Dennis,



Most ornithologists make a distinction between migration and post-breeding
dispersal. The recent occurrence of Black-headed Grosbeaks in your backyard
seems more likely, or at least as likely, to be a result of post-breeding
dispersal rather than true migration.



Migration is defined by Joel Welty in his textbook "The Life of Birds" as
being "more or less regular, extensive, seasonal movements between...
breeding regions and wintering regions". (I'm sure you will find similar
definitions elsewhere.) He goes on to state, "only the periodic to-and-fro
movements between nesting and winter quarters are considered to be true
migration", and distinguishes this from "dispersal".



The fact that you had not seen any Black-headed Grosbeaks in your backyard
since May does not prove that the birds which showed up recently were
migrants. This species almost certainly breeds within 5 miles of your home,
and is a common breeder in lowland areas of King County and elsewhere in
western Washington. It is just as likely that these birds were the result of
local, short-distance dispersal instead of migration. (The two processes can
be difficult to distinguish, and there is probably no sharp line between
them).



I've seen many people on Tweeters refer to "migration" when "post-breeding
dispersal" is probably more accurate, but I'm a bit surprised that you, as a
recognized ornithologist, would fail to make this distinction.



It may seem to many Tweeters that the distinction between "migration" and
"post-breeding dispersal", or other kinds of movements, is a slight one.
However, I think it's an important distinction, even if one that is
sometimes difficult to make. If we are going to describe any kind of bird
movements as "migration", then we might as well abandon that term and just
talk about "movements" instead.



Comments on this subject from others would be welcome!



Wayne C. Weber

Delta, BC

contopus at telus.net









From: tweeters-bounces at mailman2.u.washington.edu
[mailto:tweeters-bounces at mailman2.u.washington.edu] On Behalf Of Dennis
Paulson
Sent: August-23-09 9:20 AM
To: Tweeters
Subject: [Tweeters] migrants by the flock



Currently there are at least four immature Black-headed Grosbeaks coming to
our feeders. I say "at least," because that's how many I saw at once at one
feeder just now (Aug 23). I first spotted two of them on Aug 20, and they
have been in the yard on and off throughout each day since then. They are
surely migrants, if only short-distance ones, as there were no grosbeaks
around here this summer (I heard a few songs May 16-31, but no occurrences
between then and now). I had never seen more than two together before this.
It's obvious they are "together," as we often see 2, 3, and now 4 at once at
the feeders, then they will be absent for an hour or more, then another
visitation by more than one at a time, often appearing together and leaving
together.

Yesterday (Aug 22) there were a Rufous Hummingbird and Pacific-slope
Flycatcher in the yard as well, the hummingbird again appearing today. There
is a lot of bird activity in our yard, but not a large number of migrants,
so it's always a treat to see one.

-----

Dennis Paulson

1724 NE 98 St.

Seattle, WA 98115

206-528-1382

dennispaulson at comcast.net