To continue our series on feathers, let’s take a look at how birds grow these marvels of bioengineering. One of the best ways to understand this process, is to examine feather growth stages in altricial chicks.
Altricial chicks are often featherless (some species have sparse natal down), blind, and completely helpless when they hatch. These chicks grow rapidly, thanks to an overwhelming food supply from the parents.
Once you get past the absolute alien appearance of these little guys, you may notice small white bumps on the skin of the bird, most prominent on the neck in this picture. These bumps are called papillae and they form during embryonic development, or to put it simply, while the chick is growing in the egg. These papillae eventually form follicles, and from these follicles, similar to human hair growth, feathers form.
You may be surprised, however, at how those feathers form. The picture below is the same kookaburra chick, covered in pin feathers.
During this stage, the developing feather is nourished by blood vessels that extend into the feather shaft, giving the pin feathers a very dark coloration. These cells are alive and multiplying, but the cells eventually die as the feather grows, allowing them to harden and stand up to the pressures of the surrounding environment, and the blood supply is cut off once the feather is fully formed. These growing feathers are densely packed in to a feather sheath, and when the sheath begins to break open, the feathers unfurl from the packaging.
This eruption of feathers can be seen below, in a photograph of a Golden-headed Quetzal chick:
To best illustrate feather growth in chicks, check out this amazing series of photographs of growing Red Lorikeets by fellow bird keeper, Matt Schmit:
In all of these chicks, you can easily see that feathers are growing from some areas of the skin, while other areas remain bare. These areas, where the feathers emerge from the skin are called feather tracts, or pterylae. The bare areas of skin are called apteria.
The reasoning for this arrangement of feathers is simple. It allows the birds to function with less feathers overall, somewhat like a ‘feather comb-over’, which drastically reduces the total weight of feathers, just another adaptation for flight.
Most birds have these large areas of featherless skin, but there are a few notable exceptions. Penguins and ratites are uniformly covered in feathers. As flightless birds, they do not need to maintain their svelte figures and can boast as many feathers as possible to cover and protect their bodies.
The arrangement of feather tracts and bare skin is collectively known as pterylosis, and amazingly enough, this differs from one group of birds to another! While we do not know the reasoning behind these differences, scientists can use feather tract patterns to determine how to classify and group birds.
By this point, you may be wondering where all those cute, fluffy baby birds are. Not all chicks hatch, “naked as a jaybird”. The adorable chicks below are covered in natal down.
These natal down feathers are essential to the survival of some chicks, providing insulation and camouflage. While adult birds also have body down feathers, grown from follicles devoted to solely to the production of these down feathers, natal down grows out of the same follicles that eventually produce ‘typical’ feathers. Often, these natal down feathers are actually on the tip of the typical feathers, and as those feathers grow, the soft fuzzy down drops away.
This can lead to a very awkward stage of development, during which a chick may be partially cute and fluffy, and partially pokey and pointy, such as the ibis chick below.