Electrophysiological and behavioral correlates of sleep in the blackbird (Turdus merula)

Sleep in five blackbirds was investigated by continuous 24-h recordings of EEG, EOG, EMG, HR, and behavior. Because these recordings were similar in many respects to those obtained previously in other species, it was possible to define the electrophysiological correlates of active wakefulness (AW), quiet wakefulness (QW), slow wave sleep (SWS), and paradoxical sleep (PS). The time spent in SWS and PS was 32.2% and 5.7% of the 24-h period, respectively. The amount of SWS decreased during the course of the dark period, whereas PS exhibited an increasing trend. In addition, SWS always preceded PS, as in mammals. Thus, homeotherms may share common mechanisms of sleep regulation. Blackbirds turned their heads so that their beaks pointed backwards only during darkness. This back posture was usually associated with sleep. Head nodding, which occurred only when the beak pointed forward, was observed during 13% of PS episodes. Eye closure was a reliable behavioral index of sleep. Detailed behavioral observations alone do not provide sufficient information for the accurate assessment of sleep stages in blackbirds.