Subject: [Tweeters] Migration versus post-breeding dispersal
Date: Aug 8 11:08:01 2008
From: Wayne Weber - contopus at telus.net


Charlie, Steve, and Tweeters,



I wasn't trying to say that no passerines ever begin southward migration in
late July. In the case of some individuals, and some species, they may do
so. I would have to agree with Charlie that birds heard overhead at night
are almost certainly engaged in migration rather than post-breeding
dispersal. However, in my experience, in the Pacific Northwest, the vast
majority of passerine movements in late July and for at least the first 10
days of August appear to be short-distance post-breeding dispersal rather
than long-distance migration. However, it may be hard to tell the difference
unless you are dealing with banded birds or birds with radio transmitters,
so that you can tell exactly how far they moved.



I'm intrigued by Steve's comments about molt migration. Molt migration is
well-documented in waterfowl and in some other non-passerines, but I was
unaware that it had been documented in passerines. Steve, can you cite any
published papers that document this phenomenon in passerines?

It's easy to explain molt migration in waterfowl, which lose many of their
flight feathers at once and are often flightless for 2-3 weeks during the
summer, but I find it harder to understand why this should happen in
passerines, which molt more gradually and generally don't have a flightless
period.



I agree that Bullock's Orioles and Black-headed Grosbeaks leave quite early
(in BC, most orioles are gone by August 31, and grosbeaks by at least
September 7), but I thought this was just ordinary fall migration, not a
molt migration.



Terminology is important, and part of people's differing perceptions may be
a matter of semantics.

In the dictionary sense, almost any movement of birds could be considered to
be "migration". However, in ornithological parlance, migration is usually
considered to be a regular, twice-a-year movement between a breeding area
and a wintering area which is often hundreds or thousands of miles away.



The point I was really trying to make is that a lot of what casual
observers-and some experienced observers-refer to as "migration" is not
migration in this second, more usual, sense, but is really post-breeding
dispersal. Every year, in July and early August, there is a flurry of
messages on TWEETERS and other birding groups about bird species that popped
up in the backyard that the observer hadn't seen there since May, with the
assumption that it must be, or could be, migration.

The fact that these same species may breed less than 2 miles from the
observer's backyard is ignored, or in some cases, perhaps unknown to the
observer. An example is Kristi Hein's message about Golden-crowned Kinglets
in Anacortes. I've done lots of birding year-round on Fidalgo Island, and I
can assure Kristi and other observers that G.C. Kinglets breed commonly
there, even if they are somewhat commoner in winter. Golden-crowned Kinglets
are a good example of a species which is a "partial migrant", with a
breeding range that broadly overlaps its winter range. The Puget Sound area
is in the overlap zone, such that Golden-crowns are found commonly here in
conifer forest all year.



True migration (breeding area vs. wintering area) and post-breeding
dispersal are distinct phenomena (the first is mediated by hormones, the
second appears not to be), although they can be hard to tell apart in late
summer.



Wayne C. Weber

Delta, BC

contopus at telus.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: August-06-08 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 southbound migration, and then do not
pause for any extended period of time until they 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

.