Subject: Re: Re. Robins
Date: Nov 7 12:44:36 1995
From: Alvaro Jaramillo - alvaro at quake.net




>It may also be that the adults are just better skilled at taking
>larger prey and that the juvies drift to where they can catch
>smaller, easier prey.

I think that there is good evidence that non-breeding distributions of
raptors are influenced by prey type. Because most raptors show marked sexual
size dimorphism (one sex, the female in this case, is bigger than the
other), they are each better suited for a certain prey size. As well, size
will influence your efficiency as far as tolerating cold, or hot,
temperatures. I remember Kay McKeever (famous owl rehabilitator from
Ontario) mentioning that in a normal winter young male Snowy Owls would be
sent to her. Some winters she would get adult males and young females as
well, but only in the worst of years (from the owl's perspectives) would she
get adult females. This implies that both age and sex (size) affects how
well owls can make out during difficult winters (in terms of prey base).
Recent work on Tengmalm's Owls (=Boreal Owl, probably) in Finland has found
that size variation in males is maintained by differences in foraging
efficiency during nesting in good and bad vole years. During most (all?) of
the nesting season, the male is the sole provider for both the female and
the young. If I remember correctly during a good vole year, the smaller
males did well and raised the most offspring, but during bad years the
bigger males did best and raised the most offspring.

Ohh Jeez, I'm sounding like one of those raptor freaks :-)

Alvaro Jaramillo
Half Moon Bay, CA

alvaro at quake.net
http://www.quake.net/~alvaro/index.html