David Sibley explains variability in songbirds

Birds are variable, but they don’t have to be confusing. Learning the broader patterns of variation is one of the fundamentals of bird identification and one of the keys to understanding them.

As a rule, immatures are more variable in their appearance than adults. This has a simple explanation as birds communicate visually. The colors and patterns of an adult bird send important signals to potential mates and rivals. Immatures, however, are under less selective pressure to conform to a standard, and they are also undergoing a transition and gradually acquiring adult characteristics. All of this leads to a more variable appearance for subadult birds.

In Indigo Bunting, shown in the images above in spring, adult males (in the second or later spring) are entirely dark blue with no brown feathers. Females of all ages are almost entirely brown with only hints of blue in the wings and tail. Immature males (in their first breeding season, nearly one year old) are a patchwork of brown and blue. The amount of blue feathers acquired in the first spring molt varies widely. Most young males in spring are extensively blue like adult males, but some remain almost entirely brown like females.

This patchwork appearance is partly due to the number of feathers replaced — 1-year-old males always retain some brown immature feathers on the wings and belly — and on the timing of hormone cycles in the body.

Each feather follicle on the bird can produce blue or brown feathers, and it is hormones that “flip the switch” to change the color of new feathers. In young birds, the increase of breeding hormones can be out of sync with the growth of new feathers. If breeding hormones are already flowing before new feathers start to grow, those new feathers will be blue, but if the hormones are delayed, that bird will grow brown feathers until the hormone switch is flipped.

Think about cycles and patterns of variation as you admire the variation in birds this spring, and your understanding of that variation will surely increase.

This article was first published in the March/April 2022 issue of BirdWatching magazine.

Read more from David Sibley on his blog