 6 percent versus a 17 percent rise for Mountain Chickadees and 22 percent for the Juniper Titmouse. The body temperature of cold-blooded organisms, on the other hand, changes with the ambient temperature; they are “poikilothermic,” meaning varying temperature. Poikilotherms either cannot inhabit a cold environment or do so in fewer numbers because their BMR would be so low as to preclude activity. Very few amphibians and reptiles live above the Arctic Circle and none exist in Antarctica.
But even homeotherms have their limits. Although variables such as food, predators, nesting sites, and competitors constrain the population of birds in particular geographical areas, it is temperature (and other meteorological factors to a lesser degree) that fundamentally controls the range of the entire species.
Every bird species has its own thermoneutral zone, which provides a major explanation for the geographic distribution of bird species.
Bergmann’s Rule and Allen’s Rule
Warm-blooded animals tend to have larger body sizes in colder regions; this concept is called Bergmann’s Rule. As body sizes increase, an animal’s volume increases faster than its surface area. Because the bodies of mammals and birds generate heat and the surface area loses heat, larger animals generate proportionately more heat and lose proportionately less heat than smaller animals. Sometimes overheating is a problem; this is why elephants have big ears—to radiate heat. It also explains why hummingbirds are not typically found in cold environments—their small bodies lose heat too quickly for their metabolism to keep up. Meiri and Dayan of the University of Tel Aviv examined 94 bird species worldwide and determined that 72 percent of them adhere to Bergmann’s Rule, although a higher percentage of sedentary species reflect the rule than migratory ones. Body sizes of non-migratory Downy Woodpeckers in the northeastern United States, for example, are larger by almost 10 percent than those found in southern Florida. Similar patterns hold for the sedentary Blue Jay and White-breasted Nuthatch. It makes sense that any bird species spending all its time in one environment should be more closely attuned to that environment than a species that spends only part of the year there. Not surprisingly, 75 percent of bats also adhere to Bergmann’s Rule.
Allen’s Rule says that the appendages (arms, legs, ears, snouts) of homeotherms are shorter at higher latitudes and colder temperatures because longer appendages mean higher heat loss. Cartar and Morrison of the University of Lethridge and the Canadian Wildlife Service examined 17 species of breeding shorebirds in the Canadian Arctic and determined that the lengths of tarsi decreased as the harshness of the environment—as judged by wind speed, solar radiation, and temperature—increased. They argue that birds with longer legs are more exposed to the wind’s cooling effect and thus have to spend more energy keeping warm. Similarly, Nudds and Oswald of the University of Leeds in the United Kingdom found that the unfeathered portions of the legs of 43 species of terns and gulls are shorter in colder environments.
New evidence for Allen’s Rule came about after the discovery of the thermoregulatory capability of the toucan bill. (I’ll get to that shortly.) University of Melbourne scientists measured the beaks of 214 species of birds from across the world and found a relationship between bill size and latitude—the higher the latitude, the smaller the bill. The role of bird bills in heat regulation had received scant attention with research instead focusing on bill configurations as related to feeding habits, so this study was significant. The researchers concluded that there is a thermoregulatory cost to a larger bill so selective pressures produced smaller beaks in colder climates. Thermal imaging demonstrated that bills with a larger surface area dissipated 33 percent more heat than smaller bills. Song Sparrows living in drier habitats have bills with larger surface areas than Song Sparrows in moister habitats, which tend to be cooler. The only exception the researchers found was among the grass finches of Australia. Apparently, the finch bill’s seed-handling capability is under stronger selective pressure than its thermoregulatory costs.
Adaptations for survivability are not always neatly organized and it’s worth noting that Allen’s and Bergmann’s Rules are merely rules of thumb for birds. Many exceptions exist such as the Yellow Warbler, whose breeding range extends from the Caribbean to the Arctic, and whose characteristics do not support either rule.
DEALING WITH HEAT
Heat stress (hyperthermia)—elevated body temperature—appears, overall, to be more severe than cold stress (hypothermia) in birds. Overheating is a common problem faced by poultry producers, as too high an ambient temperature causes a loss of body and egg weight and poor quality meat. But overheating is also a concern for wild birds, more so now with increasing global temperatures. In January 2009, thousands of birds, mostly parrots, died in Carnarvon, Western Australia, when the temperature reached 113°F; the following year 208 Carnaby’s Black Cockatoos, an endangered species, were found dead when the temperature hit 118°F. A small bird can lose 5 percent of its weight every hour at that temperature, leading to dehydration and eventually death in a matter of hours.
Birds have many morphological and physiological adaptations that allow them to respond to a range of temperatures and survive with little or no stress. As the ambient temperature increases, physiological mechanisms approach their limits and behavioral mechanisms come into play. Here are some of those adaptations and mechanisms for surviving the heat:
BILLS THAT RADIATE HEAT The toucan’s bill has always been fascinating but it was made even more so when a new role for the Toco Toucan’s bill was revealed in 2009 via infrared thermography. With a large and colorful bill used for picking fruit and capturing small animals, this tropical South American bird also employs its beak to radiate body heat. The surface area of the bill covering is about half of the body’s total surface area, uninsulated, and well supplied with superficial blood vessels. While flying, the toucan might increase its metabolic heat production by a factor of 10–12, so in a warm tropical environment the bill is an important source of heat loss. Another indication that the bill radiates heat is the toucan’s common practice of tucking its bill under a wing and folding its tail forward over the bill while at rest to reduce heat loss. Relative to its body size, the Toco Toucan has the largest radiator in the animal world, even compared to the ears of elephants.
