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When

Each species migrates at a particular time of the year. Some migrate at a particular time of the day. Other species are more irregular in their migratory behavior. Red Crossbills, for example, are erratic wanderers who will settle down and breed any month of the year at whatever time and place they can find an adequate supply of conifer seeds.

Day or night migrants, including many wading and swimming birds, migrate in a continuous flight, day or night.

Diurnal migrants travel during the day. Swallows, nighthawks, and swifts are strictly diurnal migrants. Raptors rely on updrafts from thermal convection (air currents caused by solar heating), so they're also daytime travelers.

Nocturnal migrants depart after sunset and tend to stop flying by 2:00 am. Nocturnal migrants also tend to fly at altitudes below 2,300 to 2,600 feet (700 to 800 meters), although they can climb to 9,800 feet (3,000 meters) to escape turbulent air. Migrating shorebirds fly higher than songbirds. With a few exceptions, birds that migrate over water tend to fly higher than those that migrate over land.

Time of day
You might think that small birds migrate by night to avoid their enemies. To an extent this may be true, because the group includes not only weak flyers, such as rails, but also the small insectivorous (insect eating) birds, such as wrens, small woodland flycatchers, and other species that live most of their lives more or less concealed. These birds are probably much safer making their flights protected under a cloak of darkness.

But remember that night migrants also include sandpipers and plovers. Most shorebirds are usually found in the open and are among the most powerful flyers. Some of them make annual nonstop migratory flights over 2,000 miles (3,200 kilometers) of open ocean.

For most birds, night travel is probably best, chiefly because of the advantages it offers for feeding. If the smaller migrants were to make lengthy flights by day, they would arrive at their destination at nightfall almost exhausted. Since they are entirely daylight feeders, they would be unable to obtain food until the following morning. Being unable to feed would delay further flights. That could result in great exhaustion or possibly even death if they arrived at night in cold or stormy weather.

By traveling at night, the birds can pause at sunrise and devote the entire period of daylight to alternate times of feeding and resting. This schedule allows the birds to recuperate completely. They can resume the journey on a subsequent evening after they have restored sufficient fat deposits.

Another theory is that nighttime migration is advantageous because environmental temperatures are typically cooler: birds get dehydrated from regulating their body temperature and that probably limits the distance a bird can fly nonstop.(Dehydration is a bigger factor than stores of fat.) So birds fly in cooler air during the night, which increases heat loss by conduction and convection. Flight distances can be extended when less cooling is required because birds cool their body by evaporation of their limited body water.