Tweeters,

Charlie makes some very valid points in his discussion. He is correct that Black-headed Grosbeak is one of the few passerine species which has recently been shown to undertake a "molt migration" to the SW US and NW Mexico-- or at least, some individuals do. This is neither a post-breeding dispersal nor "migration" in the classic sense between breeding and wintering grounds-- it is a third type of movement, but in this case a long-distance one. Many waterfowl also perform molt migrations.

However, I believe Charlie misunderstood some of my original comments. I was not trying to say that Dennis's grosbeaks WERE NOT migrants, but rather that they could just as easily have been post-breeding dispersers that were raised within 5 or 10 miles of Dennis's home. Unless we know just where these grosbeaks were hatched, we can't say for sure whether they were migrants or dispersers.

I also indicated in my message that there is no sharp line between migration and
post-breeding dispersal, and perhaps I need to repeat this point.

There seems to be a tendency by some observers to describe all bird movements as "migration", and I was trying to point out that in traditional ornithological terminology, only long-distance, regular movements between breeding and wintering grounds are described by that term. Perhaps we are getting lost in semantics.

I would love to see more people involved in bird-banding projects, which if done on a large enough scale would reveal far more than we know about the often complex patterns of bird movements. "Migration" and "post-breeding dispersal" may be only two extremes of a spectrum of types of bird movements in late summer.

Wayne C. Weber
Delta, BC
contopus at telus.net




-----Original Message-----
From: tweeters-bounces at mailman2.u.washington.edu [mailto:tweeters-bounces at mailman2.u.washington.edu] On Behalf Of Charlie Wright
Sent: August-25-09 5:42 PM
To: tweeters at u.washington.edu
Subject: [Tweeters] Re: Migration versus post-breeding dispersal

Hello All,
This discussion seems to come up every August here, which is not a bad thing since bird migration is a fascinating topic, and the details of fall migration in some of our passerines are very poorly known and need more study.

First of all, I don't like the way Wayne uses "post-breeding dispersal" as a blanket term to refer to what birds do this time of year. I don't think there is a universal stage in a bird's life going by that term, at least not one that has been proven. Of course, some form of biological dispersal is universal, because it limits inbreeding, overcrowding, competition, and other issues. But by this I mean natal dispersal (the distance a juvenile travels between its hatching site and the place it selects to breed) and breeding dispersal (the distance some birds travel to re-locate between breeding seasons).

"Post-breeding dispersal" is sometimes used to describe what, for example, Brown Pelicans (and the Heermann's Gulls linked to them) do, moving up the coast from Mexican breeding grounds to spend the late summer and fall with us. While this makes sense on one level (they do disperse northward after breeding) isn't this really a kind of migration? I don't know if there is a better term out there for this phenomenon.

Calling the birds we see away from exact breeding sites short-distance "dispersers" without any evidence of such side-steps the issue and marginalizes the importance of trying to figure out what each species is actually doing this time of year. I find it odd that Wayne seems to detract from the observations of species in your neighborhood that haven't been around all summer. I think these are great observations which let us know when these species are in fact on the move (to/from where or for how long is another question). The fact that a migratory species might nest within a few miles of your location does NOT mean that the birds you are seeing necessarily, or even likely, come from a few miles away. This reminds me of the common misperception that individual robins in the Pacific Northwest are sedentary, because you can see them any day of the year. The reality is a lot more complex, and there is a high probability that the robin you're looking at in winter wasn't even hatched in the state. The case is likely to be similar with early fall migrants. Incidentally, one of the best ways to contribute to the effort to figure these things out is to submit frequent observations to www.eBird.org.

If Dennis' Black-headed Grosbeaks (BHGR) aren't migrants, then clearly we don't ever see fall migrant BHGR in Washington, since this species gets remarkably scarce in the state after the first week in September. In fact, there is growing evidence that BHGR is one of an increasing number of documented molt-migrants that move down to NW Mexico for the monsoon season before moving on to their wintering grounds, since adults delay molt until after migration (which begins in July). Species which have been proven to do this include Western Kingbird, "Western" Warbling Vireo, Bullock's Oriole, Western Tanager, Lazuli Bunting, and others. I have seen some evidence to make me suspect that young BHGR may actually follow the adults as they start moving, since I've seen still-dependent fledglings with adults show up in places where they do not breed this time of year. As soon as the fledglings are self-sufficient, the adults are outta here! Meanwhile the young birds may move upslope like some other species (Nashville and Orange-crowned Warbler, Western Tanager, Warbling Vireos, many sparrows) to molt in our mercifully wet and buggy Cascades. This latter phenomenon is being studied by PSBO (www.pugetsoundbirds.org) in montane Yakima County, where this month they banded Vesper Sparrow and Lazuli Bunting at over 6000' elevation. Read about some of their finds here: http://psbocascadesbandingcamp.blogspot.com/

Basically, I think the issue is a lot more complex than Wayne suggests. I don't think putting all of these birds into the catch-all category of "post-breeding dispersers" solves anything. Each species deserves to be studied at a finer detail than that.

Cheers,
Charlie Wright
Bonney Lake, Washington
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