I find there is often a tendency for people to implicate the increase of a
related immigrant species to the decline of their native relative. I have
been among those that have at times done so. I too, once thought that the
decline in Purple Finches was due to the increase of House Finches, but
later questioned this hypothesis.

While the increase of a closely related immigrant may often be an important
factor in the decline of their sister native species, I would suggest that
when we have an immigrant species increasing we look for declines in those
natives with the closest niches rather than declines in species that are the
closest genetically. Similiarly if an immigrant species is successful we may
look for declines in their prey species and then maybe those that use the
same prey. (We might also look for an increase of species that were
previously limited by the prey species of the immigrant.)

We also need to look at what landscape and habitat changes have happened
over the same period as the increase of the immigrant and the decline of the
species in question. Often it is this landscape / habitat change that has
facilitated both the immigration and population growth of a new species and
the decline of another. The fact that two related species are indeed
different species is likely because one became adapted to a different niche
from the other. On the other hand there are likely to still be some
commonalities in the niches of closely related species which would lead to
an element of competition.

Another factor where a related immigrant species may lead to the decline of
a pre-existing native species is where the new immigrant may support
increased populations of parasites, predators and other organisms adapted to
exploit both related species, but the exploiter species may impact the
pre-existing native more than the successful immigrant. Exploiter species
may also have been introduced along with an immigrant species.

-Stewart

Stewart Wechsler
-Ecological Consultant - Nature Guide
Naturalist - Botanist
206 932-7225
ecostewart at quidnunc.net
-Advice on the most site-appropriate native plants to maximize the site's
potential for native biodiversity
-Educational programs, nature walks, and field trips for schools, public and
private groups
-Botanical Surveys



----- Original Message -----
From: "Eugene and Nancy Hunn" <enhunn323 at comcast.net>
To: "'Hal Opperman'" <hal at catharus.net>; <tweeters at u.washington.edu>
Sent: Tuesday, February 17, 2009 11:19 AM
Subject: RE: [Tweeters] Anna's Hummingbird


> Hal, Dave, et al.,
>
> I have to agree with David and Hal, though without careful consideration
> of
> records. Anna's first arrived in Washington about 1965 and by the 1970s
> was
> well established in certain (but not all) urban areas as a non-migratory
> species (Hutch conducted some of the early censuses of Anna's in the
> Magnolia neighborhood), clearly heavily dependent on ornamental plantings,
> feeders, etc. Rufous Hummers have always preferred such native habitats as
> salmonberry thickets and the like (to the point that Michael Hobbs
> believes
> they emerge each spring from salmonberry buds!). Rufous Hummers have
> retreated as our built environment has hardened and expanded. I believe
> the
> House and Purple Finch distributional changes show a very similar dynamic.
> House Finches, of course, have expanded all across the US in part by
> virtue
> of introductions, such as on the east coast and in the 1930s, I believe,
> on
> Vancouver Island, though it is most likely Washington was invaded by a
> "natural" expansion from California, the historic core territory of the
> House Finch (and Anna's Hummingbird) in the US. Brown-headed Cowbirds show
> a
> similar pattern in the late 20th century. I don't believe global warming
> was
> a factor in any of those cases, at least not in any direct way.
>
> Gene Hunn
> 18476 47th Pl NE
> Lake Forest Park, WA
> enhunn323 at comcast.net
>
> -----Original Message-----
> From: tweeters-bounces at mailman2.u.washington.edu
> [mailto:tweeters-bounces at mailman2.u.washington.edu] On Behalf Of Hal
> Opperman
> Sent: Monday, February 16, 2009 9:13 PM
> To: tweeters at u.washington.edu
> Subject: Re: [Tweeters] Anna's Hummingbird
>
> Tweets:
>
> Following on David Hutchinson's recent posting, he and I have compared
> notes on the evolving status of hummingbirds in the Seattle area. Dave
> has suggested that I share my observations on Tweeters as he has done
> with his. Our experiences, while different, complement one another.
> However, unlike Dave's carefully controlled study, my evidence is
> anecdotal and I have been obliged to dredge it up from memory rather
> than from carefully tended written records; nonetheless it is
> suggestive.
>
> I moved to the Seattle area in 1967 and have always lived in what the
> real estate industry has decided to call West Bellevue - at first near
> the southeast corner of the then rather modest, spread-out business
> district, and since 1975 in Medina, a residential community one-plus
> mile west of the city center now dominated by closely packed tall
> buildings. During the first part of this four-decade period Rufous
> Hummingbird was a familiar yard bird from April into early summer. By
> about 1980 it was becoming scarcer, although one still found it
> regularly in large areas of suitable habitat such as Mercer Slough.
> Well before 1990 it had essentially disappeared from our suburban
> parks and gardens. Today I see Rufous Hummingbird in my neighborhood
> only very rarely, on spring passage.
>
> Anna's Hummingbird began turning up in our yard in the 'nineties. At
> first it was a novelty: single birds now and then. Before long,
> though, a pair set up residence; it or a successor pair has nested
> ever since, sustained by the many shrubs and perennials in bloom in
> our garden in all seasons. Other Anna's regularly test the limits of
> the resident pair's territory.
>
> In brief, Rufous has effectively been gone from West Bellevue as a
> summer resident for about 25 years. If one had good data I am certain
> one would find a 1:1 correlation between declining numbers of this
> species and increasing density of urban development.
>
> Anna's has been an established permanent resident for about 15 years.
> This means that Rufous had been essentially extirpated for a decade
> before Anna's arrived and settled in. Anna's did not drive Rufous out.
> Rufous had already retreated to the farther-out suburbs, or indeed the
> forested foothills, where it could still find acceptable habitat.
>
> I believe, by the way, that a similar dynamic is in play for the
> colonization of Seattle by the House Finch, and the deceptively
> synchronous disappearance of the Purple Finch from the urban core,
> beginning in the 1950s. Unfortunately, it is too late to gather data
> to test this hypothesis in a scientifically respectable way. I do know
> that Purple Finch was a common feeder bird at our house into the late
> 'seventies, although outnumbered 2:1 or 3:1 by House Finch. Purple
> Finches then disappeared abruptly. I now see one in our garden every
> three or four years, never for longer than a few minutes.
>
> Wouldn't it be nice if we had long-running bird census data for these
> and other species along transects crossing rural, suburban, and urban
> zones from the Puget Sound shoreline to (say) the 1,000-foot contour
> of the Cascades and Olympics? Data that could be quantitatively
> compared with measurements of land-use changes and concomitant habitat
> alteration over the years? How much more human population growth can
> the Puget Trough stand before we wake up to discover that Rufous
> Hummingbirds and Purple Finches are nowhere to be found because we
> have converted the lowlands to House Finch and Anna's Hummingbird
> habitat?
>
> Hal Opperman
> Medina, Washington
> hal at catharus.net_______________________________________________
> Tweeters mailing list
> Tweeters at u.washington.edu
> http://mailman2.u.washington.edu/mailman/listinfo/tweeters
>
>
>