Millions of migratory birds move from a warm winter range to a summer breeding range each year. Some are so predictable that their arrival in the summer range can be forecast almost to the day. We know when to expect them from long experience, but how do birds know when it's time to migrate?
Circannual Rhythm and Daylight Trigger Migration
Before a bird can migrate, it must be physically ready. In both spring and fall, birds put on weight, storing fat in the body cavity and beneath the skin. Many molt, replacing flight feathers so that their feathers are in peak condition for extended flight. And they build flight muscles for strength and endurance.
These changes are brought on by a combination of the bird's circannual rhythm (a natural annual cycle), and the shift in the hours of daylight that signals a change of season. In general, shortening days affect hormone levels and induce birds to eat more, store more fat, increase muscle mass, and molt. Later, when these physical changes have taken place, hormones also cause restlessness immediately prior to departure.
The relationship between light and the circannual rhythm is complex: birds in captivity can be induced to go through their annual changes more quickly if light levels are manipulated, yet the rhythm still proceeds if light levels in captivity never change (Elphick, p. 26). Birds migrate north from the Southern Hemisphere, where days are getting shorter, from equatorial regions where the hours of daylight and darkness change very little, and from Central America and the southern United States, where days are growing longer. Similarly, birds that migrate south to breed may begin their journeys in places where the days are getting shorter or longer, depending on where they spend the winter.
Weather Influences Bird Migration
Birds respond to weather conditions as well as light when deciding when to depart a summer or winter range. An early spring with unusually warm temperatures can trigger early departure and early breeding. Likewise, extended bad weather or a cool spring can delay things.
Birds generally wait for good weather with favorable winds - they avoid rain, overcast conditions, and winds that might blow them off course. As a result, good weather triggers a wave of departures, with large groups of birds leaving at the same time and arriving at a stopover or destination together. Most will stop to feed or wait out bad weather before moving on with another wave of migrants.
Climate and location also heavily influence fall migration for birds in the higher latitudes. These birds arrive later, breed later, molt later, and leave later than the rest of the population.
Males Go First
For some species, it's important for males to get a head start in spring in order to claim good territory in the breeding range and be ready to attract good mates when females arrive. Departing males can either delay migration of the rest if they encounter bad conditions and return, or trigger an exodus if they leave and don't come back (Elphick, p. 26).
Related Content
Sources
Atlas of Bird Migration. Elphick, Jonathan ed. Buffalo: Firefly Books, 2007.
Firefly Encyclopedia of Birds. Perrins, Christopher ed. Buffalo: Firefly Books, 2003
Migration of Birds. Lincoln, Frederick C., Steven R. Peterson, and John L. Zimmerman. U.S. Department of the Interior, U.S. Fish and Wildlife Service, Washington, D.C. Circular 16. Jamestown, ND: Northern Prairie Wildlife Research Center Online. (Version 02APR2002), 1998.
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