Drew,

Your question concerning Cassin's Vireos on the Olympic Peninsula is an
interesting one. Similar to many other passerines, the Cassin's Vireo
exhibits a decline in abundance as one travels from the dryer eastern side
of the Olympic Peninsula to wetter western regions near the outer coast.
The vireo shares this reduction in abundance with a panoply of other species
including the Olive-sided Flycatcher, Willow Flycatcher, Western Wood Pewee,
Red-breasted Nuthatch, Black-capped Chickadee, Bushtit, Bewick's Wren,
Red-eyed Vireo, Yellow-rumped Warbler, Townsends Warbler, MacGillivray's,
Yellow Warbler, Bullock's Oriole, and Spotted Towhee.
Why so many bird species exhibit this decline in abundance moving
from east to west is an open question, but probably based on several
factors. Perhaps most importantly, the coastal regions of the Olympic
Peninsula are often characterized by persistent fog, rain, and cool
temperatures during the breeding season. Studies in other regions have shown
that these conditions can substantially reduce the amount and activity of
insects available for insectivorous birds. The high tannin content of
western red cedar may suppress insect biomass, further limiting
insectivorous birds such as warblers and flycatchers. Chemical induced
insect suppression by conifers has been suspected in other areas, such as
the pine forests of the southern United States, where the high resin content
of the trees apparently eliminates many insects as food for birds.
Old-growth western red cedars are often characterized by dead tops and
relatively low amounts of foliar biomass, which reduces the amount of cover
and foraging substrate for songbirds. Cedar also possess an unusual
geometric shape of its leaflets which may effect the ability of birds to
exploit arthropods. The shrub layer of salal and evergreen huckleberry may
further contribute to paucity of birds, as their tough, leathery foliage may
inhibit arthropod production. And finally, the rot characteristics of red
cedar may make it difficult for the smaller cavity nesters to excavate holes
through the hard sapwood. One of the few species that seems to increase in
abundance in cedar stands compared to the adjacent hemlock/spruce stands is
the Orange-crowned Warbler, which appears to forage mostly the broad-leaved
understory species such as evergreen huckleberry, salmonberry, and cascara.

A closer inspection of Cassin's Vireo's distribution on the Olympic
Peninsula reveals it to be uncommon to relatively rare in most areas. This
is particularly true in cool mountain forests and the western coastal plane,
where the bird is rarely observed as a breeder or migrant. In fact, on
Washington's coast north of Grays Harbor, I only know of a single published
record fall record at Ocean Shores (1). Cassin's Vireo's have been found
slightly more frequently on the Long Beach Peninsula, where they are rare
but regular migrants and breeders (2). The species is also rare over much
of the northwestern Olympic Peninsula, with early ornithologists remarking
on the bird's absence (4). Moving eastward, the Cassin's vireo gradually
increases in numbers in the drier habitats of the Olympic Mountain
rainshadow. Rathbun found them to be "not common" around the shores of Lake
Crescent in the mid-teens, where he noted a few individuals "confined to the
east end of the lake in young firs and adjacent deciduous growth" (5). In
the Elwha Valley Cassin's vireo's are also scarce, where the species is
known only from two breeding records on the lower Elwha and a singing bird
at Humes Ranch (3). As you have noted, in the Port Angeles area the bird
can be found occasionally, particularly along the power line corridor above
town where they may be heard singing. During the spring migration of 2005,
I found five birds in a large mixed species flock at Striped Peak.
The Cassin's Vireo becomes increasingly more regular in the dryer
forests of Sequim, particularly in the mixed woodlands along the lower
Dungeness River. Along the eastern flank of the Olympics, the bird is hit
or miss with most detections coming from the valley floors of the major
river drainages such as the Dosewallips, Duckabush, Hamma Hamma and
Skokomish. Other locations were the bird has been detected singing include
Oak Head, MacWaine Prairie, Buck Prairie, and the Quimper Wildlife Corridor
in Port Townsend. Perhaps the most reliable local on the Olympic Peninsula
to find the Cassin's Vireo are in the around Shelton and along the Chehalis
River corridor.

1 Observed by Eugene Hunn on 10 September 1979 (Hoge and Hoge 1991)
2 Ralph Widrig noted the bird 8 times here between 1980 and 1986 as
both a spring and fall migrant. Two juveniles together at Leadbetter Point
on 3 July 1981 and two birds on 17 June 1976 on the LBP-BBS indicate
probable breeding (Widrig 1986).
3 Singing at Humes Ranch on 25 May 1990 (F. Sharpe). Reported from the
lower Elwha in 1940's (Kitchen 1949).
4 Reagan (1911) failed to detect any Cassin's Vireos on the west side
of the Peninsula in 1909 and 1910. Palmer (1926) also comments on the
species rarity, stating the he could not locate a single individual during
his explorations on the northwest part of the peninsula in the mid-1920s.
5 Rathbun 1916


Happy Birding
Fred Sharpe
fsharpe at sfu.ca
Sequim

-----------------
Die Stimme der Vernunft ist leise
The voice of reason is soft. But it is very persistent.
(from Christopher Hitchens)


> Message: 5
> Date: Sun, 8 Jul 2007 09:49:17 -0700 (PDT)
> From: Wheelan Drew <amazilia55 at yahoo.com>
> Subject: [Tweeters] Cassin's Vireo Status on Peninsula?
> To: tweeters at u.washington.edu
> Message-ID: <279749.74080.qm at web36906.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
>
> Hey All, I keep forgetting to ask this. The other
> day as I was getting in my car to go to work and heard
> what sounded like a Cassin's Vireo in my yard. I know
> that Hutton's can be pretty darn variable, but
> "generally" it seeems that they are mostly repeating
> the same "phrase", and not doing the question/answer
> thing. Also, I know the Hutton's in my yard pretty
> well, and I haven't ever heard them from where this
> one was singing. How uncommon are they here in Port
> Angeles, is there maybe a post bredding northward
> dispersal? Any info on this would be appreciated,
> Drew Wheelan
> Port Angeles
>
> Andrew S. Wheelan
> www.hermaninstitute.org
> The Herman Institute of Biological Studies
>