Daily Archives: December 7, 2014
Has anyone ever called you a bird brain? Maybe that term is not as nasty as intended. As we will see today, a bird’s brain is remarkably complex.
We know that birds and reptiles are each other’s closest relatives. Let’s compare a lizard brain and a bird brain.
The noses of land vertebrates are at the front of the body so it is no surprise that the olfactory lobes of vertebrates are the most anterior. The olfactory lobes of a reptile are larger than those of a bird. The sense of smell in a bird is pretty poor in most species. Exceptions occur like Turkey Vultures that locate carrion by smell and albatrosses and other tubenoses that smell the oils on the surface of the ocean released by groups of squid or small fish. Homing pigeons use smell for the final leg of a homing flight to find their roost. But smell is about the only function of the brain that is better developed in lizards.
The olfactory lobes make up a small portion of the forebrain; the cerebrum constitutes the majority of the cerebrum. The cerebral hemispheres are involved in complex behavioral instincts as well as learned intelligence. The cerebrum of a bird dwarfs the cerebrum of a lizard of similar size.
Parrots, corvids (crows and their relatives), woodpeckers and owls are regarded as the most intelligent birds. The particularly large size of their cerebrums is not surprising. Chickens and their relatives and pigeons have relatively small cerebrums.
Birds are very good at lab tests of intelligence, particularly the corvids. Birds can be taught to count to seven; researchers required 21,000 trials to teach monkeys to distinguish two from three.
In fairness to lizards, recent work at Duke University has shown that the Puerto Rican anole tested on a food-finding apparatus scored one for the reptile team. These anoles were able to solve problems as fast and sometimes faster than birds. But, the general rule is that birds are smarter than lizards.
The major part of the mid-brain of a bird or lizard is the cerebellum. The cerebellum plays a major role in muscular coordination. The demands of the complex body movements required for flight result in a well-developed cerebrum in birds. Aerial acrobats require complex control of movements to maintain equilibrium. The cerebellum of a land-restricted lizard is smaller.
The optic lobes of a bird or reptile are also part of the midbrain. Again, the optic lobes of a bird are much larger than the optic lobes of a lizard. We know that birds have the best vision of any vertebrates. The large optic lobes of a bird process the complex signals sent from the eye. Birds can certainly resolve colors much better than lizards (or humans), even seeing into the ultraviolet part of the spectrum that we humans cannot see at all. The resolution of images by birds is unrivalled. This stream of data requires a big computer for processing and the optic lobes function superbly in this regard.
The hindbrain of a vertebrate consists mostly of the medulla, the junction between the rest of the brain and the spinal cord. The hindbrain is also the site of the auditory lobes. We expect that the auditory lobes of a bird would be well developed because of the importance of vocal communication in birds. The auditory lobes are better developed in birds compared to lizards but are nowhere near the relative size of the cerebrum, cerebellum and optic lobes. Avian hearing and human hearing are pretty similar and we know that humans have a poor sense of hearing compared to most mammals.
If someone calls me a bird brain, I take that as a compliment: well-developed intelligence, the grace and control of a gymnast or ballet dancer, better vision than we humans can even contemplate and a pretty good sense of hearing. Thanks!
[First published on November 23, 2014]
In the last post, I embarked on an historical wild goose chase. I am tracing the development of our understanding of bird migration through the ages. The Barnacle Goose (this European vagrant was seen recently in Aroostook County) was the centerpiece of the last column. It’s name came from the medieval misconception that Barnacle Geese and barnacles are different stages of the same animal.
Humans did not really get a handle on bird migration until the 18th century, finally putting Greek myths about hibernation and transformations to rest. In 1749, Johannes Leche began recording the spring arrival dates of Finnish birds. As we will see, these types of records can be valuable in understanding migration.
The first published statement of bird migration appeared in Thomas Berwick’s A History of British Birds in 1798. Berwick disputed the prevalent notion that British swallows hibernated, writing “they leave us when this country can no longer furnish them with a supply of their proper and natural food …”.
