Arild Husby of the University of Edinburgh, UK, and his team looked at data gathered from a wild population of great tits (Parus major) in the Netherlands that has been monitored since 1955.
Increasingly warm springs over the past four decades mean the songbirds' biological rhythms are out of sync with nature. Plants are blossoming, fruits are blushing and caterpillars are gorging themselves earlier. Great tit chicks that hatch too late in the season miss out on this unusually early peak in food – especially the abundance of caterpillars – resulting in fewer surviving youngsters and second clutches.
The researchers correlated average daily temperatures to egg-laying dates from more than 3800 visits to the nesting boxes of nearly 2400 females recorded between 1973 and 2007.
The analysis confirmed that rising temperatures have strongly selected for great tits that hatch earlier: chicks that enjoy spring's early-bird special went on to produce more chicks of their own.
What's more, they found that the statistical correlation was strongest for the warmest springs. This, the team says, suggests that increasing temperatures are speeding up the bird's adaptation to warm springs.
Can they keep up?
It's not all good news, though. Despite this potential coping mechanism, the population overall is producing fewer and fewer offspring, leading the researchers to question whether the birds can adapt fast enough to keep pace with climate change.
"We were quite excited and surprised by these results," says Marcel Visser of the Netherlands Institute of Ecology in Wageningen. He explains that the rate of evolution compared with that of climate change will determine how well biodiversity can cope.
Timothy Coulson of Imperial College London explains that the songbirds are probably experiencing markedly accelerated rates of microevolution – change in gene frequency from one generation to the next – as opposed to long-term evolution, which spans many thousands or millions of years.
"Evolutionary biologists have known for some time that as the climate shifts it appears to lead to evolutionary change – some species go extinct, others come online," says Coulson. "What we can do now is detect microevolution – the type of very rapid evolution you might struggle to pick up in the fossil record."
With climate change, responses to environmental change are happening at an unprecedented rate, he adds, suggesting that microevolution may be happening a little faster than before.
But, he cautions: "When we see very rapid changes, it doesn't mean the individuals have developed an evolutionarily stable strategy."
Rowan Barrett of Harvard University agrees that even rapid evolution does not guarantee safe passage to species threatened by climate change. "Sometimes evolution won't be able to save a population, even if it's very fast," he says. "We are seeing more and more examples – like coral bleaching – where species just can't evolve fast enough to cope with a change in the climate."