Dennis the knowledgable sez:

>It may be that breeding success can be estimated
>from the proportion of juveniles seen in the fall, but we have to realize
>that we're getting them from all across their Arctic range, as all of them
>winter at sea. So there's not even any way of knowing which populations
>were successful or unsuccessful in a given year (it surely varies across
>the Arctic).

What do you know (in general) about using juv. vs. adult ratios during
migration to estimate breeding success? For any species, I mean.
Has anyone attempted to correlate such information for a given
species with surveys of the breeding population?

Obviously, the migrant population would need to correlate with the
breeding population, and for most species this isn't true, as
you noted with this example. It would be true for, say, Kirtland's
warbler but with such a small population direct counting of breeders
is the way to go, even if one could reliably observe them during
migration at a given site. Not likely for birds which migrate at
night!

I'm interested because our raptor migration count data for the past
two years - which were at opposite extremes of the productivity
spectrum for raptors in the intermountain west - show a major
difference in the proportion of adults and juvies.

In the poor year, we had about 65% adult and 35% kids for Coops, if
I recall correctly. Normal years run about 40% adults, 60% kids.
Of course, the poor year followed on the heels of another
exceptionally good year, and a lot of those adults were only second
year birds. So, there is feedback to be considered, including
overwintering success. Actually, in theory knowledge of the
previous year's breeding success coupled with the ratio of
returning second year vs. older birds might help one glean
information about overwintering success...

Anyway, we notice such things but consider the numbers to be merely
anectdotal.

We're interested primarily in sharpies and Coops, and unfortunately
have been unable to uncover any morphological differences which
might allow us to assign individuals to particular parts of their
breeding range (unless we someday catch a vagrant Coop from the
eastern US, I guess!).

-Don Baccus-