From around 1900, local bird clubs have been collecting arrival and departure dates for migratory birds. By reading these local reports, an observer could determine that Ruby-throated Hummingbirds arrive in the Gulf Coast in early March, around April 10 in Virginia but not until early May in New England. The wave of migration of North American birds thus becomes evident through the shared observations.
We have come a long way since Lemche’s lonely records in Finland. Central depositories like ebird.org hold millions of records so the patterns of northward spread in the spring and southward withdrawal in autumn are clearly seen. If you haven’t tried the tools under the Explore Data link on ebird, give it a try.
Plotting the arrival and departures of migratory birds gives us insight into bird populations but not individuals. Do Ruby-throated Hummingbirds that cross the Gulf of Mexico and land in Louisiana continue their migration to the Midwest while those that land in Florida migrate up the Atlantic seaboard? We must track individuals to answer such questions.
Bird banding is just the tool we need to follow individuals. Audubon almost certainly banded the first birds in North America. In 1840, he tied a silver thread around the legs of several Eastern Phoebe nestlings on his farm near Philadelphia. Two of the phoebes came back the following year. Of course, he had no idea where the phoebes went to pass the winter but he clearly established the power of banding in following individual birds
The North American Bird Banding Program (NABBP), begun in 1920, facilitates the banding of birds in the United States and Canada. After extensive training, a person is provided with a Bander’s Permit and is given aluminum bands, each with a unique nine-digit number. Banders capture birds in nets or traps; identify the species, sex and age of each bird; take various body measurements; affix an aluminum band of the proper size to one of the legs of the bird; and release the bird.
If another bander captures the bird or if a banded bird is found dead, the finder contacts the biologists at the NABBP who provide the recovery data to the original bander and notifies the finder of the original date and site of the banding. Over the 94 years of the program, over 64 million birds have been banded and 3.5 million of them have been recovered. We have learned much about subpopulations of migratory species that maintain different migration routes, as well as information on fidelity to wintering and breeding sites over the years.
Even greater detail of migration routes can be gleaned from satellite transmitters mounted on birds’ backs or from small data loggers called geolocators that track a bird’s geographic position continuously. A geolocator has to be recovered to download the data, unlike a satellite transmitter. How cool is it to monitor an Osprey’s migration from your computer desktop?
[First published on November 9, 2014]
Some birders will argue that the fall migration beats the spring migration hands-down. Sure, spring songbirds are singing with full throat, dressed in their breeding season finery. But the spring migration is relatively brief.
The fall migration is much more protracted, spanning early August into December for different species. Post-breeding dispersal of many species leads to surprising records of vagrant birds. Storms may also displace migrants.
In recent years, Old World Geese visit New England in small numbers. A few Pink-footed Geese have graced us with their presence in cow pastures in Yarmouth. A Barnacle Goose or two visit northeastern North America each fall. The nearest breeding area for both of these species is Greenland.
We occasionally see a Greater White-fronted Goose. This widely spread species occurs mostly west of the Mississippi River in North America but also in the Old World, as far west as Greenland.
On October 13, Bill Sheehan hit the goose jackpot in central Aroostook County. He found six species of geese. Three species were not surprising: Canada Goose, Cackling Goose (a smaller version of Canada Goose, now recognized as a separate species) and Snow Goose. But he hit the trifecta of rare geese finding a Barnacle Goose, a Greater White-fronted Goose and a Pink-footed Goose. A great day!
But Barnacle Goose? It’s a peculiar name because these geese are vegetarians like other geese and rarely if ever eat intertidal animals. The explanation for the name provides a good opportunity to think about the methodology of science as we seek to better understand the natural world.
The path leading to our current understanding of bird migration is a circuitous one, with plenty of dead ends. Like most scientific inquiry, observers build on the observations of those that came before them. The notion of standing on the shoulders of earlier observers stems from at least the 12th century to a man known only as Bernard of Chartres. The metaphor is best known from Sir Isaac Newton’s quip, “If I have seen further it is by standing on the shoulders of giants.”
