David Sibley explains the variability of songbirds

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

Generally, 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, experience less selective pressure to conform to a norm, and they also undergo a transition and gradually acquire adult characteristics. All of this leads to a more variable appearance for subadult birds.

Two other Indigo Bunting plumages: an immature male (1 year old, left) with predominantly blue plumage, and an adult male (2 years or older, right) entirely dark blue. Art by David Sibley

In the Indigo Bunting, shown in the images above in spring, adult males (in the second spring or later) are entirely dark blue with no brown feathers. Females of all ages are almost entirely brown with only blue highlights in the wings and tail. Immature males (in their first breeding season, nearly a year old) are a patchwork of brown and blue. The amount of blue feathers acquired during the first spring moult varies considerably. Most young males in spring are largely 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 still retain a few immature brown feathers on the wings and belly — and the rhythm of hormonal cycles in the body.