Alvaro says (see below) that sexual dimorphism in orioles and blackbirds
fits an ecological/behavioral pattern. Here's my halfpennyworth.

I won't comment on orioles, but as far as blackbirds go, I don't see the
pattern. I've only observed two Agelaius species, but both are
exceptions to the alleged pattern.

Red-winged Blackbirds - Sexes different. Males defend territories, yes, but
females also defend territories (against other females). Winter away
from breeding territories.

Yellow-shouldered Blackbird (Puerto Rico) - Sexes look the same. While they
don't migrate, they do NOT remain on territory year-round. Just like the
North American forms, they flock away from the breeding areas outside the
breeding season. I can't comment on who does more territory defense.


Chris Hill
Seattle, WA
cehill at u.washington.edu



On Thu, 27 Apr 1995, Alvaro Patricio Jaramillo wrote:

> >
> Actually, sexual dimorphism in Orioles does fit a pattern. The birds
> which are dimorphic tend to migrate in to the breeding areas, males
> establish and defend a territory and females do not. The species which
> are not dimorphic tend to be tropical, and are year round residents in their
> territory, both sexes help to defend the territory. This pattern is also
> seen in some Agelaius blackbirds, tanagers etc. In these cases it really
> does appear that the bright female plumage is selected for, rather than
> just a genetic hand me down from the males. One must remember that sexual
> selection does not just operate on males as well.
>
> Al Jaramillo
> jaramill at sfu.c a
> Vancouver, B.C.
>
>