Saturday, October 2, 2010

A Rush of Thrushes

(photo from the US Fish and Wildlife Service archives)

For some unknown reason last night was a fairly decent night for the nocturnal migration of passerine birds along the west coast of Florida. This was especially true for two species of thrushes (Swainson's thrush is shown above). From 9:30 p.m. until 11:30 p.m. last night (after I gave up in disgust while watching the Tampa Bay Rays blow another game) I recorded an average of 1.3 thrushes per minute passing overhead (I counted all call notes passing a single point for 15 minutes then waited 15 minutes and counted again).

By far the most numerous migrating thrush was Swainson's Thrush a common fall migrant (not so much in spring migration) through this part of Florida. Not far behind them in abundance was Gray-cheeked Thrush. Although getting to be late for them I also heard two Wood Thrush and if I wasn't mistaken one unusual call note was probably a Veery. This trifecta of Catharus thrushes was complimented with a nice push of migrant warblers and I was surprised that there were still two Bobolinks in the push south last night.

Despite all of the movement I heard last night there were surprisingly few migrant songbirds around this morning when I was out before dawn looking and listening. In fact among warblers this morning I saw only:

Tennessee Warbler
Northern Parula (nests here)
Black-throated Blue Warbler
Blackburnian Warbler
Yellow-throated Warbler
Pine Warbler (nests here)
Palm Warbler (common winter resident)
Black-and-white Warbler
Hooded Warbler

The highlight of the morning, however was a Philadelphia Vireo, only the third one I have seen in two years in Sarasota County (wish I could get it for my Manatee County list also!).

But back to all those thrushes.

Swainson's Thrush nests in coniferous forests across Canada and Alaska, and also in suitable habitats along the northern tier of states bordering Canada. One subspecies of Swainson's Thrush nests down the spine of the Rocky Mountains and Sierra Nevada Mountains in the western United States.

More important biologically is the winter range that extends sinuously from southern Mexico south through the spine of the Andes to northernmost Argentina.

Thrushes (except for Bluebirds and American Robins) are generally very secretive birds and seeing one is a real treat. Most of the Swainson's Thrushes I have encountered (other than ones I have caught in a net for banding) have been recorded by its voice, whether its the song or its distinctive call note. You can hear both at this link.

Swainson's Thrush has one of the most haunting, ethereal voices of any North American songbird. The first one I ever identified by voice was in late May 1968 while walking in the forest behind our farm near Rice Lake, Wisconsin. Seven years later in June 1975, my now ex wife and I stood along a road in central Douglas County, Wisconsin listening to one singing from a coniferous forest. Like it was yesterday I remember her saying that its voice was like "tinkling bells that draw you deeper into the forest." A very apt description of its voice.

The biology of migrant Swainson's Thrushes has been a research topic for more than forty years. From this species science has learned a great deal about the physiology of migration. The birds I heard last night were likely flying somewhere between 10 and 30 miles per hour. If you assume that they move at an average of 20 miles per hour and that they migrate from sunset until sunrise, the birds passing over me last night are likely resting and dining on the north coast of Cuba near Havana this morning just 260 straight line miles away from Sarasota.

From Cuba they have a long slog ahead of them to make it to up to 3,400 more miles from Havana to northern Argentina where some of these birds will spend the winter. They will remain there until next March when the migratory urge will overtake them again and they will start the push north. And just like last night when I was listening to these southbound migrants, I will be out on my lanai in early April next spring with my ear cocked toward the sky hoping to hear a rush of thrushes making their way back north.

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