Subject: FW: Re: [Tweeters] Male/Female colors
Date: Mar 12 19:06:50 2007
From: Eric Kowalczyk - aceros at mindspring.com


This is always a fascinating subject and there will always be exceptions to any generalizations made.

One generalization I have always made is that there is a strong tendency to correlate monomorphic plumage with monogamy. And monogamy can be defined as being monogamous for one clutch, for one season, or for life.

Some very migratory species such as cranes, geeses, swans, etc. are monomorphic and monogamous.There is strong selective advantage to maintain the pair bond year round. There is no need to evolve dimorphic plumage since the pairs remain together throughout the year. There is also the added advantage in monomorphic plumage since the adult pair share incubation.

As Dennis has mentioned, birds can be sexually dimorphic in ways other than in plumage (e.g. voice, size, behavior, etc.).

Similary, sexual dimorphism can be correlated with polygamy (either polgyny /polyandry). One interesting correlation is the degree of sexual dimorphism in Bowerbirds with the extravagence of the bowers built (i.e. the most complicated bowers are usually constructed by males that are not as dramatically colorful).

But there will always be exceptions to any generalization! That is what makes it so fascinating.

Eric Kowalczyk
Seattle


----- Original Message -----
From: Dennis Paulson
To: Tweeters;marianmurdoch at yahoo.com
Sent: 3/12/2007 2:49:31 PM
Subject: Re: [Tweeters] Male/Female colors


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 ca!
mouflage 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

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