Humans have certainly been aware for millennia that bird abundance changes through the year. When you depend on birds as part of your diet, failure to pay attention to changes in bird numbers can influence survival. But where did the birds go?
The notion of migration is implicit in a verse of the Old Testament: “The stork in the heaven knoweth her appointed times; and the turtledove and the crane and the swallow observe the time of their coming.” In the eighth century BC, Homer wrote that cranes flee from the coming of winter.
A couple of centuries later, Aristotle wrote, “Some creatures can make provision against change without stirring from their ordinary haunts; others migrate as in the case of the crane.” He also wrote of the migration of pelicans. So far, so good. Observers surmised that some birds come and go in response to the changing of the seasons.
Unfortunately, Aristotle also wrote “certain birds (as the kite and swallow) decline the trouble of migration and hide themselves where they are.” He went to write that some birds hid themselves in hles in the ground, sometimes without their feathers. Some Greeks also believed in transformation. In Greece, the European Redstart is a common breeder, migrating to Africa each fall. The European Robin is a winter visitor to Greece. Aristotle claimed European Redstarts transformed into European Robins.
In the Middle Ages, Europeans used Aristotle’s mistaken observations to explain the arrival of Barnacle Geese each fall from their high Arctic breeding grounds as a transformation from the stalked, goose-neck barnacles found commonly on floating driftwood. We’ve come a long way since then but the history of this misstep is perpetuated in the Barnacle Goose’s name. We’ll continue our exploration of migration next time.
[Originally published on October 25, 2014]
October is a time when we can expect to see large flocks of Dark-eyed Juncos. The ivory-colored bill is a good field mark. Adult males have gray above, extending down onto the throat and upper breast. The gray contrasts with the white lower parts of the underside. Females are brown above with some muted gray feathering as well. In both sexes, the outer tail feathers are white, flashing a warning when a bird suddenly takes flight.
Although these sparrows can be found year-round in Maine, their numbers are bolstered in our state by passage migrants (migrants simply moving through Maine) in April en route to more northerly breeding grounds and in October en route to more southerly wintering areas.
Dark-eyed Juncos breed from northern York County northward throughout the rest of the state. Breeding densities are greatest in the spruce-fir forests of the northern half of Maine. Some juncos overwinter but their numbers vary from year to year.
The entire geographic range of Dark-eyed Juncos is huge, extending as far north as the tree line in Alaska and Canada and southward throughout most of the lower Forty-eight, excepting peninsular Florida. Some even winter in northern Mexico. A recent estimate placed the Dark-eyed Junco population at around 630 million birds.
This broad range means that most North Americans see juncos during some portion of the year. The confiding nature of these birds and their willing use of human-altered landscapes increase the likelihood of our encountering these birds. John James Audubon in 1831wrote that “there is not an individual in the Union who does not know the little Snow-bird”. Growing up in North Carolina, I heard these birds referred to as snowbirds as well. Their arrival in the South coincided with the onset of winter.
Junco is the genus of these birds as well as the common name. The name seems ill chosen because Junco is derived from Juncus, an emergent reed growing on the margins of lakes and ponds. Hardly the habitat of a junco.
Juncos feed primarily on the ground and from the leaf litter. Seeds as well as insects and other arthropods like spiders make up the bulk of the diet of Dark-eyed Juncos. They will occasionally take fruit. Diet studies show the juncos prefer small seeds. Important seeds taken include those from chickweed, pigweed, knotweed, sorrel and timothy grass.
Although juncos spend a lot of time on the ground, I often find male juncos singing their hearts out at the top of a tall spruce or fir. Their trilled song is easily confused with other trillers, particularly the Chipping Sparrow. The junco song is more musical and bell-like to my ear.
Males are strongly territorial during the breeding season, defending a territory of about three or four acres. In winter flocks, a strict hierarchy or pecking order is set up. Every junco knows her or his place in the power structure of the flock.
