[Tweeters] Re: Migration versus post-breeding dispersal

Dennis Paulson dennispaulson at comcast.net
Tue Aug 25 10:52:36 PDT 2009

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.


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] 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
dennispaulson at comcast.net

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