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Well-fed robins could be lonely at Christmas

December 09, 2002

The North Wind doth blow,
And we shall have snow,
And what will poor robin do then,
Poor thing?   (Anon)

He may starve or freeze to death if he has not managed to find enough food before the snow falls.

Small birds such as robins need to carry fat reserves to keep them warm. They face a dilemma each winter, as they need to spend more time feeding. They need to eat well during the day and sudden, unpredictable, weather like a snowstorm may prevent them from finding food. So, should they spend their time building up fat, resting to conserve energy, or singing to defend their territory and attract a mate?

John McNamara and Innes Cuthill from the University of Bristol used mathematical prediction models to answer this question. The models calculated the best decision at any point in time and at any level of energy reserves. They predicted that a dawn chorus was indeed the best time for the robins to sing, not because song was a more effective signal at this time but as a response to unpredictable energy requirements.

Said John McNamara, "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."

With a colleague, Rob Thomas, the researchers tested the models' predictions on both wild and captive robins. The captive birds were trained to weigh themselves on electronic balances.

As predicted, the robins stored more fat when the regularity of their food supply was reduced. They used up more fat when the nights were cold and sang more at dawn when their reserves were high. The researchers also found that the robins anticipated cold nights and stored extra fat.

Innes Cuthill said, "Our work shows that we should think about the regulation of fat reserves when analysing daily routines, even if that regulation is not exactly as in the model. It gives us a better understanding of individual behaviour in the face of uncertainty and can help us understand population differences in species, and responses to climate change."

Natural Environment Research Council (NERC)