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. 
