The sound of a robin chirping in winter is a good sign, say scientists.
It means the bird has built up enough fat reserves to survive the cold nights and has enough energy left to defend its territory.
The bird traditionally sings in spring to attract a mate but in winter, when food is short, it faces a dilemma.
Should it spend its time hunting for food to get through the next cold snap or burst into song?
Researchers in the west of England think they have the answer: the bird sings at dawn if it has enough energy left over from keeping warm at night. It is all part of a complicated biological mechanism to regulate fat reserves. John McNamara, of the University of Bristol, said: "Because birds can't predict exactly how much energy they need to survive the night, they need to build up enough fat reserves by dusk to cater for the worst-case scenario. "And as most nights are not that cold, they should have enough energy reserves left over at dawn to sing."
The Bristol team studied wild and captive robins. The captive birds were trained to weigh themselves on electronic balances. It turned out that the birds stored extra fat when food started to become scarce and the nights got colder. When it was warmer at night, they had more fat left at dawn and were able to sing.
Mike Everett, of conservation charity the Royal Society for the Protection of Birds, told BBC News Online: "This interesting research confirms that healthy, well-fed robins have the best chance of survival in harsh weather. "It also underlines the importance of feeding robins and other small birds in our gardens during the winter - feeding which, in the most severe weather, really can mean the difference between life and death."
The research, published in the winter edition of the Natural Environment Research Council journal Planet Earth, should aid conservation efforts. According to Bristol scientist Innes Cuthill, it will give us a better understanding of how bird species react to climate change.