Every autumn, millions of songbirds embark upon long distance southerly migrations to warmer climes. Some species migrate during the day, but the majority - including sparrows, thrushes and warblers - do so at night, leaving their daytime habitats just after dusk and spending the next 8-10 hours on the wing.
Nocturnal migration has several benefits. Cooler temperatures reduce the risk of overheating; reduced turbulence allows for a smooth flight with minimal energy expenditure; and the cover of night provides good protection from predators. These fly-by-night migratory species lose substantial amounts of sleep, yet continue to behave normally, with the sleep loss having no observable effect on their behaviour.
A new study published online in the journal Biology Letters suggests a behavioural adaptation which enables the Swainson's thrush (Catharus ustulatus) to compensate for this loss of sleep. It provides evidence that these birds can rest one hemisphere of their brain for short periods of time while keeping the other hemisphere fully awake.
The Swainson's thrush exhibits several different types of sleep-like behaviour during the day. One of these, "daytime rest", consists of frequent but short episodes during which one or both eyes are closed for several seconds. These so-called "micro-naps" occur during longer periods of an intermediate sleep-like state referred to as "drowsiness". These daytime sleeping behaviours are observed almost exclusively during the migratory season, when their frequency increases significantly.
Frank Moore, of the Migratory Bird Research Group at the University of Southern Mississippi, and his colleagues set out to determine whether these sleep-like behaviours were accompanied by changes in brain activity. To do so, they implanted seven captive Swainson's thrushes with stainless steel electrodes so that they could monitor the electrical activity of the birds' brains and the movements of their eyes. Video recordings were also made, so that the patterns of brain waves could be correlated with the birds' behaviour.
This revealed that each type of daytime sleep-like behaviour was associated with a different pattern of brain activity, and showed that the micro-naps involving closure of only one eye were characterised by major differences in the brain waves in the left and right hemispheres of the brain. During the short periods of time when the birds had one eye closed (which lasted approximately 12 seconds each), the brain hemisphere which would normally receive inputs from the closed eye showed a significant increase in the slow wave brain activity associated with deep nighttime sleep, compared to the other half of the brain, or to the same hemisphere at other times.
In the birds' visual system, the vast majoity of optic nerve fibres which leave the eye cross the midline to the opposite side of the brain, and there is very little connectivity between the left and right hemispheres. As a result, almost all of the information entering the left eye of a Swainson's finch is processed in the right hemisphere of the brain, and vice versa. From their findings, the authors therefore conclude that micro-napping with one eye closed may enable the Swainson's finch to rest one hemisphere of the brain while effecetively monitoring its environment with the other.
This behaviour, known as unihemispheric (or "half-brain") slow-wave sleep, has previously been observed in aquatic mammals such as dolphins and manatees, and is likely to have evolved because of a selective pressure to remain vigilant and to keep moving while also resting. The new study therefore provides a reasonable explanation for how birds can compensate for the significant amounts of sleep that are lost during nocturnal migrations, and so remain fully alert, and may represent an example of convergent evolution, whereby the same biological phenomenon emerges in two distantly related groups of organisms.
Electroencephalographic (EEG) analyses of mammalian sleep show that the intensity of slow-wave activity in the frequency range of 1-4 Hz is an accurate reflection of the depth of sleep. These findings therefore rest on the assumption that the brain wave patterns observed in birds are the same as those of mammals, and that they can be used to accurately distinguish between wakefulness and sleep.
Fuchs, T. et al (2008). Daytime micro-naps in a nocturnal migrant: an EEG analysis. Biol. Lett. DOI: 10.1098/rsbl.2008.0405
Avian brain studies continue to produce fascinating findings. The term bird brain as a perjorative needs a rethink. The meaning and definition of sleep, as of memory, become more nuanced as the curious pursue new lines of research across species.