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Why I've Never Seen Black-throated Blue Warbler in Oregon
As is true almost every fall, tales of East Coast vagrants descending on Northern California are once again pouring in. The numbers reported can be quite astounding and routinely stimulates debate about why Oregon and Washington are not similarly blessed. One school of thought is that we don't spend enough time looking in the right spots, another is that there is something about the migration itself that causes birds to miss the northern parts of the coast. I fall into the the latter camp, and here's why....
I banded in the willows just east of parking lot C, Ft Stevens State Park, from 1989 to 2001. If ever there was a potential spot for migrant fallout, the expanse of willows and pines of Clatsop Spit should be it. I put in about 276 hrs (1460 net*hrs) in the fall season which went from the end of August to the first week in October most years. I caught 835 birds of 41 species. 218 were warblers (26%), most of the warblers were Common Yellowthroats (102 individuals or 12.2%).
Additionally, we've put in 187 hrs (1496 net*hrs) in at the Neawanna banding station, a riparian corridor at Seaside between mid-August and early October, caught 795 birds since 1998.
All those hours and the only things I caught that was even remotely vagrant were a Palm Warbler (1989) and a Rose-breasted Grosbeak (1993) at Ft Stevens and we saw, but did not catch a Prairie Warbler at the Neawanna (2002).
I've also done constant effort counts at Coxcomb Hill in Astoria (about 120 obs*hrs), a place that on a good fallout day will have 100's of Hermit, Townsend's and Black-throated Gray Warblers, plus flycatchers, vireos, tanagers and grosbeaks.... Tennessee Warbler and Red-eyed Vireo.
Compare this to the account in Warblers of North America (Dunne and Garrett 1997) for just Blackpoll Warbler:
"Well over 3000 individuals recorded in California (mainly along the coast) where they average 120 birds per year, though recent averages have dropped to about 105 per year. The maximum daily count is 23 on SE Farallon Is., on 27 September 1974."
I've been living in Clatsop Co since 1987 and saw my county first Blackpoll Warbler in 2004. According to Birds of Washington (Wahl et al. 2005), Blackpoll Warbler has only occurred 13 times in Washington and 12 of those were on the east side of the Cascades. Blackpoll Warbler is considered rare, but "nearly annual" in Oregon. Of the 29 records listed by the OBRC before the species was removed from the review list, only 7 are west of the Cascades and all but 1 of those are from Curry Co, the county closest to California.
So how do we get from rare and east of the Cascades in Washington to 120/yr and mostly coastal in California? Somehow, I don't think the lack of coverage arguement really explains the vast differences in patterns of vagrancy along the west coast, especially given that the trend holds for all vagrant warblers.
WA OR (Clat) CA
Tennessee Warbler 20 ann (4+) 2300+
Northern Parula 9 35+ (0) 800+
Chestnut-sided Warbler 14 25+ (1) 970+
Magnolia Warbler 10 20+ (2+) 985+
Cape May Warbler 1 15 (0) 209+
Black-throated Blue Warbler 8 ann (1) 630+
Black-throated Green Warbler 1 10 (0) 634+
Blackburnian Warbler 4 6 (0) 475+
Yellow-throated Warbler 1 4 (1) 75+
Prairie Warbler 1 11 (2) 355+
Bay-breasted Warbler 1 10 (0) 255+
Blackpoll Warbler 13 reg (1+) 3000+
Prothonotary Warbler 1 8 (0) 160+
Kentucky Warbler 1 5 (0) 115+
Mourning Warbler 1 5 (0) 99+
Canada Warbler 0 7 (1) 235+
No, it's hard to imagine how a 20 or 30 fold difference in the number of birds seen in the northern portions of the West Coast and the south can be purely the product of superior coverage in California. The more plausible explanation relates to latitude and a condition called mirror-image misorientation. Normal fall migrants make a beeline from their breeding grounds in Canada, SE to the Atlantic Coast concentrating at places like Cape May, NJ (~39°N) and Cape Hatteras, NC (~35°N). From there it looks like they take off over the ocean to South America.
Just as some fraction of the human population is left-handed, a portion of southbound migrants have a "left-handed" or mirror-image migration urge which sends them SW and the equivalent latitudes to Cape May and Cape Hatteras on the West Coast fall between San Luis Obispo to the south and Mendicino to the north. The Farallon Islands are at about 37.5°N, pretty much dead-center in the mirror-image flight zone.
So, Oregon and Washington are too far north to receive the bulk of the mirror-image migrants and the majority of those migrants we do get are from along the flyway as it passes over Eastern Oregon. There are always one or two birds that stray north of the main flyway and every once in a while we have a special year, perhaps weather related, which pushes the flyway north, but Oregon and Washington will probably never see the kind of annual eastern vagrant spectacular that gets reported from California.
But there's no harm in looking....
References
Dunn, J.L. and K.L.Garrett. 1997. Field Guide to Warblers of North America. Houghton Mifflin Co. Boston, MA. 656pp.
Hunt, P. D., and B. C. Eliason. 1999. Blackpoll Warbler (Dendroica striata). In The Birds of North America, No. 431 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Marshall, D.B., M.G. Hunter and A.L.Contreras, Eds. 2003. Birds of Oregon:A General Reference. Oregon State University Press, Corvallis, OR. 768pp.
Wahl, T.R., B.Tweit and S.G.Mlodinow. 2005. Birds of Washington: Status and Distribution. Oregon State University Press, Corvallis, OR. 436pp.
Posted by mbalame at October 12, 2006 3:31 AM