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The number of spiders caught by caged Great Tits Parus major in a 10 minute test increased in a sigmoid fashion with ambient temperature between 2 and 13°C. In control tests with immobile prey Calliphora pupae there was no significant effect of temperature. These results indicate that increasing activity of arthropod prey with temperature (Kacelnik 1979b) renders them more vulnerable to predators. We suggest that one reason why small temperate‐zone birds such as Great Tits do not forage but sing when they get up in the morning is because low ambient temperatures reduce prey activity and hence the profitability of foraging.

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