Marian Murdoch wrote:

While I can understand that females are more subdued
in order to help with camoflauge, why don't some
species follow that rule? Why, for example, are male
and female jays (and most of the corvidae family) the
same? Anyone?

Marian,

Most birds in the tropics are monomorphic (sexes look alike), and
this seems more characteristic of birds that are resident, both sexes
maintaining year-round territories and probably mating more or less
for life. That characterizes some temperate-zone birds as well. Note
that most of our monomorphic birds are resident, most of our
dimorphic birds migrants. One hypothesis that I favor is that arrival
on a territory in the spring and the need to defend it immediately
from other males of the same species has brought about selection for
bright plumages in males. Probably the need to attract a mate quickly
has applied the same selective pressure. On the average, these
migratory birds won't be mating with the same mate year after year,
so there is a high premium put on getting the sexes correctly
identified right away. The females probably choose the males (or
their territories), so there isn't such a high premium placed on them
being recognizable to species, and selection for camouflage in the
female, as she is the bird on the nest, becomes more important. Birds
such as corvids that are resident throughout the year already have
their territories, and the males and females have known each other
through the winter. This is presumably true for chickadees, wrens,
etc., although it's interesting that migratory wrens, for example are
also monomorphic.

In the tropics, many of the monomorphic species are dazzlingly
bright, so it's obvious being bright has been more important than
being camouflaged on the nest! On the other hand, tropical forest
birds are especially good at hiding their nests.

Seabirds as a group are monomorphic, and most of them retain the same
mate year after year. Even if they don't stay together during the
nonbreeding season, they meet at the nesting site or colony. The big
exception to this is found in ducks, and they mate during the winter
while many species are together, so again there has been a premium
put on quick species and sex recognition. Phalaropes are the other
dimorphic "seabirds," and they of course are just sandpipers that
went astray.

It remains to be seen if very many birds show sexual dimorphism in
ways not visible to us, e.g., UV reflectance. At least many of them
are dimorphic in voice, which must help getting the sexes straight.
Also, there are surely behavioral differences in the sexes in many
(most? all?) cases. When birds are monomorphic, it's harder to know!

I think this simplified model explains a lot, but certainly not all
the details.

Dennis
-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net



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