Greetings



So, I looked at Berthold's book as well as Newton's The Migration Ecology of Birds

First of all, dispersal is used rather loosely, and Berthold (at different times) differentiates it from migration and includes it as a type of migration.




The most "standard" dispersals are?

1) Young birds of resident, or mostly resident species, moving away from natal areas in a random (or mostly random) direction; this serves the purpose of increasing genetic diversity in populations, or from the other angle, avoiding "in-breeding." This is a one way form of dispersal.




2) Birds spreading out after breeding, young more so than adults typically, so as to make better use of available foraging habitats. Most often, dispersal is used to refer to fairly long range movements, but can refer to young birds moving as little as a km away to occupy a different woodlot. So, Wayne was correct, it is possible that the Western Tanagers or BH Grosbeaks that appear in your neighborhood could be "dispersants." The key is that dispersal is non-directional, at least as it is used in Berthold's book. Berthold also points out that the movements of these young birds is difficult to separate from true migration. These birds often then undertake a directional migration and in spring migrate back to their breeding grounds. Interestingly, these birds sometimes locate suitable otherwise uninhabited breeding areas and then return to them, not their natal areas, in the following spring. So, post-breeding dispersal can serve as a way to scout for future breeding location!
s as well as reducing competition for resources after breeding.




Now, dispersal is also used in a very different way. Both Berthold and Newton would label the following as dispersal: Male Snow Goose from Banks I., Canada meets female Snow Goose from Wrangel I., Russia in California during winter. Male follows female back to Russia to breed. Both authors consider this dispersal as well, but clearly its use in this manner is quite different from the 2 uses mentioned above. Newton even refers to vagrancy as dispersal, which starts (in my mind) to make the term a semantic nightmare.?




Notably, Berthold labels even short (10km) movements to lower elevations during fall/winter as migration.




Newton calls the northward movement by Brown Pelicans dispersal. I think Berthold would call it migration as it is largely directional. It is not that different from what Garden Warblers (I think I've got the right species) in the Old World. The move south well into sub-Saharan Africa after breeding, but they move around (rather large distances) throughout the non-breeding season before heading back north to breed. Other than reversing north and south, the two circumstances are fairly analogous.?




Anyway, enough of that. So, when that Rufous Hummingbird is born in your yard but then moves a mile down the road for a month before heading south, it dispersed and then migrated.




-- of course, none of this answers the question about the birds in Dennis' yard, but given how long we are after fledging time, and how close we are to the last birds departing the Pac NW, I think they are far more likely partaking in directional migration, not dispersal. Additionally, just because a flock of birds are in your yard for a few days does not indicate that they are not migrants. Most migrants move for a night, or a couple nights, then feul up for several days before starting their migration again.




Best Wishes

Steven Mlodinow

Everett WA