Dark-eyed Juncos have been the course of much debate among bird taxonomists. Until 1973, the currently recognized Dark-eyed Junco was split into five species, each with some distinctive feature on top of the generalized junco morphology. In the east, we had the Slate-colored Junco and the other species were White-winged Junco, Oregon Junco, Gray-headed Junco and Guadalupe Junco. Based on the joint possession of dark eyes and other anatomical considerations, the Check-list Committee of the American Ornithologist Union merged these five species into a single species. This decision is disputed by some bird systematists. Recent fieldwork shows that different junco forms at abutting geographic boundaries do not freely interbreed. This evidence may be sufficient to restore species status for some of these forms.
I remember the frustration of many competitive North American birders at the decision to lump the five species of juncos into one. These listers saw their life lists decrease by four species!
[Originally published on October 12, 2014]
In August, I wrote a column to commemorate the extinction of the Passenger Pigeon 100 years ago. Passenger Pigeons once numbered in the billions. Some ornithologists believe that 40% of all North American birds were Passenger Pigeons.
Errol Fuller has written an informative book called The Passenger Pigeon in this sad centennial year. The book is not an exhaustive treatment of our knowledge of Passenger Pigeons. Rather it is a celebration of this departed species through a mix of prose, paintings and photographs.
Other chapters deal with the mind-boggling descriptions of Passenger Pigeon abundance in the early 19th century, the morphology of the bird, the rapid decline from human hunting and ultimate extinction of these pigeons in the wild, the efforts to maintain these birds in aviaries, and finally the extinction of the last Passenger Pigeon in the Cincinnati Zoo in 1914.
Fuller cites Alexander Wilson, the author of the first ornithological treatise on North American birds, who calculated that over two billion birds flew over a spot in Kentucky over a period of four hours. Audubon made a similar calculation, estimating over one billion birds. The mind reels.
The book is filled with interesting tidbits. I learned that the name Passenger Pigeon is derived from the French word passager, meaning traveler. I was also fascinated by the efforts of a few men to maintain (unsuccessfully) Passenger Pigeons in aviaries around 1900 when only a few Passenger Pigeons survived in the wild.
The Unfeathered Bird
The plumage of birds, often gaudy, sometimes cryptic, is the feature most of us concentrate on when watching birds and clinching an identification. The internal anatomy of birds differs greatly among species. Differences in the skeletons of various birds are, of course, hard to appreciate in a living bird. Katrina van Grouw helps us to see the diversity of internal anatomy in her recent book, The Unfeathered Bird.
An unfeathered coot.
Van Grouw has prepared nearly 400 illustrations of 200 species of birds from around the world. We see birds doing their natural behaviors but only the skeletons are shown. The feathers are stripped away so we can really appreciate the relationship between form and function of the skeleton. Each specimen is drawn from an actual specimen.
Van Grouw is uniquely qualified to author and illustrate this book. She was formerly a curator of the ornithological collections in the Natural History Museum in London. She has handled many living birds as a bird bander and dead birds as a taxidermist. Finally she is a graduate of the Royal College of Art.
The first part of the book is a general introduction to the bird skeleton. The remainder of the book is organized into sections based on bird types (raptors, woodpeckers and their relatives, waterfowl and waterbirds, wading birds, game birds, and perching birds).
The book need not be read from front to back. I delight in just opening the book randomly to see the skeleton and read the descriptive text of a particular bird.
Here’s a taste of the book. The Secretary Bird from Africa is a snake-eater. These birds have the longest legs of any bird of prey. Unlike other birds with long legs, a Secretary Bird has a short neck. The short neck keeps the bird’s head away from dangerous snakes that have no stake in becoming lunch for the Secretary Bird. Secretary Birds have to bend their legs to reach the ground to eat or drink.
The toes of a Secretary Bird are quite short, far less formidable than the talons of a typical raptor. A Secretary Bird kills its snake prey by kicking the snake to death. Form and function meet perfectly.
[Originally published on September 28, 2014]
Every now and then, I devote a column to a different group of flying animals, the insects. We’ll recognize the insects as honorary birds today.
This column was precipitated by an interaction I saw last weekend while I was mowing the lawn. I saw a black wasp interacting in the grass with what I thought was perhaps a second wasp. Going closer, I realized the wasp was attacking a large wolf spider. This interaction was the beginning of a fascinating but macabre relationship.
