Although it was thought for a long time that migratory flights were made at normal cruising speeds, Harrisson (1931) and Meinertzhagen (1955) showed that migration speeds were in between cruising speeds and escape speeds. The theory that migrating birds attain high speeds received encouragement from the German ornithologist Gatke (1895) who, for many years, observed birds at the island of Heligoland. He postulated that the bluethroat, a species of thrush smaller than the American hermit thrush, could leave African winter quarters at dusk and reach Heligoland at dawn; this flight would mean a sustained speed of 200 miles per hour! He also thought the American golden plover flew from the coast of Labrador to Brazil in 15 hours at the tremendous speed of 250 miles per hour. Most ornithologists now consider these conclusions to be unwarranted. Reliable data on the speed of birds are accumulating slowly. Accurate measurements are difficult to obtain unless the bird travels over a measured course and wind conditions at the level of flight are known. Several subtle factors, besides wind and pursuit, can influence the speed of a flying bird. For instance, species that have a courtship flight often reach their maximum speeds then. Small woodland birds often fly faster across an open area where they might be attacked by a bird of prey than under cover where there is less danger. Birds in flocks generally fly faster than when flying alone. A thermal draft may induce an almost imperceptible air movement at the Earth's surface, but a good glider with motionless wings may make 35 miles per hour on a current of air that is rising vertically at less than 2 miles per hour. If the bird coasts downhill at a slight angle in still air, it can attain a similar speed. For sustained flight, it may be generally concluded that larger birds fly faster than smaller birds. A common flying speed of ducks and geese is between 40 and 50 miles per hour, but among the smaller birds it is much less. Herons, hawks, horned larks, ravens, and shrikes, timed with the speedometer of an automobile, have been found to fly 22 to 28 miles per hour, whereas some of the flycatchers fly at only 10 to 17 miles per hour. Even such fast-flying birds as the mourning dove rarely exceed 35 miles per hour. A peregrine falcon will have difficulty catching a pigeon during a level chase at 60 miles per hour, but this predator can probably exceed 200 miles per hour during a swoop from a greater height onto its prey.