Tweets,



Steve's post is intriguing. Lots to think about. I think there is evidence
that hummingbirds and likely many other species move up into the mountains
to take advantage of the late bloom of alpine vegetation and the bugs that
come with it, fattening up, perhaps, before begin a long flight south. I
seem to recall considerable numbers of post-breeding warblers, sparrows,
thrushes, etc., on Mt. Rainier in early September when I led a Washington
State tour that included a stop at Paradise about September 10. Sometimes
there were flocks of warblers, etc., at the base of the mountain also, which
sometimes included Hermit Warblers, likely dispersed from a bit to the
south. Raptors also hunt the alpine zone in late summer and early fall,
including such species as Prairie Falcons that nest more or less to the
east, rather than to the north, suggesting an upslope dispersal prior to the
southbound movement.



Gene Hunn

18476 47th Pl. NE

Lake Forest Park, WA

enhunn323 at comcast.net



From: tweeters-bounces at mailman2.u.washington.edu
[mailto:tweeters-bounces at mailman2.u.washington.edu] On Behalf Of
sgmlod at aol.com
Sent: Wednesday, August 06, 2008 11:20 AM
To: Tweeters at u.washington.edu
Subject: [Tweeters] migration



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

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