PLUMAGE (Feather Coloration)
Starlings are the bird most commonly mistaken for martins because they often try to nest in martin houses. That is part of the reason martins have become scarce - starlings are stronger, more abundant, and more aggressive than martins, and will kill or drive martins away. Starlings have a long yellow beak; Purple Martins have a short, wide, black beak. Martins also have a slightly forked tail.
Females, fledglings and young male Purple Martins look decidedly different from adult male Purple Martin. Only the 2+ year old adult male (a male in its 2nd breeding season) has the solid, iridescent, blue-black coloration (dark all over). Females have dull white underparts and upperparts that are more brownish.
Subadult females (left) are especially brown. Subadult males (right) look very similar to females, but will have dark blue iridescent feathers located randomly on their underside - from their chin to their undertail. Some 1st year males may only have one or two dark blue iridescent feather, while others have many dark blue iridescent feathers. The random patterning and distribution of these iridescent feathers on subadult males (abbreviated SY-M) means that every subadult male is unique, and it is often possible to identify individual SY-M's because of these feathers.
Sexing and Aging terminology
HY = Hatching Year martin; a nestling or fledgling that was hatched in the current season. Since it is not possible to determine the sex until the first molt, these are often referred to as HY-U, which means Hatching Year - sex Undetermined
SY = Second Year martin; a martin that hatched/fledged in the prior nesting season. These individuals are capable of breeding; SY-M = subadult male; SY-F = subadult female
ASY = After Second Year martin; often referred to as an adult. ASY-M = adult male; ASY-F = adult female