Sleeping Sentries

Even when apparently asleep, birds open their eyes and peek around. Peeking is limited to the phase of sleep referred to as dozing or "quiet sleep." During the remaining "active-sleep" portion of their slumber, birds' eyes remain shut. Animal behaviorist Dennis Lendrem surveyed flocks of dozing ducks until the patterns of peeking could be discerned.

Lendrem found that in the ducks peeking typically occurred about once every two to six seconds. Which birds in the flock do the most peeking depends on the number of coflockers, positions in the flock, and the time of year. Members of smaller flocks peek more, as do birds in less-protected positions and those closer to a perceived threat. During the breeding season, males peek more than females. This seems to be due, in part, to sexual activities of the flock. As the number of females in the flock increases, so do the opportunities for the males to engage in promiscuous sex, which means that males must keep an eye on each other. Similarly, there is always the threat of paired females (willingly and unwillingly) copulating with males other than their mates. That would explain why mated males peek more than bachelor males.

But being attractive to females carries some potential costs. Brightly plumed breeding males presumably are quite conspicuous to predators. Breeding males peek much more frequently than they do after they return to their eclipse plumage when, all other factors being equal, their peeking rates drop to those of females.

How much active sleep birds require is still unknown. Different sleep phases produce different heart and respiratory rates and changes in electroencephalograph (EEG) patterns, as they do in people. It has been suggested that birds that move around often and rapidly require a greater proportion of active sleep than more intermittently active birds, which exhibit more irregular sleep patterns. Dozing and peeking in flocks allow for more active sleep, reduce the threat of predation, and for some males, increase mating opportunities.

SEE: Communal Roosting; Flock Defense.
