Their precision was uncanny. A group of songbirds outfitted with pint-sized backpacks that tracked every move departed the tropics for North American breeding grounds practically the same time each year, surprising Canadian researchers and prompting them to question whether these tiny birds can adapt to climate change.
In a new study of wood thrushes, published in the online journal PLOS ONE, York University researchers found the cinnamon-brown birds with a flute-like whistle appeared to follow an internal clock in determining when to begin their 4,000-kilometre spring migration north from wintering sites in Nicaragua, Honduras and Costa Rica.
Of the 10 birds tracked via geolocators attached to their backs with a harness made of Teflon ribbon looped around their legs, the difference in departure from one April to the next was a mean of three days. One bird even left on the same date two years in a row, noted study co-author Bridget Stutchbury, a York University professor who has analyzed the behavioural ecology of birds for decades.
"This internal clock seems to be so finely tuned," Prof. Stutchbury said. "In the spring, they're in a rush to get back."
Not so in the fall, however. Autumn migration departure from the northeastern United States was far less predictable, researchers found, as they analyzed data collected by the geolocators. A key reason is likely need. In the spring, songbirds compete for prime breeding grounds and mating opportunities. Arrival times matter.
In the fall, though, they don't face the same pressures and often take their time heading south.
Although wood thrushes' migration routes varied by several hundred kilometres between the two years, likely adjusted for wind and other weather conditions, it's unclear whether the birds could easily alter the timing of their trips north, if needed.
Inflexibility in flight schedules is more common in birds that migrate long distances. It is a trait that could prove detrimental to wood thrushes as the climate warms, potentially changing when their primary food source – insects – are most plentiful, Prof. Stutchbury said.
If the birds can't adapt, their population decline could quicken even further. Already the number of wood thrushes has plummeted more than 50 per cent in Canada since the 1960s. The loss of forests for urban and industrial development is a big reason, but they're not the only bird whose population is waning.
An extensive report looking at the state of birds in Canada, which was released last month by Environment Canada and several conservation groups, has found that a large proportion of the country's bird species has declined significantly. On average, bird populations have dropped 12 per cent since 1970 and more species are decreasing (44 per cent) than increasing (33 per cent).
Prof. Stutchbury and her team are continuing to study the migration patterns of wood thrushes, which help control insect populations and distribute fruit seeds in forests. The researchers want to know whether this spring's hotter-than-normal weather affected the timing of the birds' flights in any way.
The geolocators they are using were developed in 2007 by York University scientists and the British Antarctic Survey, an environmental research centre, so that the movements of small birds could be studied. The high-tech devices, which weigh no more than 1.9 grams and are smaller than a dime, have sensors that measure light, allowing researchers to estimate latitude and longitude by recording sunrise and sunset times.
The geolocators helped solve one uncertainty in 2009. Data from the bird backpacks showed songbirds can fly more than 500 kilometres in a day, far more than the 150 kilometres estimated in previous studies.