A Synthesis of Sleep in Wild Birds

Abstract

In this synthesis we have attempted to survey and reanalyse the current literature concerning bird sleep. This was achieved by first reviewing present theory describing functions of sleep. Two general models surfaced which encompass many of the specific sleep theories, namely homeostatic and ecological theories. The terminology used to describe sleep and other associated behaviour was organized into an operational hierarchy. We defined loafing behaviour as a general state of immobility. This functional heterogenous group may include comfort behaviour, feeding, social interaction, wariness, staring down and inactive behaviour. Sleep and associated behaviour are considered inactive behaviour. We next described the common sleeping postures as follows: 1. Bill on back or under scapulars; the classical sleep posture. 2. Bill forward or rest-sleep posture. 3. Head on ground. Specific sleep characteristics were also identified as: 1. Quality of sleep based on duration and frequency of eyelid closure. 2. Length of sleep, called sleep session time which is based on the summation of sleep bouts. A sleep bout is defined as the time elapsed during eye closure and opening. 3. Temporal patterning of sleep sessions which are suggested to be dependent on timing of other higher priority behaviours. 4. The resilience (flexibility) of sleep session time. An original analysis was conducted on these sleep characteristics and other environmental, constitutional, behavioural and predation factors. The results indicated that birds experiencing longer daylength slept less each day. Further, birds slept less if (1) overall dangers were greater and (2) they were more communal sleepers. Longer sleep session times were associated with greater exposure while sleeping (i.e. on a lake or mud flat). Several possible functions of sleep were then suggested. Protection theories encompass choice of a safe sleep site while addressing the significance of communal roosting. Recuperation theories were not adequately supported by quantitative studies. Energy conservation theories imply that birds sleep when doing other behaviour might be non-productive. We suggest a model based on eyelid blinking which allows for a degree of vigilance during sleep but which is also compatible with minimizing energy expenditure.