Subject: Re: gull plumages...
Date: Aug 29 13:41:25 1994
From: Dennis Paulson - dpaulson at ups.edu


>what is the selective advantage for young gulls to have different
>plumage patterns as they mature? is this a visual cue to adult
>birds so they won't be so mean to the youngsters when they are
>fighting over a tasty french fry? does this grey plumage make the
>young birds less visible to predators? why is it that these birds
>maintain this "immature" plumage for several years (depending upon
>the species, of course)?
>
>deborah wisti-peterson
>nyneve at u.washington.edu

The brown plumage of juveniles may well serve to make them more cryptic
when foraging in the intertidal, where many gulls forage--or for that
matter, anywhere that brown is less visible than white (almost any
environment). Note that young glaucous and ivory gulls, which live in icy
regions, are white. But it's not camouflaged from predators, it's
camouflaged from other gulls! Full-grown gulls don't have too many
predators (peregrines and eagles get them in flight, when any color is
conspicuous), but they have much to fear from other gulls, which are superb
kleptoparasites; they'll take already captured prey from any other bird
they can overpower. It's an easy thing to see almost anywhere gulls hang
out. It's characteristic of the whole family--not only the jaegers we
discussed before but even small terns (but black skimmers don't have the
bill for it....).

It's probably not a cue so adult birds won't be aggressive toward young;
they don't hesitate being aggressive toward young! That's probably why you
typically don't see any immature gulls around nesting colonies. I do think
it's interesting that the plumage changes gradually. The final color change
certainly correlates with reproductive maturity, but why not just stay
brown until you're ready to breed and therefore have to compete with the
other adults around the colonies? I can't hypothesize an answer to that
one, unless gulls actually keep track of each others' maturity to
facilitate their interactions. There may have been selection for getting
lighter with age because--with a little boost of diet and hormones--some
individuals can actually breed a year earlier than normal.

There's a lot of research yet to be done on the interactions among adult
and various-age immature gulls; most of the research so far has been done
at dumps.

Dennis Paulson