To all interested: my 2 cents on "migration" vs. "dispersal":

I suspect this recent discussion on dispersal or migration got started with my post below inquiring if others had any idea if an apparent juvenile Western Tanager that showed up at Camp Long in West Seattle, where I don't think they bred this season, was more likely a "migrant" or if it was more likely there as a result of post fledging dispersal. In the post I used the term "post nesting dispersal", but "post-fledging" dispersal might have been a more specific term. Since I wasn't quite certain at the time that it was a juvenile, "post-nesting" may indeed have been a better term that would apply to both parents done feeding young and to fledged young. In the case of the 4 juvenile Black-headed Grosbeaks that showed up at Dennis Paulson's feeder, it would be my guess that they were more likely nest mates that had started moving away from their nest site, but were still hanging out together, but not yet on a consistently directional flight south. That said, that is only a guess. If they had started heading south, I would guess they would be flying in a more random mixed songbird flock, but again, only a guess. For all I know, nest mates start their southward flight together with nestmates, and then join up with more random songbirds as they continue south, only becoming part of a mixed species flock sometime after the the beginning of their southward journey. I haven't specifically studied this and don't know what literature there already is on this.

I made my earlier post (below) in part to ask if others thought this might be the beginning of migration for the Tanagers and other birds. If not migration, but a more local and less directional movement, it seemed of general interest to the group that we start to keep our eyes open for birds that had not been around a given spot during the nesting period.

Though I don't know what an ornithology text would say on this, I would define "migration" as a fairly consistent directional movement from breeding areas to wintering areas or visa versa. I would not define it by the distance moved. If the late summer or fall movement is not reasonably consistent in the direction of the wintering ground, or the late winter or spring movement is not reasonably consistently in the direction of the breeding grounds I wouldn't call it "migration". On the interesting issue of birds moving south to Mexico then spending a period of time there during a molt and then continuing further south, I would think of that as a "staged migration". I only just learned of this 2 stage "migration". I just checked my ancient Pettingill "Ornithology" (first written 1939 - last edit 1970) and found little that I thought helpful.

-Stewart

Stewart Wechsler
-Ecological Consultant - Nature Guide
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----- Original Message -----
From: Stewart Wechsler
To: tweeters
Sent: Thursday, August 13, 2009 1:13 PM
Subject: Seattle Western Tanger - dispersal or migration?


I heard a repeated unfamiliar multiparted calls / vocalizations at Camp Long yesterday afternoon. After a while trying to get a look at it I concluded it was a Western Tanager, probably juvenile rather than a fall plumage adult. The 2 parted call "note" was different from the usual W Tanager call I know best (I would write that "Chih-tuk!"). There sometimes was an added scratchier sound or sounds after the 2 parted call also. I expect this call / vocalization was a juvenile call form that would not be the usual for an adult, but it might be a variation that I don't know as well as the "Chih-tuk!" that I know better. Either the same one or more likely 2 or 3 were calling the rest of the afternoon, audible and sometimes visible from around the lodge building there.

Though I didn't spend that much of the nesting season looking and listening there, I didn't note any Tanagers nesting at Camp Long this year nor have I in previous seasons. Would others agree that it was most likely there from post nesting dispersal rather than being a migrant? I expect it is still early to find Tanagers that have started migrating south.

-Stewart

Stewart Wechsler
-Ecological Consultant - Nature Guide
Naturalist - Botanist
206 932-7225
ecostewart at quidnunc.net
-Advice on the most site-appropriate native plants to maximize the site's potential for native biodiversity
-Educational programs, nature walks, and field trips for schools, public and private groups
-Botanical Surveys