Warmer nights caused by climate change are leading forest birds to build nests and lay eggs earlier in spring, according to Scottish researchers.
Scientists warned the chicks might be hatching after their main source of food is most plentiful.
This is because warmer temperatures are also causing caterpillar numbers to peak earlier.
A team from the University of Edinburgh analysed data from 40 Scottish sites over a five-year period.
The team discovered birds decide when to reproduce based on night-time temperatures in the spring.
They said as climate change causes temperatures to rise, the breeding patterns of birds such as blue tits are being altered.
While previous studies have shown warmer springs have led birds to begin breeding earlier, until now scientists had not identified the key factors causing this behaviour.
Peak caterpillar numbers
But the team warned the warmer temperatures are also causing peak caterpillar numbers to occur earlier in the year, so chicks may still begin hatching after periods when their main food source is most plentiful.
While birds like the blue tit are responding to this change, the researchers warned this is often not fast enough.
Dr Jack Shutt, of the University of Edinburgh's school of biological sciences, said: "Working out what information birds use to time breeding is key to us accurately predicting how this may change under future conditions, and what effect this will have on them."
Trees 'a cue' to breeding
The team said their findings suggest colder temperatures may delay the birds from building nests and laying eggs.
Blue tits were also found to lay eggs sooner if birch trees come into leaf earlier.
The researchers said they had found some of the first evidence that birds use trees as a cue to timing breeding.
Using data gained from two national citizen science projects, the researchers found night-time temperature and birch leafing have very similar effects on the breeding behaviour of woodland birds across the UK.
The study, published in Proceedings Of The Royal Society B biological science journal, was funded by the Natural Environment Research Council and involved researchers from the Woodland Trust, British Trust for Ornithology, University of Exeter, and the Royal Society for the Protection of Birds.