I remember a similar thing with tern bills. Three species in one colony:
Common, Arctic, Roseate. In the early summer, the bill colors were
easily diagnostic. However as summer went on and breeding ended, they
all returned to a color scheme similar to the Common. The Roseate bills
started to red and the Arctic bills started to get black. By summer's
end bill color was worthless. They were all half black, half red.

We focused on bill color as a character out there because it was one many
of our tourists could see, as they had trouble picking the terns apart in
flight. However, by August they had to be weaned from bills to wings and
jizz.

____________________________
Mike Smith
Univ. of Washington, Seattle
whimbrel at u.washington.edu