The Toco Toucan’s bill was discovered to be a radiator.
EVAPORATION VIA THE SKIN When we sweat, moving air wicks the water off our skin to provide a bit of cooling. Birds do not have sweat glands but their skin contains water and cutaneous water loss assists in cooling; how much cooling depends on the thickness of the skin, the amount of fat, and the blood supply. In birds, the fat (lipid) layers of the skin minimize water loss in a thermoneutral environment, but also function to facilitate heat loss during episodes of hyperthermia. Verdins, like many small birds, stay within their TNZ by losing water, along with accompanying heat, from their skin. This cutaneous evapotranspiration accounts for 50–65 percent of the bird’s total water loss. However, as the temperature rises, respiratory water loss—from lungs and air sacs—becomes increasingly important. Diamond Doves of the arid deserts of central Australia have little access to water. They feed in the heat of the day without shade cover but to survive they conserve water by allowing their body temperature to rise, like camels, but remain within the TNZ. The more heat that can be stored, the less water is needed to dissipate it. As the air cools later in the day, the bird will need to use a lot less water to dispose of the excess body heat.
AVOIDING OVERHEATING As birdwatchers know, early in the morning and late in the afternoon are far better times to watch birds than midday. Near the equator in places like Bolivia or Uganda, little or no bird activity occurs in the hours around noon as the heat and humidity make it nearly impossible for a bird to engage in any serious movement. Conversely, in cold environments, birds are more active because they need more food to survive and the energy they expend in foraging helps to generate body heat.
PANTING AND GULAR FLUTTERING Panting, the rapid increase in respiratory rate and air volume, is an additional mechanism for heat loss in birds subjected to heat stress. Songbirds lose excess heat by cutaneous water loss as long as the birds are sufficiently hydrated. When dehydration occurs, loss of water from the respiratory system comes into play, and if that isn’t sufficient, panting begins. Panting movements may be as much as 16–27 times the normal respiratory rate. If the heat load continues to increase, some non-passerines (non-songbirds) may also employ gular flutter. In these birds the gular or neck region of the skin from the bottom of the jaw to the throat is featherless or nearly so. The floor of the mouth and throat vibrate by the use of hyoid bones and muscles, helping to dissipate heat from the bare skin. Gular flutter is common in seabirds such as pelicans, cormorants, anhingas, boobies, frigatebirds, and ground-dwelling birds like turkeys, pheasants, and roadrunners. These large birds generate considerable body heat for which overheating could be a problem without gular fluttering. The flutter rates of various bird species vary from 235 to 735 flutters per minute. Panting and gular fluttering may be synchronized as they are in owls and domestic pigeons or uncoordinated as in pelicans and cormorants. The difference appears to be that the gular region of birds that show synchronization is comparatively small and may simply be driven by the movement involved in panting while unsynchronized panters have large gular regions that are controlled independently of panting. The energetic cost of panting is high, as is the amount of water lost, so birds may use the more efficient mechanism of gular flutter instead to conserve both energy and water. Monk parakeets employ lingual flutter, vibrating their tongue in coordination with their respiratory rate. The cloaca, the common urogenital opening, can also be used to dissipate excess heat; this is probably an emergency measure as it only occurs at particularly high ambient temperatures. Cloacal evaporation in Inca Doves has been measured at 21 percent of total evaporation at a temperature of 107.6°F.
Gular flutter in a cormorant.
SEEKING SHADE When we get too warm we can seek shade, fan ourselves, find a breeze, drink water, or jump in a pool—birds can employ similar behaviors. Desert-dwelling birds probably face the greatest challenges. Rock Doves in the Negev Desert move among rock formations to seek shade on hot days; when they sit, they spread their wings and erect their dorsal feathers to dispel heat from their backs. Birds in arid areas like the Sonoran Desert minimize heating and water loss by installing themselves in tree crevices during the hottest part of the day, forgoing foraging activity to decrease their exposure to high temperatures. The Crowned Plover, which lives in eastern and southern Africa in short grass habitats, raises itself on its legs above its nest and spreads its wings to expose more body surface area to the air. Although the bird shades and cools its eggs while doing this, the primary point of this behavior is to disperse heat from the bird’s body. The Hoopoe and Crested Larks of the Arabian Desert (where more than a year can pass without rainfall and the average summer humidity might reach only 15 percent) confront air temperatures of more than 113°F and ground surface temperatures as high as 140°F. To avoid the heat of the day, the larks forage in the early morning and late afternoon and spend the rest of the day resting in the shade of shrubs. During the hottest times of the day, the birds often hole up in the burrows of Egyptian spiny-tailed lizards, scrape away the top layer of sand, and prostrate themselves with their neck and chest against the ground to conduct heat away from their body—this reduces their potential water loss by 81 percent. The birds are capable of lowering their basal metabolic rates by about 40 percent to decrease the loss of water from their skin and respiratory systems. Early in the nesting season while it is still relatively cool, most lark nests are built on the open ground, but as the nesting season progresses, the majority of nests are built in or under shrubs.
A Crested Lark in the cool early morning hours.
