Tweeters,



I'm glad that my message from a couple of days ago has generated some
discussion. There seems to be some agreement that "migration" should not be
applied uncritically to all kinds of bird movement, which is the point I was
trying to make in the first place.



I would agree with Steve that a songbird in late summer that has moved 60 or
100 miles south should be considered to be "migrating" rather than
"dispersing".



However, the case of the White-crowned Sparrows in Marymoor Park, raised by
Michael Hobbs, is interesting. I'm surprised to learn that White-crowns do
not breed in the park, which seems to have suitable habitat (hedgerows or
shrubbery next to open fields). I am sure that White-crowns breed within 2
or 3 miles of the park. (Around Vancouver, they breed in almost every
shopping center with clumps of shrubs or trees, if not in residential areas
per se.)



However, if White-crowns show up in Marymoor in late August, these would
have to be dispersing rather than migrating birds. "Pugetensis" White-crowns
are short-distance migrants, with birds breeding in Washington travelling
usually no farther than California, and at least a few of them wintering
locally (along with "gambelii" White-crowns from farther north).
Furthermore, White-crowns don't even start migrating until well into
September. Pugetensis arrives early in the spring (late March and April),
and leaves later in the fall than many other species. So, if White-crowns
are showing up in late August at Marymoor, it would have to be a result of
post-breeding dispersal. One of the things that characterizes migration is
that each species has its own migration schedule, and that even closely
related species may have quite different migration schedules (e.g. Hermit
versus Swainson's Thrushes).



Migration may be hard to distinguish from post-breeding dispersal- as I and
others have noted, there is likely no clear line between the two-but I do
think it's worth trying to distinguish them.



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 Steven
Mlodinow
Sent: August-26-09 1:58 PM
To: birdmarymoor at verizon.net; Tweeters at u.washington.edu
Subject: Re: [Tweeters] dispersal



Hello Michael (and others)



A BH Grosbeak that has started its southbound migration to wherever and has
made its first 60 mile jump (or 100, etc) would be a migrant. A WC Sparrow
that has just shifted a couple miles from where it was raised would not
really be considered a migrant. I am not sure where a real ornithologist
would draw the line. Now, your WC Sparrows could in theory be from 100 miles
away, as pugetensis is migratory, and I guess that would be a migrant. So,
local movement vs long distance. Directional, vs nearest decent habitat. I
guess those would be part of the distinguishing features.



Best Wishes

Steve Mlodinow

Everett WA