As climate change forces animals to shift their breeding schedules, one group of British birds has been able to quickly adapt to the warmer weather without having to rely on slower evolution.

There are limits to their flexibility however, and when those limits are reached, global warming could hit the population hard, researchers say.

Since 1961, ecologists have been tracking the population of great tits that breed in Wytham Woods, near Oxford, UK. "It's only in the past 30 to 35 years that you see this increase in temperature in early spring to which the birds have responded," says study leader Ben Sheldon of the University of Oxford.

The birds now lay their eggs 2 weeks earlier than they did in the 1970s, tracking a 2-week shift in the emergence of their favoured food – the caterpillars of the winter moth.