RAISING OR FLUFFING OF FEATHERS Birds use the technique known as ptiloerection to both increase and decrease heat loss, the difference being in the position of the feathers. To increase heat loss, the body feathers are lifted high enough so that the skin is exposed and cutaneous water loss increased for cooling as the blood flow to the skin is increased. Australia’s Spinifex Pigeon lives in an arid environment where half of the year the temperature in the shade exceeds 100°F. The bird survives by having a low metabolic rate and manages its cutaneous heat loss by elevating its skin temperature and raising its feathers. Great Knots wintering on the north coast of Australia prepare for their long migratory flight to the Arctic by adding layers of fat, but because they are doing this preparation in a warm environment they face overheating. To avoid heat stress, the birds raise the blackish feathers of their back to reduce absorption of solar radiation and increase both convective and cutaneous cooling. An alternative to raising feathers is flattening them, as the Curve-billed Thrasher of the southwest United States and Mexico does. Compressing the feathers against the body reduces their insulating value by eliminating most air spaces. In addition to gular fluttering, the Great Frigatebird engages in three heat-dissipating postures. When heat stress is minimal, the birds erect their feathers on various parts of the body; as the temperature increases, they droop their wings; and during the hottest temperatures, they spread their wings for maximum cooling. European Bee-eaters at the southernmost tip of Israel have been observed diving into salt ponds and the Red Sea. After emerging from the cooling water, they perch in a tree and spread their wings to take advantage of any breeze and the low humidity. Occasionally birds get waterlogged and have to lie on the beach with outspread wings to dry before they can fly. Besides that inconvenience, bee-eaters have also been found in the stomach of sharks.
UROHYDROSIS Wetting the legs by excreting on them is sort of like spraying your legs with water and letting evaporation cool you off. The Turkey Vulture spreads its wings, extends its neck and head, and, engaging in a behavior that adds to the bird’s rather unpleasant reputation—it poops on its legs. An interesting problem was discovered several years ago when a bird bander found that several of the Turkey Vultures that he had banded developed lesions on their lower legs because of the accumulation of waste matter on the leg bands. (The U.S. and Canadian banding offices no longer allow leg bands on vultures.) Urohydrosis is rather rare in the bird world because it requires easy access to drinking water, but New World vultures, herons, condors, storks, gannets, and boobies also engage in this unsavory activity.
PLUMAGE COLOR White-feathered birds reflect more solar radiation than darker birds so we expect white birds to be more common in hot environments. Herring Gulls nesting in the open are subject to intense solar radiation, so when they not able to leave the nest when incubating or brooding, they orient their bodies so that their whitest plumage, with the highest reflectance, faces the sun. So why aren’t all birds in hot environments white or at least a light color? The size of the bird, the flight functions of the feathers, the microstructure of the feathers and skin, the range of ambient temperatures, the bird’s migratory habit, the plumage’s social or sexual signals, and visibility to predators are all factors that could affect the color of a bird’s plumage. To complicate things further, Gloger’s Rule says that birds and mammals living in humid environments tend to have darker plumage or fur than animals of the same species living in drier habitats. For example, Song Sparrow populations of the Pacific coast show increasingly darker plumages from southern California deserts north to the moister and cooler Pacific Northwest forests. The same pattern of south to north darkening is seen in House Sparrows in the Rocky Mountains and the Midwest, and in more than 90 percent of the bird species of North America. One explanation is that melanin, the black-brown pigment, makes feathers stronger and more resistant to degradation by feather bacteria, which grow well in humid environments. Also, perhaps, the paler colors of birds in drier and lighter-colored environments help in concealment.
SURVIVING THE COLD
People in cold environments, unlike birds, can take advantage of a variety of clothing to keep warm—mukluks, parkas, sweaters, and thermal underwear. It makes me cold just watching geese sitting on frozen ponds, shorebirds pecking away on mud flats in a biting wind, and gulls flying through heavy rain over winter ocean swells, but it appears that they are more physically, physiologically, and behaviorally suited to avoid hypothermia than hyperthermia, and not just because some of them migrate to the tropics for the winter.
George Murray Levick was a surgeon and zoologist on Robert Scott’s disastrous expedition to the Antarctic in 1910–1913. Scott did not survive but Levick and his notebooks did, in which he described the sexual behavior of Adélie Penguins. He was so taken aback by their “astonishing depravity”—homosexuality, coercive sex, and sex with dead females—that he wrote his observations in Greek so that only “learned gentlemen” could understand. Some of that depravity results in baby penguins, such as those in March of the Penguins. The film’s depiction of fuzzy young penguins being whipped by the blistering winds of the Antarctic makes one wonder “how do they make it?” It takes about 15 days after hatching for young Gentoo and Chinstrap Penguins to develop a sufficiently insulating layer of down, so they need to be brooded by their parents for that time; when the offspring are 25 days old, their downy coat is quite adequate for the task. The feathers are stiff and short, and not arranged in tracts like other birds—instead the feathers cover the entire body, overlapping like roof shingles. The wind, rather than ruffling the feathers, compresses them, increasing the young birds’ insulation. In fact, when the winds bluster over the colony, the young are comfortable because the insulating efficiency of their coat is not only ample, but 136–178 percent better than in calm winds! The contour feathers of the adults are similarly compressed when the birds are swimming, increasing both insulation and aerodynamics. In addition, the deeper feather layers are composed of ever-smaller feathers that produce insulating pockets of air.
As winter approaches, many birds increase their body fat for energy storage and insulation and thicken their plumage with extra down feathers. The Common Eider, a northern sea duck, increases the insulation value of its plumage by 25 percent for the winter by adding down. When molting in the breeding season the eider uses its down feathers for nest lining. Vikings learned to make down comforters out of eiderdown for their long voyages. Today the only eiderdown harvested from wild birds is in Iceland, where it is collected from nests after the breeding season and a small truckload exported annually, which is why eiderdown comforters are so expensive. The Rock Ptarmigan, a grouse of the Subarctic and Arctic Eurasia and North America, puts on extra fat, comprising up to 32 percent of its winter body mass, almost doubling its weight; that fat plus down results in an insulation-efficient body. American Robins might increase their feathers by 50 percent as winter approaches; American Goldfinches nearly double their body feather count.
