Dear Gail and Tweeters:

Gail is quite correct in her conclusion that the Swainson's Thrush
"appears to occupy a broader range of habitats" than other brown
thrushes. Swainson's is what ecologists would call a "habitat
generalist". Species such as this often occupy a broad range of
habitats in areas where there are no competing species, but can be
excluded from some habitats by closely-related species with narrower
habitat preferences (e.g. Veery).
Don't be misled by some of the statements about habitat in
Campbell et al. (The Birds of British Columbia). These tend to
emphasize the range of habitats occupied, rather than the typical or
preferred habitat. The statements about the habitats in which nests
were found tend to reflect the distribution of observer effort more
than of the birds. For example, Swainson's is mainly a conifer-forest
bird throughout its range, yet Campbell et al. report only 17% of
nests in conifer forest, versus 17% in deciduous forest and 39% in
"mixed" forest. This may result partly from a large percentage of nest
records being from the southern coast, where Swainson's Thrush nests
in deciduous/mixed stands much more often than in the rest of B.C.
Also, I suspect that most of the "mixed" stands were mainly
coniferous. (Would 6 aspen trees within a bird's territory qualify it
as "mixed" forest?). In addition, Campbell et al. report that nearly
40% of Swainson's nests were found in riparian situations, regardless
of forest type. Overall, I doubt if 10% of Swainson's nests are in
such situations. However, in most parts of B.C., activities of birders
and ornithologists are strongly concentrated in riparian areas,
because, in mountainous areas, most of the roads follow river systems!
(The Veery, on the other hand, is a truly riparian species.)
The statements on habitats of nesting birds in Campbell et al. can
be misleading, because of bias in observer coverage, and because the
habitat descriptions in most nest record cards are anecdotal, not
detailed. A much more reliable source of information on habitat use by
breeding birds is the breeding bird censuses that were published for
many years in the journal "American Birds" (more recently in the
"Journal of Field Ornithology") and elsewhere. These include detailed
descriptions of the vegetation as well as of the breeding birds.
Results of breeding bird censuses in B.C. and elsewhere in Canada
have been summarized by Anthony Erskine and others in a series of
Canadian Wildlife Service publications. These give an accurate picture
of what the commonest birds are in each habitat type. However, the
surface has barely been scratched in B.C. in terms of the number of
censuses that need to be done to accurately describe the breeding bird
populations in different parts of this very diverse province. We need
a lot more of them (bird counters, where are you?).
For the same reason, in my original posting on the relative
abundance of Hermit versus Swainson's Thrushes, I deliberately avoided
quoting provincial average numbers of these species recorded on
roadside Breeding Bird Survey routes. These numbers (which can be
useful in indicating the relative abundance of two species in a flat
Eastern state) would undoubtedly underestimate the abundance of Hermit
Thrushes relative to Swainson's, because there are few roads in the
high mountain areas generally preferred by Hermit Thrushes.
My statement that Swainson's Thrush is one of the 5 commonest
breeding birds in B.C. as a whole was largely based on the results of
breeding bird censuses, plus my personal experience in many parts of
B.C.

Sincerely,

Wayne Weber
114-525 Dalgleish Drive
Kamloops, B.C. V2C 6E4
Phone: (250) 377-8865
wayne_weber at bc.sympatico.ca


-----Original Message-----
From: Spitler <spitler at wkpowerlink.com>
To: tweeters <tweeters at u.washington.edu>
Date: Thursday, June 17, 1999 8:03 AM
Subject: Re: Swainson's Thrush and friends (long)


>Hi All (particularly thrush-ophiles)
>The discussion so far has sent me searching the literature. Let me
>summarize, mostly from Campbell, Birds of BC, v.3; although I have
also
>checked Godfrey (Birds of Canada) and Cannings (Birds of the
Okanagan),
>Kaufman (Lives of NA Birds).
>
>Veery
>Found nesting between 105m and 1050m elevation, with 80% of the
observed
>nests below 650m. Predominantly, the Veery inhabits shady, riparian
>woodlands with a dense understorey.
>Nests are generally found near or on the ground in shrubs or thickets
or in
>dense vegetation or leaves. Most (63%) were situated in shrub
thickets. The
>heights from the ground ranged from ground level to 3m, with 57%
between 0.1
>and 1.2m.
>
>Swainson's Thrush
>On the coast the nests are found from sea level to about 930m
elevation. In
>the interior it ranges from valley bottoms to 1850m. Mixed forest was
the
>most frequently reported nesting habitat class (39%), followed by
deciduous
>stands (17%) and coniferous stands (17%). Most nests were in trees
(30% in
>deciduous and 24% in conifers). Shrub and brush tangles accounted for
38%.
>The heights of the nests were from ground level to 24m, with 60%
between 1
>and 2 m.
>
>Hermit Thrush
>Breeds from sea level to 1500m elevation on the coast and from 435 to
2200m
>in the interior. During the breeding season this thrush occupies a
wide
>variety of forest types, but prefers areas with dense shrub
understorey.
>Most nests (64%) were found in trees including snags or tree roots.
>Coniferous trees (42%) were preferred over deciduous trees (8%).
>The heights ranged from ground level to 6m, with 57% between 0.3 and
1.5m.
>
>For me, the next question was how does the Varied Thrush fits into
this
>picture. On the coast it nests from sea level to at least 1500m
elevation;
>in the interior it nests from 420 to 1860m elevation. Most nesting
occurs in
>coniferous forest (78%), mixed forest (11%) and deciduous forest
(6%). Nests
>are in trees, including coniferous (46%) and deciduous (35%).
>The heights ranged from ground level to 11m, with 60% between 2 and
4m.
>
>For me, the Varied Thrush, Veery, and Hermit Thrush appear to have
more
>defined habitat requirements; whereas the Swainson's Thrush appears
to be
>occupy a broader range of habitats.
>
>Sorry this got so long
>Cheers
>Gail Spitler
>Johnsons Landing, BC
>spitler at wkpowerlink.com
>
>
>
>
>