Birdwatch News Archive

One of the study's trapped Swainson's Thrush fitted with its geolocator. Photo: Kira Delmore (University of British Columbia).

Swainson's Thrushes keep their distance

Posted on: 28 Sep 2012

Scientists have found out how the two subspecies of Swainson's Thrush in British Columbia have been able to stay isolated from each other.

Using state-of-the-art geolocators smaller than a postage stamp, researchers from the University of British Columbia, Canada, have been able to map the two subspecies' completely geographically separate migration routes.

Twenty individuals of each subspecies of the small, nightingale-sized Catharus thrushes were trapped in their breeding areas in Pacific Spirit Park, Vancouver, and Kamloops, British Columbia in June 2010, by luring them into mist nets using tape playback of mating calls. Geolocation devices weighing less than a gram – about four per cent of the bird's total body weight – were then attached to each. The newly-developed light-level geolocator devices record sunrise and sunset times, which when analysed enable scientists to work out the position of single birds along their route over a complete year, covering their departure and return to their breeding areas.

"Birds of a feather do not necessarily flock together," said Kira Delmore, lead author of the paper on the Science Daily website. "Our teams of thrushes took dramatically different routes to get to their wintering grounds, either south along the west coast to Central America, or southeast to Alabama and across the Gulf of Mexico to Columbia. This detailed level of migration and stopover data helps us pinpoint vital feeding and rest habitats that the birds rely on at key points during their long journey, just before crossing the Gulf of Mexico, for example."

Their results confirm the routes that had been suspected from ringing recoveries and genetic analyses, with the west coast subspecies heading directly south and back, to and from Central America, hemmed in by the Rocky mountains, and the inland subspecies changing its strategy to a longer, looped route south-east straight over the Gulf of Mexico in autumn but travelling around the gulf in spring, hugging the coastline at the start.

Their results suggest that migratory behaviour may play an important role in speciation, keeping populations isolated throughout their winter and summer ranges. Migratory behaviour itself is strongly influenced by genetic factors, and maintaining these differences would strengthen the genetic uniqueness of each population. The researchers have speculated that hypothetical hybrids between the two subspecies might take an intermediate migration route.

Reference
Delmore, K E, Fox, J W, and Irwin, D E. 2012. Dramatic intraspecific differences in migratory routes, stopover sites and wintering areas, revealed using light-level geolocators. Proceedings of the Royal Society B, DOI: 10.1098/rspb.2012.1229