I was once asked about pregnant birds. After I thought a bit, I realized the inquirer was looking at a Hermit Thrush that had ruffled its feathers, giving it an apparently larger body. Ptiloerection, used for cooling, is also employed in reducing heat loss. How many holiday greeting cards have you seen with a fluffed up bird pictured against a snowy background? A moderate amount of fluffing traps sufficient air to form layers of feathers and increases insulation value by 30–50 percent. However, on a rainy day, this might not be the best strategy. Laboratory studies of American Kestrels demonstrated that fluffing during a rainstorm allows water to penetrate to the skin and lower their body temperature, so the birds slick down their plumage instead.
Hermit Thrush fluffed up for warmth.
Instead of adding extra feathers for cold weather, Eurasian Bullfinches increase the amount of fat ingested, not for insulation but for food reserves. Birds need to eat enough food during the day to put on enough fat to allow them to survive the night. The colder the temperature, the more foraging required to accumulate fat, but there is a tradeoff between expending the energy necessary to accumulate fat stores and being more exposed to the weather and predators while feeding. Heavier birds are also more at risk from predators as they move a bit more slowly. Every day is different because of variation in the weather, competitors, predators, and the food supply.
One strategy for minimizing energy expenditure and maximizing food resources is to establish a territory. Both male and female European Robins hold winter territories. The male holds the same territory all year round and the female establishes one to protect a winter foraging area. Singing to advertise their protected area is important but it takes energy. Robins with a higher body mass sing more when they greet the dawn than thinner birds. The birds were less apt to sing at sunrise if a cold night required them to use their fat stores. Similarly, the amount of time nightingales spend singing during the night is directly related to their latitude (higher latitude, lower temperature, less singing).
Countercurrent Heat Exchange
Losing body heat through respiration is handy in a hot environment, but not so helpful when it is cold, so physiology has had to adapt. When we, and birds, inhale cold air, it is heated as it passes through our nasal passages, throat, trachea, and bronchi. When the air reaches the lungs it is almost body temperature; then we exhale warm air. In birds, a countercurrent exchange of air occurs in the nasopharyngeal passages so that some of the heat from the expiring air is recaptured. Depending on the ambient temperature and the species of bird, the amount of heat recaptured can be as high as 80 percent, as it is in penguins.
Sanderlings standing on one leg.
The surface area of a beak may be a compromise between heat loss and foraging requirements. Likewise, leg and toe lengths are compromises between locomotory needs and heat loss. The old joke I lay on beginning birdwatchers is “Why do some birds stand on one leg?” The answer: “Because if they lifted it up they would fall down.” After the groans subside I explain that to reduce heat loss in cold temperatures, some birds like the Sanderling tuck one leg up into the feathers of their abdomen while resting, or both feet while sitting down or floating on the water. But it is not always possible to tuck one or both legs away. Instead, ducks, gulls, tinamous, and many other birds have a countercurrent heat exchange system between the arteries and veins of their legs. Blood in the arteries from the heart is warmer than venous blood returning from the extremities. The arteries and veins are intertwined in a structure in the lower leg called the rete tibiotarsale (net of the tibiotarsus) which may consist of as few as three arteries and five to seven veins, as in owls, or up to 60 arteries and 40 veins as in flamingos. Birds without this net have two veins, one of which runs closely on either side of an artery. The warmer arteries pass on some of their heat to the colder veins, returning some warmth to the heart but still providing enough heat to the legs and feet to keep them from being frostbitten. In some species of gulls and guillemots, the arteries and veins are closer to each other in the birds of more northern areas than in those of more southerly locations.
Countercurrent heat exchange operates very efficiently. A duck standing on ice will lose body heat, but only 5 percent of that loss will come from its feet. Of course, blood has to supply oxygen and nutrients to the feet and legs, but only a small amount of blood is needed because the muscles that operate the feet and legs are mainly concentrated in the upper leg and utilize long tendons for mobility. At lower temperatures that threaten the feet with frostbite, blood flow is increased to the lower limbs by opening special valves in the arteries. By the same mechanism, birds can lose body heat in a hot environment by shunting more blood to their legs and feet.
Heat exchange in lower leg. In countercurrent heat exchange, blood flowing in the arteries toward the foot is warm; the blood gives up some of its heat to the veins going toward the heart.
Huddle Pyramids and Other Behavioral Adaptations for Cold
Physiological and physical adaptations diminish the effects of cold, but behavioral adaptations can be just as effective. Penguins of the Antarctic are exposed to temperatures as low as -94°F but yet their feet do not freeze. Again, countercurrent exchange, minimal muscle in the feet, and feathered legs all help, but the birds might also squat down and let their abdominal feathers droop over their feet. Penguins can also lean back on their heels and tails and lift the front of their feet up off the ice and against their body. Loons and grebes, spending most of the day on the water, extend a leg, shake the excess water off, and tuck one foot or both feet under a wing. Hummingbirds crouch so their downy belly feathers cover their feet. Some sparrows and other songbirds drop to the ground and hunker down. Shorebirds, gulls, ducks, herons, and other birds may rest with their bills tucked under a wing or scapular feathers, mimicking what we do when we wrap a scarf around our faces. Infrared studies of heat loss indicate that one of the areas of greatest heat loss in birds is around the eye, one reason why a cold bird, especially a small one, tucks its head under a wing.
