Greetings All



Unfortunately, I keep reading these emails on dispersal vs migration when at work, away from my reference sources. I recommend, strongly, that anyone interested in such topics get Berthold's, Bird Migration, the lastest edition of which was ca. 2005.



Anyway, there are specific definitions for all of these terms, and I can't remember them with certainty.



Dispersal refers, in most cases, to a non-directional movement. Young female RC Woodpeckers disperse from natal colony at some point during first year of life (can't remember precise details). This ensures genetic diversity, as colonies are now, and have traditionally been, somewhat isolated. The distance of dispersal, as I recall, is typically in the 5-30 mi range, but don't quote me. In any case, the direction they travel appears random.



That is one kind of dispersal.



Some interior species (I think Am Avocet and Franklin's Gulls are among these) disperse after breeding, presumably because some habitats are hospitable in general, but not for nesting. In any case, these dispersals are also somewhat non-directional, and as I recall, involve young more than adults.



That is another kind.



Interestingly, crossbill movements were formerly called dispersal, but I think are now called non-directional migration. Can't remember details on that.



I believe the downhill migration in winter, which varies from year to year based on conditions, of many montane species such as Varied Thrush, is called facultative altitudinal migration. Not sure. But I don't think dispersal is used in reference to this movement, but I am not?absolutely certain.



The movement of Brown Pelicans and Heermann's Gulls, to me, seems to far better fit the concept of migration After all, molt-migration in waterfowl is called precisely that, molt-migration, even though it most often involves northward movement. The pelicans and Heermann's Gulls are purposefully (though this may well be genetically programmed) moving a specific direction (northward up the coast)?to take advantage of better fishing to the north of their breeding colonies.



As for BH Grosbeaks, they definitely are molt migrants. It was most interesting to be in yhe mountains of southern Baja's Cape?in early August, seeing dozens of tattered adult BHGR and Western Tanagers, and then returning to Washington, where I saw small numbers of fresh plumaged juvs (or is it immatures?) in my neighborhood (loosely speaking). Interestingly, I saw hundreds of freshly plumaged OC Warblers, all of the lutescent (pacific coast) race and dozens of freshly plumaged adult BT Gray Warblers down there at same time. Apparently, the Rocky Mtn race of OC Warbler is a later migrant (did not know that, but it is mentioned in Garrett and Dunn's superb WARBLER book). The OCWA and BTGray Warblers clearly molt before migration. There still are some OCWAs in w. Snohomish County as well as BTGrays. I am not good at ageing OCWA, but all the BTGrays I saw (only a dozen) were juvs/imms (unlike Baja, where they were almost all adults).



Best Wishes

Steven Mlodinow