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During migration, birds must travel long distances between their breeding and wintering grounds. The journeys frequently are thousands of miles from start to finish, and some birds may travel over 1,000 miles in nonstop flights. Some species travel during the day. Hawks, for example, glide from one thermal or updraft to another as they make their journey; these flying conditions are strongest during the day. Most birds, including songbirds and shorebirds, use more direct, powered flight. For such flights, the best conditions are at night, when heat is less intense and winds are weaker.
For this reason, many birds need to reverse their sleeping patterns during spring and fall. Instead of sleeping through the night and staying active during the day, they need to fly all night and then rest for the next flight during the day. Unfortunately migrating birds also need to eat, so the entire day cannot be spent sleeping.
It appears the solution is to take a series of cat naps during the course of a day. A study of caged Swainson's thrushes showed that sleeping patterns change during spring and autumn.
To find out how the birds get through these tiresome periods, scientists observed caged thrushes for an entire year, recording when and how long they slept. They found that during autumn and spring, when the birds are normally migrating, they reverse their typical sleep patterns, staying awake at night and resting during day.(Thanks to squeakysoul for the tip.)
But instead of sleeping for long stretches at a time, the birds took several naps a day, each one lasting only 9 seconds on average.
The thrushes also mixed up their shut-eye sessions with two other forms of sleep. In one, called unilateral eye closure, or UEC, the birds rested one eye and one half of their brains while their other eye and brain hemisphere remained open and active, keeping them semi-alert to danger.
The birds also occasionally slipped into another state, one that any college student who has ever been stuck in a boring lecture can relate to. Called drowsiness, this state is characterized by a partial shutting of both eyes that still allows for some visual processing.