Birds huddle together to conserve body heat. Flocks of Pygmy Nuthatches, sometimes more than 100 birds, spend the night close together in a tree cavity. Dozens of Tree Swallows survive snowy nights in Canada by perching close together on a wire while tucking their beaks into their feathers. Typically solo foragers, Snow Goose goslings spend more time huddling together when the ambient temperature drops. European Long-tailed Tits roost tightly together in a line at night, competing for the middle positions as the birds on the ends get colder and lose more weight overnight. Bald Eagles prefer the middle of a roost and the colder the ambient temperature, the more birds seek the middle, although the maintenance of dominance hierarchies has a role as well. Inca Doves will stand on each other’s backs, up to twelve birds three layers high, to form a huddle pyramid for warmth. Penguins too group together tightly in the chilly Antarctic environment. With temperatures down to -60°F and winds up to 100 mph, the temperature in the middle of the penguin pack might reach 70°F. The birds continually shuffle around so that penguins on the edges move toward the middle and vice versa and the entire group survives by sharing heat loss.
Being wet adds to the problem of retaining body heat. Anhingas (also called snakebirds, darters, and water turkeys) are found in many shallow warm water habitats of the world. They have wettable feathers, allowing the birds to make quick dives. Although they have fairly low metabolic rates, their aquatic lifestyle causes a loss of body heat, so when they leave the water they perch in a wings-spread posture with their backs to the sun, both to dry their plumage and absorb solar radiation. Bank Cormorants, endemic to the cold ocean waters of Namibia and western South Africa, allow their body temperatures to drop to nine degrees below normal while foraging continuously for nearly an hour. Like anhingas, they recover their body heat by basking in the sun.
A population of Great Cormorants winters in Greenland near the Arctic Circle, with water temperatures near freezing and sub zero air temperatures. They live there even during the polar night (24 hours of darkness) when they are unable to sunbathe after foraging. How do they survive? Great Cormorants are the most efficient foragers of all water birds ever studied, capturing 0.6 ounces per minute, or about 2.2 pounds of fish per hour, so they only need to spend 2 percent of their daily activities fishing and 3 percent of their time flying. Eating plenty of fish and resting most of the day keeps their body temperature within the TNZ. Even when the sun is shining, they do not sun themselves with spread wings. It is so cold, humid, and windy that if the birds spread their wings to dry, they would lose body heat. On the other end of the world, the related Antarctic Blue-eyed Shags behave similarly.
Emperor Penguin chicks huddling.
Lots of birds sunbathe, though. African Penguins (formerly “Jackass Penguins” because of their braying call) orient their black backs and flippers to the sun soon after sunrise for an hour or two before becoming active. Herons, egrets, pelicans, vultures, hawks, and hawks engage in sunbathing, but we can only assume that this behavior is to absorb solar radiation when the ambient temperature is low and/or the birds are wet. Other reasons for wing spreading include losing heat, drying out, or allowing the sun to kill skin parasites and bacteria. An experiment in which both louse and mite populations in a colony of Violet-green Swallows were reduced with the application of pesticides resulted in the birds sunning themselves considerably less than birds in a colony of swallows that was not sprayed. Birds get some of their vitamin D from sun exposure, but since the sun can’t reach their skin, they use another mechanism. The precursor chemical for producing vitamin D is found in the uropygial or preen gland. During preening the chemical is spread on the bird’s feathers and when exposed to ultraviolet light, changes to vitamin D. The next time the bird preens, it ingests the vitamin D. Water birds like the Shoebill Stork preen more often to stay waterproof.
The unusual Shoebill Stork of Africa, ugly as it may be, still needs to preen.
Shivering and Non-Shivering Thermogenesis
When you spend too much time in the cold, you began to shiver—the body’s physiological response via small contractions of mostly skeletal muscles to produce more heat. Birds do the same thing; this is shivering thermogenesis. Most of this energy comes from fat stores and much of the shivering occurs in the pectoralis muscles. As winter approaches, birds acclimate to the increasingly colder weather and don’t shiver as often, although in severe weather, survival might require shivering for days. Overwintering birds obviously have the greatest need for the extra heat produced by shivering, but shivering also occurs in some migrants traveling north from the tropics. One study of a vireo indicated that on spring migration the bird had the capacity to generate 17 percent more heat from shivering than it did during the summer. On their northward migration, birds move rather quickly to their breeding grounds but may run into cold weather along the way, so this extra heat-generating capacity might come in handy.
Non-shivering thermogenesis is the generation of body heat without muscle contraction. The digestion and absorption of food requires energy and produces body heat. The Tawny Owl of the northern forests of Eurasia is nocturnal, feeding primarily on small birds and mammals. These feeding bouts produce enough body heat so that the bird generally avoids shivering even on the coldest nights. A bird can maximize its heat generation by “choosing” the best schedule for food intake. Instead of using energy to generate heat, some birds, such as the Gray Jay of the Arctic, enter a state of hypothermia by letting their body temperature, normally 107°F, drop to about 99°F; this is torpor.
Torpor and Hibernation
It took awhile to establish the basic facts about hibernation that we now know. Twenty-four centuries ago, Aristotle watched summer swallows as they flew over a marsh and dipped into the water’s surface. Noticing their absence from Greece in the winter, he deduced that the birds must have dived into the water and buried themselves in the mud below to spend the colder months. Sure enough, next spring the swallows were flying above the water. This speculation spread widely and for many years fishermen reported that birds emerged from their nets along with fish and that chunks of mud dredged from the bottom of a swamp contained hibernating birds that flew off after they were released from the muck and warmed up. This bizarre myth steadfastly persisted into the early 19th century. Others who witnessed the disappearance of birds speculated that they hibernated underground.
