Perceived predation risk affects sleep behaviour in free-living great tits, Parus major

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Sleep is of major importance to most organisms but insights into how sleep is affected by ecological processes are largely lacking. Perceived predation risk constitutes a major factor that should shape adaptive phenotypic plasticity in sleep but it is unclear to what degree an individual can tailor sleep to different types of risk. If animals base behavioural decisions on the predation landscape then we would expect individuals to adjust their sleep behaviour when exposed to changes in predation risk. Here we investigated the plasticity of phenotypic sleep in wild great tits roosting in nestboxes and exposed to different types of predation risk. Following our prediction, when exposed to experimentally increased perceived predation risk from owls, Strix aluco (a bird that can prey on birds solely outside their roosting cavity), individuals increased total sleep duration. Contrary to our prediction, when exposed to experimentally increased perceived predation risk from martens, Martes martes (a mammal that can prey on birds inside cavities), individuals woke up less often during the night, but otherwise did not change their sleep behaviour. Birds did not alter total time spent awake during the night in response to predator exposure. Our findings demonstrate that individual great tits modify their sleep behaviour in response to changes in predation risk. Ecological factors including exposure to predators, resource availability and reproductive competition may act as significant constraints on natural sleep patterns and warrant further investigation with free-living individuals.