These soaring birds must migrate by day, since the rising currents of air which enable them to soar form only during the day as the sun s rays heat the earth. Like airplane pilots, birds choose a flight altitude depending on at what height the best wind conditions are found. This can vary according to time of day, time of year, features of the earth below, and the weather. In general, nocturnal migrants travel at higher altitudes than diurnal migrants. Of the nocturnal migrants, most shorebirds and waterfowl fly higher on average than do songbirds.
Most birds tend to fly higher when crossing large bodies of water than when flying over land.
Slower and faster flight speeds have been recorded, but they are exceptions. In general, larger birds fly faster than smaller birds. The speed and direction of the wind influences how fast a bird travels. A one-way migration can take anywhere from several weeks to 4 months. The pace of migration tends to be faster in the spring, with the pace picking up as a bird gets closer to its breeding area.