Nothing as spectacular as mud hibernation occurs in the bird world, but at least 29 species of birds demonstrate a lowering of their metabolism and body temperature for periods of time. If a hypothermic bird exhibits a significant depression of metabolic function and little or no response to external stimulation, it has entered torpor. German researcher Elke Schleucher, in her review of avian torpor, tells us that the major reasons for entering that state are a lack of food and cold weather. Torpor has arisen over evolutionary time as a sort of emergency measure for many birds facing low temperatures and/or temporary food shortages and for a few birds as a way of saving energy to make it through the night. Some think that birds might use torpor during migration to reduce the amount of foraging necessary en route.
Virtually all birds that enter torpor are insectivorous, frugivorous, or nectarivorous because their food sources are so unpredictable. In a laboratory study of Red-backed Mousebirds, researchers found that if the birds were deprived of food until they lost 35 percent of their body weight, their metabolism would drop to one-third of normal and the birds would enter torpor. Torpor in free-living mousebirds is rare, however, as they cluster together to share body heat on cold nights. The normal daytime body temperature of hummingbirds is about 100–104°F, but during nighttime torpor, their body temperatures can decrease to the ambient temperature if it is not too cold. A couple of hours before dawn, with no apparent stimulus except its innate circadian rhythm, a hummingbird will begin to arouse from torpor. Respiratory and heart rates increase, the bird vibrates its wing muscles, shivers, and after 20–60 minutes the bird is ready to begin feeding. The mainly insectivorous female Puerto Rican Tody may exhibit torpor even with abundant food and tropical conditions, perhaps because of the stresses of breeding, and can lower her body temperature by 25°F although she remains awake and alert.
At least one bird goes into such a deep state of torpor it resembles hibernation. The Poorwill was called holchko, the sleeping one, by the Hopi Indians. Lewis and Clark may have found one of these birds in a state of torpor in 1804 although a confirmed sighting was not until much later, in 1879 California. The Poorwill is a robin-sized insectivorous bird of the American southwest; all its relatives—nightjars, potoos, and nighthawks—enter torpor, but the Poorwill is the only one to do so for extended periods. Torpor is used extensively by Poorwills when the ambient temperature is below 50°F. Typically roosting under a cactus or next to a rock while facing a southerly direction, they might awake if they are warmed by sunlight and spend the day foraging, only to reenter torpor at night. Some Poorwills spend 10–20 days in a state of torpor, with no stirring at all. This behavior is very much like that of hibernating small mammals that wake occasionally to snack on their stored food cache. There has been speculation for many years that some swifts and swallows hibernate but most likely they are entering a moderate level of torpor.
Common Poorwill sitting on a gravel road.
HOW WEATHER AFFECTS BIRDS: MIGRATION, FOOD SUPPLY, AND DISEASE
The Old Farmer’s Almanac says “If crows fly in pairs, expect fine weather; a crow flying alone is a sign of foul weather.” No evidence supports this particular saying, but some bird behavior can be used to predict bad weather. Have you ever noticed birds, often swallows, perching on power lines as a storm approaches? That’s because birds perch more as a low-pressure center (cold front) approaches. Low barometric pressure is a reflection of the reduced density of air molecules that makes it energetically more expensive to fly. In one study, captive White-throated Sparrows were experimentally subjected to different pressure regimes. Under high pressure the birds awakened in the morning and began to preen themselves in preparation for the day’s activities. When the pressure was lowered, the birds awoke and immediately began to feed, expecting bad weather. Apparently a sensory organ in the middle ear can detect changes in pressure because of shifting weather conditions as well as altitude, but little is known about that mechanism.
Every day the weather is different and birds have to face extremes of temperature, wind, rain, snow, and solar radiation and try to stay in their zone of thermoneutrality. When they cannot, nature has provided birds with an array of adaptations to survive until they again reach TNZ.
Bank Swallows gathering before a storm.
Weather-Related Mortality of Migrating Birds
The impetus to migrate is not the weather, although weather unquestionably affects the migratory behavior of birds. Heavy rain, hail, snow, and wind can take a heavy toll on migrating birds. Storms can delay departure and winds can slow or speed up their passage or blow birds off course. A snowstorm interrupted a spectacular migration of Lapland Longspurs in northern Iowa and southern Minnesota on March 13–14, 1904. It was not a particularly cold night and the wind was calm, but heavy wet snow was falling. Birds fell from the sky and landed everywhere—in towns, on the roads, on roofs, and especially around streetlights. Floating on the surface of two small lakes in the area were an estimated 750,000 birds. The total estimate for this disaster was 1.5 million dead Lapland Longspurs.
