Greetings All

Wish I knew more about this topic.

First of all, true passerine migration begins in July for many western species and some eastern species. BH Grosbeaks, Western Tans, Cassin's Kingbirds start to appear in Baja's Cape and in Sonora in July, and these locations are quite distant from their breeding grounds.

This is because many western passerines engage in a molt migration. They leave the (usually) very arid (in July/August) Cascades and Sierras and head for the sw US and nw Mexico, where monsoon rains are usually going on and insect life is abundant. Clearly some members of these species linger, but it is amazing how few BH Grosbeaks and Bullock's Orioles we see after 1 September, or even 15 August.

After molting at these locations, many "molt migrants" continue on to their wintering grounds.

In the east, most species molt before sbound migration, and then do not pause for any extended period of time until the reach their non-breeding/wintering/resting grounds. Admittedly, their southbound migration is more leisurely than their northbound migration, but I bet birds rarely spend more than a week at a given stopover.

Some western migrants do the same.

One of the differences between Bullock's and Baltimore Orioles, and between eastern and western Warbling Vireos, is that the eastern equivalent molts on breeding grounds, and?the western versions head south first.

So, a migrant Warbling Vireo in mid-July would not be surprising in the west.

As for "post-breeding dispersal".
I am nearly totally ignorant as to how much this occurs in passerines. Certainly, sagebrush sparrows flock up and move about a bit.... but how much is a bit? Don't know. A couple miles? Dozens of miles? Random direction?

I've not seen something among woodland passerines that really matches what one sees in sagebrush sparrows. My neighborhood does not fill up with flocks of wandering Wilson's Warblers for a month before they depart as one might find with Brewer's Sparrows in e. Washington. In June, and to a lesser extent in early July, I do encounter wandering individuals, not in great numbers, that I suspect represent birds that failed to find mates or had their nests failed. Many of these birds are singing males that hang around for a day or two then disappear.

So, what do Wilson's Warblers (not molt migrants, I believe) do after their young are independent? I have no idea. Do they start south? Do they disperse (which implies a somewhat random direction) before migrating? If so, how far do they disperse? Perhaps they move up into the mountains and flock up there before heading south.

There may well be someone out there reading this listserv that knows, or a peak at the Birds of North America accounts for some of these species may provide insight.

And I see no reason why dispersal would be diurnal vs nocturnal.....

Blah, blah, blah, blah, blah

Signing out
Steven Mlodinow
Everett WA