When the time comes for songbirds to migrate to their breeding or wintering grounds, extraordinary changes happen: normally active during the day,migrating birds journey at night, often in long stretches of non-stop flight. Plumped up through pre-migratory binges, these birds rarely stop to eat and,instead, maintain a singular focus on reaching their destinations as quickly as possible. This frenzied pace of migration seems to leave little time for sleep, which is problematic. Deficits in sleep are known to have serious consequences, from decreased cognitive performance to physiological impairment and, in extreme cases, death. Given such dramatic consequences, can, and do,migrating songbirds really get by with little or no sleep? If so, how? Niels Rattenborg and colleagues from the University of Wisconsin set out to answer these questions by studying a well-researched migratory songbird species, the white-crowned sparrow.
First, Rattenborg and his co-workers used a combination of behavioral and electrophysiological recordings to test whether or not captive sparrows in a migratory state slept less. Tracking seasonal changes in activity with an infrared video camera and activity monitoring system, they found that in the migratory season the birds spent a lot more time hopping around and flapping their wings when the lights were off. These behaviors, known as migratory restlessness, reflect the duration of migratory flights in the wild, and are never observed in non-migratory birds. To confirm the behavioral results suggesting that migratory birds spend less time sleeping, the authors measured sleep patterns with electrophysiological recordings from the birds' brains. These recordings confirmed that birds in a migratory state sleep on average 63% less than non-migratory birds, and found that they enter REM sleep (the dream stage of sleep known as Rapid Eye Movement) more quickly.
Could these birds somehow compensate for sleep reduction during migratory periods? Electrophysiological recordings confirmed the behavioral observations that birds do not compensate for their night time activity by sleeping during the day. Nor do they compensate for quantity by improving quality: a component of slow-wave sleep that indicates sleep intensity did not differ between migratory and non-migratory nights.
With such apparent sleep deprivation one might expect a decline in cognitive functions for migrating birds. Using tests that measure learning and psychomotor performance, the team found that despite drastic reductions in sleep, migratory birds were cognitively and physically unimpaired. However,non-migratory birds that were forced to sleep as little as birds in a migratory state performed significantly worse on these tasks.
These results raise fascinating questions. Have migrating birds found a way to circumvent the need for sleep, as these results suggest? If so, how do to they consolidate memories, a process that is thought to require sleep? In addition, the authors suggest that studying the mechanisms that coordinate the seasonal rhythms of migration with changes in sleep patterns may provide insight into seasonal mood disorders in humans; people with these afflictions show striking similarities in sleep patterns with migratory birds, including reduced amounts of sleep and the tendency to fall into REM sleep more quickly. Understanding how migrating birds manage their tremendous journeys with little sleep will shed light on the very nature of sleep itself and why it is something that we can't seem to do without.