Hi Tweets,

Jim Rosso writes:

>It is also interesting that the female/size difference exists in owls and
>skuas. Also I read that the difference in some species varies with the
>diet.

When we see in a field guide that the measurements of male and female
differ, are we then looking at a ratio averaged for the various races and
populations with a given species? Or are sexual size-difference *ratios*
different within the same species, depending on race?

Does the question of why there's sexual dimorphism at all within a species
need to be answered on a species-by-species basis? The following example
suggests not.

>In Kestrels, if I remember correctly, the difference in size
>increases as the prey items increase in size. So there are regional
>variations in the size difference.

But why was it the females and not the males which took advantage of larger
prey? And why are *all* or nearly all raptor females larger? If morphology
reflects, among other things, adaptation to environmental constraints, given
the huge range of habitat-types with raptors in them, surely at least one
raptor species would have found it advantageous for males to be larger? So
the question as well is why is large female/small male dimorphism *so*
general among predatory birds?

But then the converse appears: in shorebirds, within even one genus,
Calidris, it's as impossible to generalise about male/female size
diffferences as it is possible to do so within raptors: some species'
dimorphism has the females larger, others the male. Why are female gulls
smaller than males but female jaegers larger? If environment influences
morphology, why are raptor, shorebird and gull species living cheek-by-jowl
in the same habitats so widely different in their dimorphic selections or
lack of them in a way that in seems in some to depend on the family to which
they belong and in others not at all?

Michael Price
Vancouver BC Canada
at mindlink.net