Nest Cams

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Royal Society for
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Home Adaptations Migration When

The day migrants include some of the ducks and geese as well as loons, cranes, gulls, pelicans, hawks, swallows, nighthawks, and swifts. Soaring birds, including Broad-winged Hawks, storks, and vultures, can only migrate during the day because their mode of flight makes them dependent on updrafts created either by thermal convection or by wind deflected by topographic features like hills and mountain ridges.

Swifts and swallows feed entirely on diurnal flying insects and are often seen in circling flocks in late summer feeding as they travel gradually southward. Likewise large flocks of Franklin's Gulls in the Great Plains feed on insects caught in thermals. They use these updrafts as a source of food as well as a means of achieving the soaring flight that carries them on their journey with minimal expenditure of muscle power.

Nonstop three- to four-day journeys across open ocean or desert regions are fueled by reserves of fat. Small land birds have a maximum flight range of about 1,553 miles (2,500 kilometers), while shorebirds can fly 1,864 to 2,485 miles (3,000 to 4,000 kilometers) nonstop.

Strong flyers like Snow Geese can make the entire trip from their staging area in James Bay, Canada to the wintering grounds on the Louisiana Gulf coast in one continuous flight. These birds are seldom shot by hunters while they're en route between these two points. They're often observed migrating by aircraft pilots.

Speed of flight
Generally, the flight velocity of birds ranges from 20 to 50 miles per hour. In sustained flight, larger birds typically fly faster than smaller birds. The common flying speed of ducks and geese is between 40 and 50 miles per hour, but among the smaller birds typical flying speed is much slower. Timed with an automobile speedometer, herons, hawks, Horned Larks, ravens, and shrikes 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.

During a level chase at 60 miles per hour, a Peregrine Falcon will have difficulty catching a pigeon. However, this predator can exceed 100 miles per hour during a stoop (a diving attack from above) from a greater height onto its prey.

Directions of migrants in the spring are less diverse than in the fall, which suggests they lose less time in passage. Also, a species' fat stores in the spring are greater than during their fall migration. Spring migrants would have greater energy reserves for longer flights at the beginning of their migration. In fall, the flights are more leisurely, so that after a few hours of flying, birds often pause to feed and rest for one or several days, particularly if they find themselves in suitable surroundings.

Wind currents
For migrants, the primary stimulus for departure is a following wind – in the spring the following wind is from the south, and in the fall it's a wind from the north.

Clear skies (useful for providing orientation cues) are less important than wind currents, since major flights will occur under an overcast sky if adequate tail winds are blowing.

It's a mystery exactly how migrants forecast weather conditions, but birds are sensitive to changes in barometric pressure and feed more intensely when storms are approaching and barometers are dropping.

 

 


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