Dennis, Wayne. Tweeters.

Points are well taken towards either possibility, personally I lean towards
migration. The best example of post-breeding dispersal. are the great
numbers of BROWN PELICANS and HEERMANN'S GULLS from mainly Mexican breeding
grounds, we find here in the Northwest this time of the year. Of course I
may wrong and we are seeing "reverse migration" with these species?

Cheers,

Hans-Joachim Feddern
Twin Lakes - Federal Way, WA.

On Tue, Aug 25, 2009 at 10:52 AM, Dennis Paulson
<dennispaulson at comcast.net>wrote:

> Hello, Wayne.
> Thanks for your comments, and I agree that it would be easy to confuse
> migration with postbreeding dispersal. As you yourself wrote, the two
> processes can be difficult to distinguish, and there is probably no sharp
> line between them.
>
> Nevertheless . . . .
>
> If I had been writing about Black-capped Chickadees or Downy Woodpeckers,
> largely resident species, I would have used the term "postbreeding
> dispersal" and felt that it covered the ground. But as Black-headed
> Grosbeaks are migratory, as they are surely on their way to their wintering
> grounds in Mexico when appearing in my yard in late August, and as I have no
> idea where they came from (they do breed well to the north in British
> Columbia, by the way), I do believe that "migration" is an appropriate term.
>
> Dennis
>
>
> On Aug 25, 2009, at 8:12 AM, Wayne Weber wrote:
>
> 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<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
>
>
>
>
>
>
> -----
> Dennis Paulson
> 1724 NE 98 St.
> Seattle, WA 98115
> 206-528-1382
> dennispaulson at comcast.net
>
>
>
>
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