The researchers, led by ecologist Kevin Fraser at York University in Toronto, reached their conclusion after tracking purple martins, members of the swallow family that range as far north as southern Canada, all along their spring migration route of more than 7,000 kilometres.
The results of the study were published in the journal PLoS ONE.
Fraser and his colleagues found that two populations of birds that their research team was tracking took off from their wintering grounds in Brazil's Amazon Basin for their breeding sites in the northeastern U.S. at the same time every year for five years in a row, no matter what the temperature was like at both their summer and winter homes.
Record high spring temperatures at their breeding grounds in 2012 didn't break the pattern.
"There wasn't really any difference in what they did in the really warm year compared to other years," said Fraser in a phone interview Monday afternoon from Texas, where he is in the process of tagging a southern population of purple martins.
Fraser said that means the martins likely arrive at their breeding grounds "too late," after the spring population of the insects they eat has peaked and started to decline, and the reduced food supply could hamper their ability to breed successfully.
Songbird populations falling
Populations of many songbirds have fallen drastically in North America since the 1960s — more than 75 per cent in the case of some species, such as the rusty blackbird, the olive-sided flycatcher, the Canada warbler, the American black duck and the evening grosbeak. A variety of factors may play a role, including habitat destruction and climate change.
Currently, purple martin populations are declining by three to four per cent per year in the northern part of their range, around the Great Lakes, said Fraser, a postdoctoral researcher working with Bridget Stutchbury, a York University ecology professor who co-authored the paper.
Some scientists have suggested that an increasing mismatch between the birds' migration times and the start of spring may be to blame. European studies show insect populations have been peaking earlier and earlier each year, and that places where the peak happens earliest are where populations of migratory insect-eating birds called pied flycatchers were declining the most. The flycatchers did not appear to migrate earlier to compensate.
Fraser and his team wanted to see if they could get more direct evidence about the timing of the spring migration of purple martins in relation to spring weather conditions all along their 7,300-kilometre migration route.
Fraser said there is some evidence that certain weather patterns in the Amazon and the northeastern U.S. are correlated. He wanted to see if individual birds could respond to those cues, or perhaps change their migration speed in response to conditions along their route.
In order to find out, he and his colleagues captured purple martins at their breeding sites and attached tiny tracking devices to their backs using a little "backpack," starting in 2008. The devices log light levels, providing data about latitude from the timing of sunrise and sunset, and data about longitude from the timing of the solar noon, which varies with longitude. Studies show that the backpacks do not appear to affect the birds' ability to fly, breed or survive.
In 2012, the researchers managed to recapture 52 purple martins that had been tagged in Erie, Penn., and Woodbridge, Va., and to retrieve the tracking devices, with their data. They then matched the tracking data with data from weather stations along the route to find out what conditions were like as the birds flew by, and compared it to data from previous years.
Fraser said he was surprised to find that the birds did not seem to respond to temperatures or rainfall.
"I thought there would have been some climate signal somewhere along the route," he said. "I thought ... that if they found warmer temperatures, they could pick up the pace a little bit."
The findings suggest that individual purple martins can't compensate for short-term changes in the timing of spring, unlike species with shorter migrations, such as tree swallows.
Fraser said purple martins and the dozen or so other songbirds in North America that migrate to very distant wintering sites "might be particularly at risk with changes in climate because they're further away and less likely to get the signals they might need of changes up north."
Scientists expect that natural selection will eventually cause the birds to shift their migration times, but Fraser said that so far, that's happening too slowly.
"It could take lots of generations of birds," he added, "and if they're declining at such a strong rate, 'Will that be too late?' is the question."Suggest a correction