Date: 6/26/17 8:44 am From: Wayne Hoffman <whoffman...> Subject: [obol] Re: Swainson's Thrush singing in marginal habitat in Benton County
Thanks, Joel, for the good words. Stephen Fretwell was actually working with grassland birds when he and Lucas developed the model. He (Fretwell) had Dickcissels as a long-term study subject, and i think they were a major inspiration for the model.
I have had the same experience this year on the coast, with Swainson's Thrushes.
My home is embedded in good Swainson's Thrush habitat, and they have been present and easily heard daily(in season) every year I have lived here. This year I am hearing a couple of males singing in small patches of woody vegetation out on Yaquina Head, which look pretty marginal to me.
On 6/25/2017 4:35:05 PM, Joel Geier <joel.geier...> wrote:
The recent OBOL discussion of Swainson's Thrush abundance was
interesting. I especially appreciated Wayne Hoffman's explanation of how
declines in populations can be obscured by surveys that focus on
relatively "high-quality" habitat for a given species::
There is a paper published about 1972 in the American Naturalist
by Fretwell and Lucas that models how returning migratory birds
occupy nesting habitat, that I think likely applies fairly well
to Swainson's Thrushes. According to the F&L model the first
returning males occupy the best habitat patches. The next birds
also go to these places until they are full, and then later
arrivals (are forced to) occupy progressively less-favorable
habitats. The females arrive after the first males, and pick
mates at least in part based on habitat quality.
This model lines up well with what we've been seeing in some other
species or subspecies that are declining, for example, "Streaked" Horned
Larks and "Oregon" Vesper Sparrows, plus Chipping Sparrows in the
Willamette Valley. Perception of declines by individual observers can
vary greatly depending on how much time an observer spends in prime
habitat vs. marginal habitat.
Turning to Swainson's Thrushes, today I was out in on site in the Soap
Creek Valley north of Corvallis where the habitat structure is basically
grassland, with widely scattered oaks. So imagine my surprise when I
heard a SWAINSON'S THRUSH singing from an isolated copse of Oregon-ash
and hawthorn trees, plus a few serviceberries, no more than 50 m long by
20 m wide, and surrounded on all sides by pasture lands (grazed, but
with a good native forb component).
Of course it would be unwise to draw broad inferences from a single
bird. But seemingly that particular bird felt a need to occupy a very
small, marginal patch of habitat, perhaps due to competition for better
habitat in local forests. This might just reflect local conditions,
rather than the range-wide situation. But at least on the local level,
it seems positive.
Camp Adair area north of Corvallis