Frequent fliers

by Jim MacInnis

“One of the big questions in songbird conservation is the role of breeding versus wintering grounds in driving widespread songbird declines,” observes Bridget Stutchbury, a biology professor at York University and the author of Silence of the Songbirds. “Though we are fairly sure of the wintering range of a species as a whole, we have never been able to link individual breeding and wintering populations.”

Researchers are closer to finding an answer to the question Stutchbury alludes to. Stutchbury heads up the first team of scientists to successfully track a group of songbirds along the entirety of their respective migratory routes, and discovered that migratory songbirds fly faster than previously thought possible.

The findings, first published in the journal Science, were made possible by revolutionary, feather-light gadgets called “geolocators.” Unlike heavy satellite encumbrances that transmit location information directly, these tiny, unobtrusive devices can easily be fitted to songbirds and capture data about light levels for future analysis. By retrieving geolocators and studying the exact times of sunrise and sunset along the migration route, researchers are able to determine where individual birds are on any given day.

In June 2007, Stutchbury and her team, which includes colleagues from the Purple Martin Conservation Association, fitted 20 purple martins and 14 wood thrushes with geolocators at an avian research station in Edinboro, Pennsylvania. The following spring, seven of the geolocators were retrieved and their data analyzed. The results were astonishing.
Birds were observed to travel three times faster than previously thought, with some covering distances of more than 500 kilometres in a 24-hour period. “The rapid migration of purple martins was a huge surprise,” reports Stutchbury. “The official Birds of North America species account describes them as ‘leisurely migrants!’” The slowpoke of the test group travelled 159 kilometres per day; this sluggishness was attributed to a route around, instead of over, the Gulf of Mexico.

Researchers were also amazed by the huge discrepancies between fall and spring migration times. One purple martin returned to the Pennsylvania station from its wintering grounds in Brazil in a mere 13 days, compared to the 43 days it spent flying in the opposite direction the previous fall. “The fast spring migration, compared with fall [migration], in both martin and wood thrush is no doubt due to the mating game,” explains Stutchbury. “Early arrival means first pick of the older and best mates – and the best territories.”

Perhaps the most important findings involve where the birds are going. Once thought to spread themselves across vast areas in South America, the data collected suggests that wood thrushes winter in a fairly concentrated area in Honduras and Nicaragua. The mapping of exact wintering locations will be valuable for future conservation efforts, particularly in the quickly dwindling forests of Latin America. “We can now find out what is happening in the regions where a particular breeding population overwinters. If a breeding population overwinters in a tropical region with extensive forest cover but is nevertheless declining, this suggests that the problem lies during migration or on the breeding grounds.”