The spider, at least five times the size of the spider wasp (family Pompilidae), tried to escape as the wasp injected a neurotoxin into the spider. The toxin quickly took effect and the spider was paralyzed. The wasp then quickly dragged the spider to the side of our house. It walked up a granite foundation stone and underneath the lowest cedar shingles.
The rest of this tale will take place sight unseen. The wasp will lay one inside the paralyzed spider. Though paralyzed, the spider will live. The wasp egg will hatch inside the spider and the larva will eat the internal organs of the spider. The wasp larva will eventually pupate, later emerging as an adult. Maybe I should have saved this story for a Halloween column!
Entomologists classify wasps like the one I saw as a parasitoid. Unlike a true parasite, a parasitoid either kills or sterilizes its host. The wasp I saw is hardly unique. Many wasps, in dozens of families, are parasitoids. Some lay eggs on other insects but wolf spiders are commonly used. Wolf spiders typically do not build webs. A spider’s web is used primarily to capture prey but offers a secondary benefit of protection from wasps. The free-ranging wolf spiders are therefore at risk to parasitoid wasps. I may have unwittingly caused the demise of the spider by cutting the grass and making the spider easier to see.
Bird migration is near its peak now. We can predict the order of departure in the fall: first the swallows and flycatchers, then the warblers, then the sparrows and hawks. Migration implies a predictable, seasonal movement. Animals may engage in nomadic wanderings are not predictable enough to warrant as migration. For instance, White-winged Crossbills wander widely to find bumper crops of conifer cones.
Although birds are the best migrants, they do not have a monopoly on migration. You have no doubt seen videos of wildebeest, zebras and other mammals migrating to and from the Serengeti Desert.
Some insects migrate as well. A widely distributed dragonfly, the Wandering Glider, occurs on six continents. Populations in Africa migrate to India after the monsoon season starts. The abundant rains provide ample opportunities for the females to lay eggs in aquatic habitats.
Some butterflies migrate as well. In North America, the best known migrant is, of course, the Monarch. Any Monarchs you see this fall will attempt to fly south to a small pine forest in mountains in northwestern Mexico. Even they can make it to the wintering grounds, they will not return. The complete migration from Mexico back to Mexico the following year requires five or six generations!
Originally published on September 14, 2014]
We are coming up on the 100th anniversary of one of the sad days in ornithology. On September 1, 1914, the last remaining Passenger Pigeon, a female named Martha, died in captivity in the Cincinnati Zoo. A species that had been astoundingly abundant had vanished from the earth.
Photo of Martha taken by Carl Hansen, Smithsonian Insitution, 1985.
Passenger Pigeons had a large range in eastern North America, spanning eastern Canada south to the Gulf coast and west to western Texas and Montana. They resembled the Mourning Dove but were much larger, up to 17 inches long. Their feathers glowed with red, gold and blue iridescence. The word passenger in their name refers to their migratory tendencies, not to their use in carrying messages for humans.
Passenger Pigeons were undoubtedly once the most abundant bird in North America. In the early part of the 19th century, Passenger Pigeons were more abundant than all other North American birds combined! There is a report of one flock that contained over two billion birds. Audubon reported a migrating flock of Passenger Pigeons in Kentucky that blackened the sky for three days. Some nesting colonies were 20 miles across. Such numbers boggle the mind.
In Maine, Passenger Pigeons were summer residents. They were abundant until 1820 or so and were common enough to provide successful hunting until around 1850. The first record we have of Passenger Pigeons in Maine dates from the French explorer Samuel Champlain who found them on islands near Cape Porpoise in what is now York County.
Why did this species go extinct? The answer is probably obvious: humans. A large industry developed in the 19th century to provide Passenger Pigeon meat for the tables of European immigrants to the United States. Because of the pigeons’ flocking and colonial nesting behavior, they were easy targets. Large numbers of pigeons could be harvested with ease. Some were caught in nets, others fell to the ground after being smoked with sulfur fires, yet others were killed by guns. Special guns, precursors to machineguns, were developed to allow large numbers of pigeons to be killed quickly.