If it is raining, feathers can become saturated and migrants might have to interrupt their journey. Birds may have the option to stop if they are traversing land but over water many birds would die. Land birds passing over large bodies of water are often lost without a trace, drowned or eaten by scavengers on the water or after being washed ashore. An enormous loss of migrating birds, estimated at 40,000 migrants of 45 species, occurred during a tornado on April 8, 1993, off of Grand Isle, Louisiana, a barrier island at the northern edge of the Gulf of Mexico that is reputed to have the highest density of birds in the United States during a spring migratory landfall. Indigo Buntings appeared to have suffered the greatest losses, but Cerulean and Swainson’s Warblers and the Seaside Sparrow also showed significant mortality. Water birds such as ducks, geese, and grebes don’t face an overwater problem as they can rest on the water but may have the opposite problem while crossing over large expanses of land. Eared Grebes winter in the southwestern United States to southern Mexico, but have to cross large expanses of desert to get to their nesting sites in the western United States. Even if the weather is reasonable, strong headwinds can prevent the migrants from reaching their destination, causing the birds to expire from exhaustion or starvation. Small songbirds are most vulnerable to the vagaries of the weather, but one incident of strong storms passing over the Mediterranean killed at least 1300 birds of prey. Neither are seabirds immune from ocean storms. In the first two months of 2014, more than 20,000 seabirds, mainly puffins, auks, and murres, died in the seas off the Atlantic Coast of France, primarily because of starvation and exertion as the birds tried to avoid storms and seek food. According to the French Society for the Protection of Birds, this was the largest die-off of birds in France since 1900.
Hurricanes, low-pressure storms with winds exceeding 74 mph, do not usually seriously affect birds even though they often occur in the Atlantic during the peak of migration season, mid-August to late October. Generally, migrating birds do a pretty good job of avoiding or evading storms. But birds die, of course, some from hypothermia because of wet plumage, especially young chicks still in the nest. High winds alone can kill birds if they are hit by flying debris, thrown into objects, or blown so far off course that they become disoriented and exhausted. Hurricane Sandy in 2012 produced some unusual sightings such as Northern Gannets, normally found in the North Atlantic, in New York Harbor, and a Pomarine Jaeger off Cape May, New Jersey, far from its usual migratory path in the Atlantic. A 2005 study in Quebec found that the local Chimney Swift population fell by 50 percent after Hurricane Wilma blew the birds astray, some as far as Western Europe. (These swifts are typically only found in the eastern part of the United States while breeding, and the western part of South America while wintering.) In 1989 Hurricane Hugo hit the shores of South Carolina with winds of 87 mph. An analysis of the bird populations afterward, albeit rather crude and incomplete, indicated only minor mortality. Seabirds moved northward before the storm and land birds stuck close to the ground. A study of the birds of a Puerto Rican forest immediately after Hurricane Hugo revealed that nectarivores and frugivores declined precipitously while omnivores and insectivores increased and granivores only slightly declined. Less than a year later, the numbers returned to their pre-hurricane abundance, indicating that the mortality rate was low—the birds just temporarily moved elsewhere.
The major effects of hurricanes (as well as tornadoes, fire, flood, and earthquakes) are more significant if the habitat is significantly altered. Normal food sources might be gone, nest sites or protective tree cavities damaged or destroyed, and there might be increased exposure to predators. For any creature acclimating to a new environment, the first survival tactic is to find alternative food sources. Before Hurricane Hugo, the Antillean Euphonia, a colorful tanager-like bird, specialized on mistletoe berries; after the hurricane, it ate berries from at least eight other plants. Typically, for a year or two after a hurricane, the reproductive success of birds is low; after three or four years reproduction recovers to normal levels or even higher because of the growth of new vegetation. Adapting to changed environments is critical to survival, but not every species can respond quickly. Populations of woodpeckers, owls, nuthatches, and others that depend on tree cavities for protection and nesting recover more slowly.
Weather, Foraging, and Food Supply
The time period at which birds migrate evolved as the most advantageous time to find an adequate food supply. So when an atypical cold and wet storm hit New England in the spring of 1974, it resulted in an unusual mortality of insectivorous birds, especially Scarlet Tanagers, which normally prey on large, heavy-bodied insects. The population of Scarlet Tanagers decreased by 33 percent the following year and 67 percent the year after that as reproduction was minimal and some adult birds died.
Birdwatching on a cold, brisk, windy day is usually a challenge. In a forested environment, birds will move away from the edge and into the denser forest. Woodpeckers shift from foraging on small branches to larger branches or the trunks of trees. Nuthatches stay on the trunk but birds like chickadees and titmice forage closer to the ground, where it is less windy, and dive into shrubs with each gust of wintry air. In more open areas birds may try to forage in the warming sunlight. Ospreys feed less in windy weather because the roiled water surface interferes with seeing potential prey. Smaller birds like the Tricolored and Little Blue Herons and Snowy Egrets may suspend foraging activity for as long as three days in severe weather, losing 6–12 percent of their body weight. Great Blue Herons, and probably other herons, feed more frequently when the sky is overcast than when it is sunny or raining. Presumably their prey of aquatic animals is more active on cloudy days or less able to spot the predator standing above them. Or the birds simply need more food.
Predators complicate the challenges posed by the vagaries of the weather. In a winter study of Common Redshanks, Eurasian shorebirds, the birds could choose to feed in one of two places: a productive salt marsh or less-productive intertidal flats. The colder the temperature, the more the birds preferred the salt marsh, but that habitat attracted predators like the Eurasian Sparrowhawk. At lower temperatures, sparrowhawks were more successful at catching their redshank prey because the redshanks had to spend more time feeding, but the redshanks still chose the more hazardous but more productive salt marsh. Better to obtain sufficient food at the risk of being eaten than face sure starvation.
