Birds sing at dawn and tend to go quiet after dusk. Presumably during darkness they sleep. But the further from the equator one lives, the greater the disparities between the hours of darkness and daylight. So in summer, in temperate or polar areas, do birds not get enough sleep? And do they get too much in winter? How do they deal with the changes?
• Birds do sleep mainly at night, but only in very short snatches. They will also sleep during the day provided they feel secure enough, so the length of daylight makes very little difference to them.
They sing for several reasons, including to attract a mate and to keep in touch with the flock. But the principal reason is territorial. We hear them mostly at dawn because it is quiet and therefore the best opportunity to re-establish territories. Most sing during the day, too, but this tends to be drowned out by a cacophony of traffic, music and the ubiquitous power tool.
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Some birds do sing during darkness: robins, thrushes and nightingales are common examples. But other birds such as rooks and magpies will chatter away in the dark, too, and I have heard sparrows chirping very quietly in the hedge at night, particularly when sitting on eggs.
Street lighting can confuse birds. Last year when the lights in our village were on all night, birds sang in the middle of the night.
Tony Holkham, Boncath, Pembrokeshire, UK
• Birds cannot risk the long, deep sleep to which we are accustomed. They need to be alert to danger, so they sleep in short bursts. Moreover, many species have long migration flights, which represent an even bigger assault on their sleep than seasonal changes in the hours of darkness. It is said that the common swift spends virtually its whole life in the air, landing only to lay its eggs and raise its chicks.
“Birds cannot risk the long, deep sleep to which we are accustomed. They need to be alert to danger”
Many birds migrate at night to avoid predators and overheating. Individual legs of a migration may exceed 24 hours, especially when passing over an ocean or other geographical barrier. Instead of allowing the whole brain to sleep, birds can let each hemisphere of the brain and its associated eye take turns to rest, in a process called unilateral eye closure. This is the biological equivalent of autopilot – the active half of the brain can still alert them to danger.
Between flights, Swainson’s thrushes and other migrating birds catch up with missed sleep by taking significantly more “micro naps”, lasting just 9 seconds each, and drifting into an intermediate sleep-like state referred to as drowsiness, when they keep a lookout through drooping eyelids. They supplement this with unilateral eye closure. Although not as rejuvenating as normal sleep, this has the advantage of decreasing the risk of being caught and eaten.
Amba Sanghera, Mtarfa, Malta