The harvest of what seemed like a nearly infinite resource was not regulated. The development of railroads, providing a fast way to get pigeon meat to eastern markets, drove the hunting to an even greater intensity. By the middle of 19th century, several thousand people derived a livelihood from harvesting and selling Passenger Pigeons. One processing plant in New York handled 18,000 birds a day. A billion birds were harvested in a single year in Michigan.
But how could the most abundant bird in North America be harvested to extinction? As the population started to decline, one would expect the industry to collapse, giving the ravaged Passenger Pigeon population a chance to recover. The answer appears to lie in the highly social nature of the species. The gonads of most birds regress to about a fraction of their active size during the non-breeding season. Changing day lengths cause birds to increase their sex hormones and their gonads enlarge. Rather than increasing day length, the cue for Passenger Pigeons to enlarge their reproductive organs and initiate nesting was the social stimulation of lots of other Passenger Pigeons in a local area. Without a large enough concentration of birds, the reproductive cycle did not begin.
The collapse of the Passenger Pigeon started around 1880 and commercial hunting, no longer profitable, ceased. Unfortunately, the large flocks of Passenger Pigeons were dispersed across the continent in smaller groups. These flocks may never have gotten large enough to induce the beginning of the nesting cycle.
The population continued to decline and was extinct in the wild until 1900. The last Passenger Pigeon reported in Maine was shot in Dexter in 1896. Efforts to breed the species in captivity failed. Martha was the last of her species, gone 100 years ago.
[Originally published on August 17, 2014]
In the last column, I wrote about the melanin and carotenoid pigments that birds embed in their feathers to impart color to the plumage. Melanins are manufactured by the bird while carotenoids must be acquired from the diet.
Porphyrins are a third type of feather pigment. Porphyrins can make red, brown, green and even pink colors in a variety of bird species. The brown colors of many owls are caused by a combination of melanins and porphyrins. Pigeons and grouse have porphyrins as well. Porphyrins produce the green plumage in a group of African birds called the turacos.
Porphyrins are created by birds by stringing together amino acids. The structure of porphyrins is actually quite similar to hemoglobin.
One cool aspect of porphyrins is that they are bright red under ultraviolet light. Birds’ color vision allows them to see into the ultraviolet portion of the spectrum, an ability we mammals do not have. Porphyrins must be much more vivid as seen through the eyes of a bird.
A group of birds from New Guinea called the pitohuis have black heads and an orange body, an indication that they deposit both melanins and carotenoids in their feathers. But the Hooded Pitohui and Variable Pitohui go one step further by adding poisons to their feathers!
This remarkable phenomenon, well know to New Guinean natives, was only appreciated by western scientists about 20 years ago. Jack Dumbacher, then an ornithology graduate student, caught some pitohuis in his mist nets. After handling the birds, he had a sneezing attack as well as tingling in the hands. He was able to show that the feathers and skin of the pitohuis have a potent toxin related to the deadly toxin produced by some poison arrow frogs. The pitohuis acquire the toxins from their favored beetle prey and sequester it for their own use. The nasty toxins in the feathers may function in repelling ectoparasitic mites and insects as well as discouraging snake, raptor and mammal predators.
But how about the blue coloration of a Blue Jay or Indigo Bunting or the green coloration of a parrot? Those colors are produced by light interference rather than by an actual pigment. We classify the blue of a Blue Jay as a structural color.
Small pockets of air within the feathers of any bird with blue coloration scatter the incoming light (a mixture of all the colors of the rainbow). Shorter wavelengths are scattered more than longer wavelengths. So, the blue wavelengths are scattered to a greater degree than the other colors so the blue wavelengths are the ones we see.
In parrots, the vacuoles have a different shape to maximize the scattering of the green wavelengths. The primary color scattered back from such parrots is therefore green.
Because the light is scattered in all directions, the blue or green color will appear equally vivid from any viewing angle.