Perhaps 60 million people in the United States and 25 million in the United Kingdom feed birds in the winter. Many people who maintain bird feeders around their home are concerned that they not only attract seed-eating birds, but also predators, and they do. Cornell Laboratory of Ornithology’s Feeder Watch program determined that of all the predation incidents around bird feeders, the Sharp-shinned Hawk was responsible for 35 percent, Cooper’s Hawks 16 percent, and cats another 29 percent. Predation by the Red-tailed Hawk, American Kestrel, and Merlin combined only accounted for 12 percent. The data suggest that although home feeders expose small birds to some risk from predators, they are no more dangerous than the wild and may actually provide some safety, as more birds are present to warn of a predator’s approach. And the additional food provided may lessen the amount of time a bird has to forage elsewhere, reducing its total exposure to predation. Besides the predation question, people are concerned that the birds will become dependent on the feeder food in the winter, and fear that if the feeders become empty when the owners go on holiday, the birds will starve. A study of Black-capped Chickadees in the northeastern United States found that they obtained only 21 percent of their daily winter calories from bird feeders. Another study involved withdrawing food from feeders in areas where birds have been fed for 25 years and compared the survival rates of birds in those areas to birds that had never received supplementary feed. No difference. So go on a holiday and don’t worry about feeding your birds.
Weather and Disease
Birds can contract about 60 diseases. Some are mild, others devastating. Birds may or may not develop an immunity and different species may respond differently to infection. The causes and effects of diseases are extremely varied and not everyday occurrences so birds often do not have the mechanisms to survive them. Avian deaths from disease may be closely linked to weather conditions, but most epizootics (diseases in animal populations) among birds tend to be localized, and are most frequent in birds that winter together in close proximity, such as waterfowl. Here are some of the more common and well-known avian illnesses:
AVIAN CHOLERA, caused by a bacterium, affects about 150 bird species, but is especially deadly to waterfowl in their wintering areas where the birds congregate and the bacteria are disseminated by bird-to-bird contact or ingestion of contaminated food. The bacteria also collect on the water’s surface and spread into the air when the birds fly off. In the late 1970s cholera caused high mortality in Common Eiders wintering in open areas in the ice off the Netherlands and in the year 2000, 10,000 Baikal Teal died in Korea of the disease. Cholera outbreaks can happen anytime but are most common during the winter when cold conditions make waterfowl more susceptible to the disease as well as reducing the number of ice-free areas, causing the birds to crowd together. Fog and rain discourage birds from flying, further encouraging bird-to-bird contact. In March of 1994, an outbreak of avian cholera in severe cold weather killed an estimated 100,000 waterfowl in the Chesapeake Bay region. Cold temperatures along with a low-pressure center prevented the birds from dispersing northward. Earlier outbreaks in 1970 and 1978 killed up to 100,000 waterfowl each time. Avian cholera is detected virtually every year in waterfowl of the Mississippi Flyway. Researchers there do not think that cold weather initiated the epizootic but determined that colder temperatures were responsible for a higher mortality of infected birds. Warm weather can also exacerbate cholera epidemics. In 2012 the reduced snowmelt from a dry year was insufficient to freshen the water in the Klamath Basin of Oregon and the disease killed an estimated 10,000 waterfowl. The continuing drought in California is forcing more birds into smaller and less abundant bodies of water so the potential for future severe cholera epidemics looms.
AVIAN BOTULISM, caused by ingesting the toxin produced by the bacterium Clostridium botulinum, is responsible for hundreds of thousands of waterfowl, shorebird, and gull deaths in the western United States each year. An episode of the disease in Russia in 1981 killed more than a million birds. A study of the causes of mortality of 600,000-plus near shore and freshwater aquatic birds of 153 species over a period of 30 years in the United States indicated that botulism was the leading cause of death. The toxin acts upon the nervous system and kills birds by interfering with nerve impulse transmission. As paralysis sets in, birds can no longer fly or even hold their head up and often drown. The epidemics tend to be greatest in the late summer and early fall, especially with high ambient temperatures. Warm weather and heavy rain may precipitate botulism epidemics, but predicting outbreaks is difficult.
WEST NILE VIRUS (WNV) is a fairly new disease on the bird scene in the United States. Since first being found in New York in 1999, it has spread to all 48 contiguous states, infecting at least 300 species of birds with varying degrees of susceptibility. During the years 2001–2005, warmer weather, higher humidity, and increased precipitation contributed to a 30–80 percent higher incidence of WNV. Mosquitoes spread the virus to birds (and occasionally humans and other vertebrates). Most birds recover after about a week and become immune to another infection although crows and jays suffer a near 100 percent mortality. The disease is usually mild and rarely fatal in humans, with a mortality rate of about one in 1000. WNV is different from many other mosquito-borne illnesses because an urban-dwelling mosquito (Culex pipiens) is the primary transmitter. As the climate changes and we experience warmer winters and hotter summers, more mosquito eggs and larvae survive. In the spring, the few puddles or ponds that are available are saturated with nutrients that support mosquitoes. The adult mosquitoes emerge in large numbers because water levels are lower and there are fewer mosquito predators like frogs, lizards, and damselflies.
All environments undergo changes, both long and short term. Birds can adapt to these changes over evolutionary time. The soil, the vegetation, the topography, food supply, and potential nesting sites just don’t change much year to year under natural conditions. The weather pattern changes from year to year and day to day so it is more difficult for birds to develop adaptations to deal with this moving target. But weather in any particular geographic area has parameters and limitations and it’s the continuum of weather conditions birds have evolved to deal with. Climate change, global warming, however you define the increasing heat of the planet, has already modified some bird habitats and behavior and we will no doubt see more alterations. A disease, weather-caused or exacerbated by weather conditions, is one of those things that birds seem to have few adaptations to survive. It is often just a matter of bad luck. The same is true for many sources of human-caused bird mortality, which we will get to later in the book.