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Several bird species display periodic nocturnal sleep–wake patterns, resembling the ultradian rhythms expressed by mammals. Although relatively little is known about the underlying molecular properties of ultradian biological clocks, field observations demonstrate that the frequency of nocturnal awakenings (which may relate to rhythmicity) increases with ambient environmental temperature. To understand how ambient environmental temperature conditions affect the nocturnal sleep–wake pattern of birds, we experimentally heated nestboxes during the night and monitored the frequency and rhythmicity of awakening behaviour of roosting great tits. More than 80% of great tits displayed ultradian rhythmicity in major nocturnal awakenings, with awakenings occurring every 50–170 min. Experimental increases in temperature, on average, 5 °C, caused birds to wake up approximately 30% more frequently over the course of the night, with the strongest temperature effect occurring during the first part of the night. However, the period length of the predominant nocturnal awakening rhythm was unaffected by increased temperature, likely because most additional awakenings were arrhythmic and clustered during the beginning of the night. We suggest that short-duration awakenings elicited primarily during the first part of the night may not be regulated by an ultradian biological clock, and may respond directly to current environmental conditions, such as the risk environment. Longer-duration awakening bouts, which were not affected by experimental heating, may instead be endogenously regulated by an ultradian clock to optimize clock-controlled sleep patterns and sleep homeostasis. [ABSTRACT FROM AUTHOR]

Ecologists have recently begun to recognize sleep as a behaviour that is important in animal ecology. The first steps have been taken to characterize sleep in free-living birds, but it is unclear to what extent these results can be generalized between species. To describe sleep behaviour in the wild, we videorecorded great tits, Parus major , in their roosting boxes during two consecutive winters and individuals in captivity for comparison. Here, we examined endogenous and exogenous correlates of sleep behaviour in free-living great tits and addressed the potential confounding issues of studying avian sleep in captivity. Like that in blue tits, Cyanistes caeruleus , sleep behaviour in great tits was strongly related to season, and was affected by sex, age and the environment. Although literature suggests relationships between sleep and risk-taking behaviours, possibly arising from stable differences in physiological state, sleep behaviour appeared to be plastic in great tits, and was not predicted by between-individual variation in exploratory tendencies. Captive tits initiated sleep later than wild individuals, even under natural photoperiodic conditions, suggesting that captivity alters timing and duration of sleep in great tits. Long-term repeatability in sleep behaviour was low for all variables, except morning latency (high repeatability) and evening box entry time, evening latency and frequency of awakenings (no detectable repeatability). Variation in sleep behaviour may largely represent within-individual differences in daily sleep requirements. Our study describes how different observable components of sleep are intercorrelated by providing evidence for significant within-individual correlations between sleep behaviours, which represent the integration of plasticity between traits. Consistent with low repeatability, low between-individual correlations suggest substantial behavioural plasticity in sleep, rather than a correlational structure leading to clear sleep ‘syndromes’. Our study provides quantitative evidence for the factors producing phenotypic plasticity in behavioural sleep in an ecological context. [ABSTRACT FROM AUTHOR]