Tuesday, February 28, 2017

Looking more closely at feathers

The Black Vulture above shows off the feathers that allow it to soar the skies. The feathers of the wings and tail are major reasons a bird can generate lift and thrust and also maneuver in shifting air currents. You can count the feathers- about 70- that make up the flight feathers.

And yet, according to Feathers by Thor Hanson, hummingbirds have around 1,000 feathers and swans have up to 25,000 (apparently mostly in the neck) (2011). Most birds have less than 10,000 feathers, with smaller birds having in the lower thousands (Wetmore 1936). Even if the Black Vulture above had only 1,000 feathers, its feathers primarily dedicated to flight represent less than 10% of the total. Although all feathers contribute to flight by streamlining the bird and reducing its weight relative to other substances, they do so much more.

Down feathers on a baby Chipping Sparrow.
A feathered dinosaur on exhibit at the American Museum of Natural History.
Although there is still much unknown, according to Hanson, it is generally
accepted by most scientists that birds evolved from theropod dinosaurs.
Additionally, fossil evidence points to feathers evolving earlier than
any known birds, which means that birds were not the only organisms
to have them (some non-avian theropod dinosaurs had them too).

A contour feather from a Helmeted Guineafowl in Uganda. Contour feathers
are most of the feathers that are visible on a bird and contribute to the
patterns we see.  

One of the most fascinating aspects I learned from this book was that feathers grow in tracts. Nearly all birds have the same pattern of tracts, where the feathers grow in specific rows separated by bare skin. The overlapping feathers arranged in tracts contribute to plumage patterns that span diverse species, such as breast streaks on hawks and sparrows.

What's more is that birds also have control of their feathers, so that they can raise their feathers to expose the bare skin to allow heat to escape. High-speed photography has detailed how birds like Peregrine Falcons can individually adjust feathers in flight to adjust their course or speed. Indeed, it appears that nearly all flying birds can adjust their feathers to "alter the turbulence patterns around their wings."

One of the useful aspects of Feathers is an illustrated guide to feathers in the appendix. I found the Great Blue Turaco feather in Uganda long ago, and after reading this book, I have a better understanding of what I found and photographed. Some of the feather types could be elminated alone by sight, but I will outline the process fully. The above feather has a rachis (the central, rigid structure), which narrows it down to either a flight feather, contour feather, semiplume, bristle, and filoplume (eliminating down feathers). Semiplumes, filoplumes, and bristles can be ruled out because they do not contain a vain; the vain is made up of barbs and each barb is like a mini-feather, with even more barbules coming off each barb. Barbules interlock to form the overall structure seen in the above picture, which appears to be one whole connected piece. This leaves contour feathers and flight feathers. The rachis is offset from the center of the feather. Whereas contour feathers are essentially symmetrical, flight feathers have an offset rachis. Thus, this flight feather is from the tail or wing of a Great Blue Turaco.

Snowy Egret in Mexico

The feathers on the Snowy Egret above are part of the their breeding plumage and are semiplumes (not flight feathers, not contour feathers, not down feathers, but semiplumes). Semiplunes have a rachis (the central, rigid structure) but the barbs (the structures that come off the rachis) do not interlock and thus do not form a vane like the turaco feather above.

This Turkey Vulture is missing some feathers- but birds can grow
feathers back! In fact, birds have cycles of molting in which they replace
their old feathers sequentially with new ones.

Birds grow many feathers over their lives, and all of the feathers form in feather follicles in the bird's skin. While the feather is forming, it is connected to blood flow and is a living structure. However, once the feather is completed, the live tissue recedes and the feather vane is left hollow. Thus, the feathers you see are no longer alive, but are "dead" structures made of keratin.

Golden-crowned Kinglet in Central Park, NY

Feathers allow birds to help survive outdoor temperatures and regulate their body temperature. The Golden-crowned Kinglet (above) is a bird species that lives in North America. Hanson writes about how feathers enable Golden-crowned Kinglets to survive outdoor temperatures below zero degrees, despite having a very small body mass. In contrast, birds can adjust their feathers to expose bare skin and dissipate heat from their bodies.

Black Vulutre in Ecuador

One of the chapters of Hanson’s book is dedicated to discussing that even though feathers convey many survival advantages to birds, they are not always a winning physical trait. The Black Vulture, of North America, is one species that does not have feathers on its head. Vultures stick theirs head in already dead animals; the detritus that would accumulate and further decay on feathers of the head with this feeding strategy may put individuals at a disadvantage. Thus, Hanson, writes, vultures over time evolved to have bare skin without feathers.


  • Hanson, Thor. Feathers: The Evolution of a Natural Miracle. New York: Basic, 2011. Print.
  • Wetmore, Alexander. "The Number of Contour Feathers in Passeriform and Related Birds." The Auk 53.2 (1936): 159-69. Searchable Ornithological Research Archive. Web. 19 Apr. 2017.

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