We used light-level geolocators to describe the migratory geography of Swainson's Thrushes (Catharus ustulatus) from a breeding population in central coastal California. Between 11 June and 20 July 2010, we attached geolocator tags to 35 Swainson's Thrushes at three sites in Marin County, California, and recovered 12 in the following two breeding seasons. All 12 birds spent the winter in Mexico, north of the Isthmus of Tehuantepec. Eleven of the birds went to the Jalisco region in western Mexico, but two of those birds (18% of total) moved east in midwinter to either the Sierra Madre Oriental or Sierra Madre del Sur; the 12th bird went directly to one of the latter regions. The return rate over the next 2 years for tagged birds (34%) was not statistically lower than that for control birds (45%). We combined our results with the results of an earlier study that used geolocators on Swainson's Thrushes from British Columbia to quantify the strength of migratory connectivity using Mantel's correlation coefficient. Our estimate of the strength of migratory connectivity for Swainson's Thrushes (rm = 0.72) was much stronger than that reported using band returns of Barn Swallows (Hirundo rustica) in the Eastern Hemisphere (rm = 0.03). To our knowledge, this is the first quantitative measure of migratory connectivity derived from geolocators.
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