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Many birds fly non-stop for days or longer, but do they sleep in flight and if so, how? It is commonly assumed that flying birds maintain environmental awareness and aerodynamic control by sleeping with only one eye closed and one cerebral hemisphere at a time. However, sleep has never been demonstrated in flying birds. Here, using electroencephalogram recordings of great frigatebirds (Fregata minor) flying over the ocean for up to 10 days, we show that they can sleep with either one hemisphere at a time or both hemispheres simultaneously. Also unexpectedly, frigatebirds sleep for only 0.69 h d−1 (7.4% of the time spent sleeping on land), indicating that ecological demands for attention usually exceed the attention afforded by sleeping unihemispherically. In addition to establishing that birds can sleep in flight, our results challenge the view that they sustain prolonged flights by obtaining normal amounts of sleep on the wing.

The functions of sleep remain elusive. Extensive evidence suggests that sleep performs restorative processes that sustain waking brain performance. An alternative view proposes that sleep simply enforces adaptive inactivity to conserve energy when activity is unproductive. Under this hypothesis, animals may evolve the ability to dispense with sleep when ecological demands favor wakefulness. Here, we show that male pectoral sandpipers (Calidris melanotos), a polygynous Arctic breeding shorebird, are able to maintain high neurobehavioral performance despite greatly reducing their time spent sleeping during a 3-week period of intense male-male competition for access to fertile females. Males that slept the least sired the most offspring. Our results challenge the view that decreased performance is an inescapable outcome of sleep loss.

Twice a year, normally diurnal songbirds engage in long-distance nocturnal migrations between their wintering and breeding grounds. If and how songbirds sleep during these periods of increased activity has remained a mystery. We used a combination of electrophysiological recording and neurobehavioral testing to characterize seasonal changes in sleep and cognition in captive white-crowned sparrows (Zonotrichia leucophrys gambelii) across nonmigratory and migratory seasons. Compared to sparrows in a nonmigratory state, migratory sparrows spent approximately two-thirds less time sleeping. Despite reducing sleep during migration, accuracy and responding on a repeated-acquisition task remained at a high level in sparrows in a migratory state. This resistance to sleep loss during the prolonged migratory season is in direct contrast to the decline in accuracy and responding observed following as little as one night of experimenter-induced sleep restriction in the same birds during the nonmigratory season. Our results suggest that despite being adversely affected by sleep loss during the nonmigratory season, songbirds exhibit an unprecedented capacity to reduce sleep during migration for long periods of time without associated deficits in cognitive function. Understanding the mechanisms that mediate migratory sleeplessness may provide insights into the etiology of changes in sleep and behavior in seasonal mood disorders, as well as into the functions of sleep itself.

Birds have overcome the problem of sleeping in risky situations by developing the ability to sleep with one eye open and one hemisphere of the brain awake. Such unihemispheric slow-wave sleep is in direct contrast to the typical situation in which sleep and wakefulness are mutually exclusive states of the whole brain. We have found that birds can detect approaching predators during unihemispheric slow-wave sleep, and that they can increase their use of unihemispheric sleep as the risk of predation increases. We believe this is the first evidence for an animal behaviourally controlling sleep and wakefulness simultaneously in different regions of the brain.

Externally mounted transmitters or loggers may adversely affect migration performance for reasons other than the effects of added mass. The added frontal area of a payload box increases drag, and if the box triggers separation of the boundary layer over the posterior body, the drag coefficient could also be increased, possibly by a large amount. Any such effects would lead directly to a decreased migration range and reduced energy reserves on completion of migration. We measured the body drag coefficients of Rose-coloured Starlings in the Seewiesen wind tunnel by the wingbeat-frequency method. The speed at which the wingbeat frequency passed through a minimum was taken to be an estimate of the minimum-power speed (V mp ), from which the body drag coefficient was calculated in turn. Dummy transmitter boxes were mounted on the bird's back by attaching them with Velcro to a sideloop harness pad. The pad alone projected 6 mm above the bird's back, and increased the drag coefficient by nearly 50%, as compared to the ''clean'' configuration with no harness. Adding boxes (square-ended or streamlined) produced no further significant increase in the drag coefficient, but the addition of a sloping antenna increased it to nearly twice the clean value. These increases are attributed to separation of the boundary layer over the posterior upper body, triggered by the payload. We then ran computer simulations of a particular Barnacle Goose, for which detailed information was available from an earlier satellitetracking project, to see how its migration range and reserves on arrival would be affected if its transmitter installation also caused flow separation and affected the body drag coefficient in a similar way. By representing the range calculation in terms of energy height, we separated the effect of the transmitter's mass, which reduces the fat fraction (and hence also energy height) at departure, from that of flow separation, which steepens the energy gradient. The effect of the mass is small, and increases only slightly with increasing distance, whereas a steeper energy gradient not only reduces the range but also reduces the reserves remaining on arrival, to an extent that increases with migration distance. Energy height is related to the fat fraction rather than the fat mass, and is therefore preferable to energy as such, for expressing reserves in birds of different sizes.

SUMMARY Birds provide a unique opportunity to evaluate current theories for the function of sleep. Like mammalian sleep, avian sleep is composed of two states, slow-wave sleep (SWS) and rapid eye-movement (REM) sleep that apparently evolved independently in mammals and birds. Despite this resemblance, however, it has been unclear whether avian SWS shows a compensatory response to sleep loss (i.e., homeostatic regulation), a fundamental aspect of mammalian sleep potentially linked to the function of SWS. Here, we prevented pigeons (Columba livia) from taking their normal naps during the last 8 h of the day. Although time spent in SWS did not change significantly following short-term sleep deprivation, electroencephalogram (EEG) slow-wave activity (SWA; i.e., 0.78-2.34 Hz power density) during SWS increased significantly during the first 3 h of the recovery night when compared with the undisturbed night, and progressively declined thereafter in a manner comparable to that observed in similarly sleep-deprived mammals. SWA was also elevated during REM sleep on the recovery night, a response that might reflect increased SWS pressure and the concomitant Ôspill-overÕ of SWSrelated EEG activity into short episodes of REM sleep. As in rodents, power density during SWS also increased in higher frequencies (9-25 Hz) in response to short-term sleep deprivation. Finally, time spent in REM sleep increased following sleep deprivation. The mammalian-like increase in EEG spectral power density across both low and high frequencies, and the increase in time spent in REM sleep following sleep deprivation suggest that some aspects of avian and mammalian sleep are regulated in a similar manner.k e y w o r d s bird, evolution, pallium, phylogeny, sleep function, slow-wave activity