The shimmering of a hummingbird’s throat (the gorget) or the shimmering of the eyespots of a peacock’s tail relies on a different kind of interference. The iridescent colors in these birds change with the angle of view. These colors are also quite bright. The shimmering of the colors of a male hummingbird’s gorget as it twists is head is simply stunning.
Rather than having simple vacuoles, a hummingbird feather has a complex vacuole system consisting of several sheets of vacuoles stacked one on top of the next. As light is absorbed by the vacuoles, light scattered from one portion of the structure interferes with light scattered from another portion. Sometimes, these interactions cancel each other out and sometimes they are in perfect synchrony, leading to bright colors. The gorget of a hummingbird can range from black to dull red to bright red depending on the angle of the observer.
[Originally published on August 3, 2014]
The topic of this column and the next was inspired by an email from Lisa Jones. She sent me a video of an albino American Robin in her backyard in Clinton. You can see the video in another post on this website.
The robin that Lisa found is a pure albino. All of the feathers are white and eyes are red.
Many birds, of course, have feathers of strikingly different colors. Rose-breasted Grosbeak males are stunning birds with black, white and red feathers. Ducks, especially drakes, often have many different hues in their feathers. A Green-winged Teal is nice example.
How are these feathers colored? We need to realize that a fully formed feather has no living tissue associated with it. During the formation of the feather, tissues in the feather-forming structures called follicles form the intricate shape of the feather to be. The cells in the follicle tissues secrete keratin, the same basic material in your fingernails. Once the complex feather is formed from the secreted keratin, the follicle cells withdraw, leaving the non-living keratin behind. A feather is produced!
Often, follicle cells lay down pigments in the developing feather. The most common type of pigment is melanin. Melanin gives a feather dark coloration (black, browns, grays).
A bird is able to manufacture the melanin it needs to color it feathers. In the process of digestion, proteins are split into their basic units of amino acids. A protein is nothing more than a long chain of amino acids. One of these amino acids is tyrosine. Birds use tyrosine to manufacture melanin.
The follicle cells intersperse melanin granules in the keratin secreted to make a feather. A feather with no melanin or other pigments appears white because all wavelengths of light are reflected back to a viewer. Dark melanins absorb all the wavelengths of visible light and hence are black to our eyes.
In addition to providing color, melanins also appear to strengthen the feathers. In a flying bird, the primary feathers at the tips of the wings undergo pronounced deformation during the downward stroke of powered flight. The tips of larger birds are often richly endowed with melanin. Snow Geese, American White Pelicans, Northern Gannets, Wood Storks, American Bitterns, and most species of gulls provide only a partial list.
The glorious yellow of an American Goldfinch, the brilliant orange of a Baltimore Oriole and the vivid red of a male Northern Cardinal are produced by a different type of feather pigment called carotenoids. Carotenoids are plant pigments used in the process of photosynthesis. Birds cannot manufacture carotenoids so must recycle the carotenoids from their food. The pathway of carotenoid acquisition can be complicated. For instance, flamingoes are pink because they sequester carotenoids from their small shrimp prey, which in turn get the carotenoids from the unicellular algae growing in saline lakes.
Have you ever seen a House Finch male that was dull orange rather than red? Such a washed-out bird is not acquiring enough carotenoids in its diet to make deep red breast and head feathers. Animal behaviorists call the intensity of the coloration an honest signal. A Scarlet Tanager with stunning red colors is advertising its ability to forage effectively. That red coloration can’t be faked. On the other hand, a Scarlet Tanager with a washed out appearance is advertising its poorer foraging abilities. Which male do you think a female Scarlet Tanager would choose for a mate?
Scarlet Tanagers use their carotenoid resources wisely. Their contour feathers only have red carotenoids at the tips. Since the contour feathers overlap like shingles, only the outer tip of each feather is seen. There is no benefit in coloring parts of feathers that will never be seen. Female Scarlet Tanagers have an olive-green color. This color represents an interaction of melanin and carotenoid pigments in the same feathers.
[Originally published on Jul 20, 2014]