Dear Gail,

My experience is generally that, in the Pacific Northwest, one
does not normally find two or more species of "brown thrushes"
(Catharus and Hylocichla species) breeding in the same forest stand.
In the southern B.C. interior, I have found that purely deciduous
stands, at least at lower altitudes, are usually occupied only by
Veeries. Swainson's are almost always found in conifer stands, or
deciduous stands that have some conifers in them. (And Hermit
Thrushes, as I noted, are usually in higher-altitude forest stands
where there are openings and the forest cover is discontinuous.)
Each of the 3 species seems to have its own preferred ecological
niche, to which it is best adapted and where it can breed most
successfully. To me, this seems like a good illustration of one of the
basic laws of ecology, "Gause's Law" or "Gause's Principle", which
states that two closely-related species rarely both persist for long
in the same environment. This "competitive exclusion" could operate
either by active behavioural interactions, such as interspecific
territoriality, or by one species simply out-competing the other for
resources, or both. Without detailed study, it is hard to say which of
the two mechanisms (if not both) is operating between Veeries and
Swainson's Thrushes.
West of the Cascades, the picture changes, as Veeries are
generally absent. In southwestern B.C., Swainson's Thrushes breed
commonly in deciduous stands, including the kinds of stands (e.g.
riverine cottonwood forests) that would be used by Veeries in the
interior. I have to conclude that some kind of competitive exclusion
is operating between the two species in the Interior (and also between
Swainson's and Hermits).
I hasten to add that my experience that 2 species of brown thrush
do not usually breed in the same forest stand does not always hold in
other parts of North America, where different habitat types occur, and
other species of thrush (Gray-cheeked Thrush, Bicknell's Thrush, and
Wood Thrush) are added to the equation. The range of habitat types
used by a species depends on the range of habitats available, as well
as on the presence of other potentially competing species.
There is an old paper by William Dilger (Wilson Bulletin
68:171-199, 1956) which discusses habitat preferences and competition
among brown thrushes in New York. I'm sure there are some more recent
papers that deal with the subject, but I don't recall any at the
moment, and haven't time to look right now. Can any of our readers
help me out?
One of the things that I find intriguing about the
Veery/Swainson's Thrush/Hermit Thrush situation is that there is, in
general, an altitudinal succession of species, with Veeries at the
lowest altitudes, Swainson's in the middle altitudes, and Hermits at
the higher altitudes, although there is a lot of overlap.
A similar situation, where you get an altitudinal succession of 3
closely-related species (over a narrower range of altitudes) was found
by Robert Weeden, who studied ptarmigan (Willow, Rock, and
White-tailed) in the Chilkat Pass area of extreme northwestern B.C. He
found that Willow Ptarmigan nested at the lowest altitudes, in the
dense willow thickets above the limit of conifers; that Rock Ptarmigan
occurred higher up, in alpine vegetation above the willow thickets;
and that White-tailed occurred highest, where the landscape was
dominated by rocks and scree rather than vegetation. Once again, the
picture changes in areas where one of the 3 species drops out, such as
the northern Rockies of B.C. (where there are White-tailed and Willow
but no Rock Ptarmigan), or in the Coast Range between the Skeena River
and Vancouver (where there are Rock and White-tailed Ptarmigan but no
Willows).
When I say that two species of "brown thrush" do not usually breed
in the same forest stand in our area, I'm trying to describe the big
picture, and there may be some local exceptions. Would anyone else
like to share their experience with these species?

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

-----Original Message-----
From: Spitler <spitler at wkpowerlink.com>
To: tweeters <tweeters at u.washington.edu>
Date: Wednesday, June 16, 1999 7:16 AM
Subject: Re: Swainson's Thrush


>Hi All
>
>Wayne wrote:
> In the B.C. southern interior, the picture is complicated by
>Veeries, which generally exclude Swainson's from cottonwoods and
other
>riparian vegetation below about 2500 feet.
>
>I was a bit surprised by this sentence. Wayne are you saying that
Veeries
>are dominant over Swainson's below 2500 feet and are victorious in
winning
>nesting sites coveted by both species? My interpretation of
Campbell, Birds
>of BC, v.3 is that Veeries and Swainson's coexist as the Swainson's
forages
>higher above the ground and prefers habitat with less undergrowth
than the
>Veery. As breeding bird survey data for the interior (1968 to 1993)
seems to
>show stable populations of both species, I wonder if they are
competing or
>have simply found their niches?
>
>Comments,please.
>
>Cheers
>Gail Spitler
>Johnsons Landing, BC
>spitler at wkpowerlink.com
>
>
>