> A couple of questions occur, namely
> 1. It would seem that birds in the northern part of the breeding range
> winter in the northern part of the wintering range, and a similar
> case for southern birds.
> Is this documented ?

I'm involved with a raptor organization that has banding operations in
Nevada (bording the Great Salt Desert) and New Mexico (Sandia mountains
in the spring, and Monzano mountains - SW of Albq. - in the fall).

Band returns for southward headed fall migrants would indicate that
migrant accipiters passing over both fall sites winter in
mid-western Mexico in large part.

We also have some good reasons to believe that the two populations are
separate: we've never recaptured an Arizona bird in Nevada, nor vice-versa;
we HAVE recaptured birds banded at one AZ site at the other in the
opposite season (despite the fact that the spring site bands sporadically
as they've only managed a couple hundred captures in good seasons,
compared to 3,850 in Nevads last year alone); fall-banded Nevada birds
recaptured nesting have all been north of the site; spring-banded
AZ birds recaptured during nesting season have all been south of the
Nevada site.

Recapture rates of living birds, either as migrants or on territory,
are quite low (10 < recaptures < 20 per year). Recoveries of bands
from dead or injured birds brought into rehab facilities support
this notion as well, though. Currently a woman at UNLV is investigating
morphological differences between birds captured at the two sites,
including looking for DNA differences.

Again, this applies to accipiters (actually, sharpies and coops - most
gos don't really migrate at all in our area, just spread out when the
food thins out).

>
> 2. Do northern and southern birds both begin migrating at the same time ?

I find it interesting that Cape Flattery's peak redtail dates normally
happen in early-ish April, and the past two years have seen big flights
in the last week of March. This is about as early as our redtail
peak dates in New Mexico.

Of course, the migrants in New Mexico are mostly wintering in Mexico
(from banding data), whereas I imagine the ones seen at Cape Flattery
are mostly wintering in the PNW or Central Valley, etc of CA.

Since winter is typically harsher the further north one goes, perhaps
(for these raptors) southern climes ala AZ and NM allow earlier breeding
than, say, the Sawtooth mountains in Idaho, which kind of balances out
the extra time needed for migration?

Also, it doesn't necessarily take a migrating bird much time to boogie
home. Coops migrate a couple hundred miles a day in the intermountain
west. Falcons probably more than that - we caught a peregrine in
Nevada that had been banded as an about-to-fledge bird still in
the nest three weeks earlier in Prudhoe Bay (a mere 2,500 miles north
of us!), so it fledged and fled 2,500 miles in about 21 days.

Passerines make overwater passages of over 500 miles in a single night
flight, shorebirds I imagine more (but Dennis would know all about that,
wouldn't he?)

So a bird doesn't necessarily need to leave much earlier to migrate, say
another few hundred miles, though these marathon flights by active
fliers like passerines require some time to refill the tank afterwards!


-Don Baccus-