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Monday, 20 July 2009

Reasons for this year's unprecedented BROWN-HEADED COWBIRD arrivals in Britain

Bill Sheehan of Woodland, Maine (NE USA) amongst others, has very kindly responded to my plea for more information on BROWN-HEADED COWBIRDS this year and has provided this additional information which clearly has a direct link to the 4-5 occurrencies in Britain this spring/summer.

''Northeastern North America experienced at least two large waves ofmigrant overshoots in April 2009. These events were apparently caused by massive, powerful but extremely slow moving fronts that extended from Florida and the Gulf of Mexico northwards along the entire Eastern Seaboard up into Nova Scotia and Newfoundland. The consequent avian results were dramatic and substantial fallouts of large numbers of migrants of southern species that do not typically stray this far north in North America. These included large numbers of Moorhens, egrets, Indigo Buntings, Summer Tanagers, Blue Grosbeaks, White-eyed Vireos, Prothonotary, Hooded and Kentucky Warblers etc.

Bill lives in northernmost Maine, US, near the New Brunswick, Canadaborder and experienced only a fraction of this fallout, but this did include unusualnumbers of INDIGO BUNTINGS and BROWN-HEADED COWBIRDS.

Cowbirds are fairly common in this area but they were particularlyabundant at this time. It appears that easternmost coastal Maine,southern New Brunswick and Nova Scotia were directly under the weathersystem that was pumping the migrants north and had the most profoundbird fallouts. It was certainly best documented in NS as they havemany more active birders than ME or NB. Check the Nova Scotia RareBird alert archive for April to see the full effect.

Bill further comments

''Regarding the British stray cowbirds, the species was definitely a significantcomponent of the spring overshoots that we saw, but I am guessing thesewere overshadowed by the abundance of "real" rarities and less likelyto be remarked upon by most North American birders during the event.I only noticed them because I was on the periphery. My hypothesis (guess) is that a large number of these overshootBrown-headed Cowbirds ended up either well north of their range or outover the Atlantic during these weather events. These were among thehardiest of the species that were involved and I assume the survivorswere what ended up crossing the Atlantic.

As Alex Lees has commented, BROWN-HEADED COWBIRDS would most likely struggle to cross the Atlantic unaided and it seems likely that a percentage of birds that found themselves out over the sea perhaps 'hitched a lift' on an ocean-going vessel and in turn appeared in western Europe. It is likely however that the Fair Isle bird made a direct crossing, after overshooting North America.

I am further grateful to the following commentators who very kindly responded to my cowbird request - including Julian Hough (CT, USA), Greg Hanisek and David Sonne (Anchorage, Alaska)