The inspiration for this edition of Dawn Chorus was the horn of the Rhinoceros Auklet.
As you can see, it is a remarkably unlikely feature for a bird, and especially in that it’s so temporary. This Pacific Ocean alcid looks radically different in winter compared to its summer incarnation. Consider the summer look in the picture above, then compare it to what they look like from early fall through winter:
Between now and spring, the Rhino Auklet won’t just molt into some sleek facial racing stripe plumage, but more noticeably, its bill will turn bright orange, get thicker, and it will grow a horn! And then after the breeding season the horn falls off! How can a hard part of its body vary so much?
My go-to resource Cornell Ornithology’s Birds of North America website goes into exhaustive detail about every single molt through a bird’s life as well as phenology, breeding, migration etc. It also has a section for each species called Bare Parts. Feathers take center stage in a bird’s appearance but the bare parts — beaks & bills, feet & legs, plus a few others — are pretty fascinating too, and are clues about its life.
The bare parts can be soft like the skin of their legs (and head, for some) or hard like the keratin of their beak and talons, but all of those take their colors from living tissue. A bird’s beak feels hard like bone but it’s a dynamic feature even more changeable than a bird’s molt, which only occurs once or twice a year. For beaks and feet, the skeletal framework is covered by a layer of keratin — same material as our fingernails and hair — which grows continuously and can take on colors generated by the living tissue between it and the underlying bone. Most birds wear down their beak keratin (Rhamphotheca) as they use their beaks for feeding, digging, grooming, fighting, etc and that keeps up with the steady growth. In pet birds who don’t chew enough to keep up, a vet may need to be called on to trim their beaks to avoid excessive tissue that can interfere with their health.
While in some cases skin color helps in thermoregulation, keeping a bird warm or cooling it off, the primary function of bare parts color is the same as for plumage color. It’s a way of communicating with other birds. It takes extra energy and resources to color bare parts, so there must be a significant payoff. In what situations do birds benefit from colored bare parts?
Maturity
In the bird world, adults are dominant. Juvenile birds who depend on their parents to feed them often have mouths that are brightly colored inside, a message as compelling to a parent as its cheeping and fluttering: FEED ME.
But otherwise, a young bird’s chance of survival improves if it keeps a low profile, both as camouflage from predators and to make it clear it’s low on the social scale of its group and species.
Eagles change appearance from first year to adult, which takes about 4-5 years, including their beak. Legs are yellow from the start, but the beak changes from black to yellow.
More young and adult bird contrasts:
Social dominance, especially for breeding
Sometimes birds will use their dominance for having the upper hand at the dinner table, like turkey vultures at a carcass. It’s not obvious to me why the one eating is able to prevent the others from feeding at this baby seal carcass, but studies of vultures elsewhere, such as Namibia show those with more intense head coloring get first dibs on a carcass. They do that by “flushing”, ie suffusing their skin with increased blood flow making it redder. Evidently the bare head of a vulture isn’t only to keep it cleaner.
Flushing is one of three sources of bare-part coloring, and the most ephemeral.
The primary biological imperative for birds as other kinds of creatures is to reproduce successfully, keeping their genes in play even after they as individuals die (and the dinner table dominance feeds into that breeding dominance). Birds want to mate, and they want to have babies with the most biologically attractive mates possible. That means competition. Mostly we hear about plumage and behavior as the factors females judge to pick a worthy mate, and that both males and females use to establish social dominance. But bare parts figure into the equation too.
The brightly colored blue feet of the Blue-footed Booby have been studied quite a lot, with the general conclusion that the more intense the blue, the more attractive a booby is, and the more worthy of parental investment in offspring, by both moms and dads. Among several other cited studies at the Wikipedia page,
chicks raised by foster fathers with brighter feet grew faster than chicks raised by foster males with duller feet.[18] Females continuously evaluate their partners' condition based on foot color. In one experiment, males whose partners had laid a first egg in the nest had their feet dulled by make-up. The female partners laid smaller second eggs a few days later. As duller feet usually indicate a decrease in health and possibly genetic quality, it is adaptive for these females to decrease their investment in the second egg. — wiki en.m.wikipedia.org/...
Many aquatic birds in my area have bright red feet and legs. I haven’t tracked down any studies to explain why. Perhaps there’s a similar dynamic going on there.
But a bright colored bare part we see across the whole range of birds is beaks, and there are hundreds of studies seeking its significance. In general, it plays the same role as the brightly colored feet of the Blue footed booby: signaling health and superior reproductive potential. The primary coloring agent in bare parts is carotenoids, generating various shades of yellow, orange and red. Birds can’t make their own carotenoids, obtaining them from the food they eat, so a brighter color = better food collecting ability = healthier = better genes. Terrestrial birds get carotenoids from plants, aquatic birds from fish like anchovies.
Black and brownish colors come from melanin, the third source of coloring, which is formed by the bird’s metabolism.
Carotenoid and melanin pigments are deposited in skin and keratin and last in a beak until the sheath grows out and wears down, months to a year or so depending of the species. In comparison, flushing as seen in Wild Turkeys can be as brief as minutes, expressing situational dominance. In general males are the brightest red, females less so, immatures the least (the blue color on their heads, as on booby feet, is a structural color, perceived as blue by how light refracts from the layering of pigmented skin, similar to iridescence in feathers).
The peak colors in birds coincides with breeding season, both plumage and bare parts. A metareview of 321 published studies at BioOne, “The role of bare parts in avian signaling”, describes many examples across the bird kingdom, primarily in mate choice during breeding season. The intensity of color correlates closely with an individual’s vigor, and both males and females prefer to maximize their chances of producing strong healthy offspring. Bare part color is like a shortcut marker, and the subtleties are left to plumage and behavioral performances.
Bare-part signals are likely to occupy a signaling niche distinct from that of both plumage and vocalizations. For instance, their relative inconspicuousness probably makes bare parts most effective at close range. Unlike in plumage, patterning is almost entirely absent from bare parts, making them poor signals of individual identity as well. Instead they may be more accurately viewed as pluripotent social signals of condition, reflecting a variety of different socio-environmental inputs on relatively short timescales.
You have probably seen these changes in beak color. Non-breeding season on left, breeding on right. These are obviously taken at different times, so there are lighting variations, but you can see how the carotenoid color brightens up in breeding season, fading once nesting is over.
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Females pick mates based on perceived fitness, and among sexually dimorphic birds the contrast between the males’ flamboyant color and females’ camouflaged look can be subtle or extreme.
Right now birds are in the quiet time, feeding heartily and staying safe in preparation for the peak of their year: breeding season. Now is the time the need to pack on the weight and accumulate ample supplies of pigments. The colors they turn those into — their bare parts as well as feathers — may well be the difference between getting the chance to breed or not this spring and summer.
What bare parts draw your attention among the birds you see? Do beaks or feet vary seasonally or between members of a species?
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Caribbean beauties
To round out my selection of birdy examples, I’m appending a little gallery of some tropical birds I see in the Caribbean whose bare parts are pretty gorgeous. I’m there at the beginning of the breeding season, so I’m probably seeing them at their most spectacular. Perhaps one day I’ll take a trip there in fall and find out.
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What’s up in your birdy world this week?