In a groundbreaking study that wouldn't have been possible even a few years ago, scientist Kevin Fraser from York University in Toronto says that shifting seasons due to global warming have thrown the delicate timing of bird nesting and food availability out of whack.
"Selection has favoured birds arriving at the perfect time, so they get there right at the peak of insect food, the big flush of all different kinds of insects we get in spring," said Fraser, whose paper is published in the online journal PLOS One.
"There is a real sharp peak in that. And birds ... have timed their migration to match that exact peak so they produce a lot of young and do really well."
But spring has been arriving earlier and earlier. Fraser and his colleagues wanted to find out if migratory birds are able to adapt and leave their wintering grounds in time to take advantage of the seasonal feast they need to build nests, mate and lay eggs.
Using tiny "backpacks" containing GPS recorders that weren't developed until 2007, the scientists focused their attention on purple martins, a common insect-eating songbird that migrates from the Amazon basin to breeding grounds in the United States and Canada.
Spring 2012, the warmest and earliest since record-keeping began in 1895, gave them the perfect opportunity.
Fraser found that not only did the purple martins not leave early, their departure from the tropics was actually later than average.
"Birds departed significantly later, not earlier, from wintering sites and there was no difference between years in the timing of crossing the Gulf of Mexico or arrival at breeding sites," the paper says.
Some species of migratory birds that don't roam as widely have been able to adjust their flight schedules, Fraser said.
"They were actually able to flexibly adjust what they do year-to-year and change their egg-laying date and change what they do breeding-wise to capitalize on those changes. It's the long-distance migrants that are having trouble."
Purple martins may just be too far away to receive signals about conditions on their breeding range. The same issue may be affecting other bug-eating long-distance migrators, which are all suffering from population declines as large as three per cent annually.
"Birds that feed on insects on the wing like purple martins and swallows and flycatchers and swifts, they're showing the strongest declines of any type of bird," said Fraser.
"They're all very different birds, so we're wondering what type of big thing could be affecting them all. It couldn't just be habitat loss in one particular area.
"We're thinking that climate change could be a big contributor."
Fraser said more research is needed. Other bird species need to be considered and the purple martin research needs to be extended over more years.
The work needs to be done to better understand the implications of the earth's shifting climate, he said.
Aside from providing important services such as eating nuisance insects such as mosquitoes, birds are easily studied and are common indicators of where things are headed.
"They're good canaries in the coal mine. They can tell us about environmental health."