Bird Migration


Zugunruhe, often called migratory restlessness, is a phenomenon exhibited by migratory animals—birds in particular. Before migration, the organisms begin to show anxiousness or agitation. In nocturnal migrants, for example, this includes increased activity around dusk and abnormal sleep patterns. Shortly after exhibiting Zugunruhe, birds increase their rate of energy intake to accumulate fat deposits to fuel their migration. Environmental cues such as changes in temperature and photoperiod (the duration that an organism is exposed to light) are believed to be some of the most important exogenous factors in the onset of Zugunruhe.


There is also a genetic component regarding migratory readiness.  This was proved in a German study where blackcaps (Sylvia atricapilla) from northern Europe and blackcaps from southern Europe were crossbred. In nature, blackcaps from the northern populations migrate early and southern blackcaps are late migrants. The offspring in the study, however, exhibited intermediate behaviors, showing Zugunruhe later than the northern populations but earlier than the southern populations.        



Migration is a very energetically expensive behavior and the benefits of migration must outweigh the costs. The primary force behind migration is food supply. It has been found that if humans provide a food source year-round, a number of species that would otherwise migrate remain in their breeding grounds. Many species are obligatory migrants that would perish if they did not seek more favorable conditions.









A White-winged crossbill feeding on conifer seeds.

This species is often an irruptive migrant.


Migration habits vary between species. The most common migration patterns include complete migrations, partial migrations, and irruptive migrations. For complete migrants, all members of the species leave their breeding grounds and migrate elsewhere to winter. Partial migratory species have only a portion of the population making seasonal migrations; this often results in an overlapping range where the species can be found year-round. Irruptive migrants are usually more unpredictable in terms of movement and interval. Crossbills are notorious for their irruptive migrations. As conifer seed specialists, crossbills are known to irrupt in  southern parts of the United States, well out of their normal wintering grounds in years of poor seed production.



The timing of migration also varies between species. Bird migrations can generally be divided into either prebreeding or postbreeding migrations. Because of variable breeding season length and because different species breed at different times of year, migration times throughout the year also vary. For most of the United States, there is likely at least one guild of birds that is either migrating to or from their breeding grounds in any given month of the year.  For example, many waterfowl species and some raptors can be seen migrating north in the first few months of the year.  Neotropical migrants and most shorebirds, however, are not usually seen in the temperate parts of the United States until April or May. Late summer and early fall are particularly good for finding large numbers of south-bound waterfowl.

Additionally, birds do not often arrive at their breeding or wintering grounds at the same time. Many migrants will establish themselves in the southern parts of their breeding range months before others become established in northern parts of the range. It is also not uncommon for one sex to reach a breeding or wintering ground before the other. In some species, male and female birds may even winter in separate regions. 

Relevance to Airports


Many species migrate in large flocks. Large-bodied shorebirds and waterfowl in particular are known for their large flocks. Line formations (like the familiar flying V or J formations seen in geese) are thought to increase aerodynamic efficiency for the birds, but these and other flocking formations are also an increased risk to aircraft. Because of the large flocks and the overall increased volume of birds in the air, airports and pilots should be alert and proactive during the migration season. Using data from 1990-2003, roughly 51% of all reported bird strikes occurred between the months of July and October. These months coincide with peak postbreeding migration numbers. Postbreeding migrations are often larger than prebreeding migrations because of the addition of first-year birds and also because the winter’s relatively high mortality rate depletes the number of returning breeding birds.          

Flocking Canada geese like these are a hazard to aviation.



Airports should be vigilant with respect to migrating birds and there should be good communication between the grounds crew, the tower, and pilots when potential wildlife hazards are found near the airfield. Any potentially hazardous wildlife that is loafing near the airfield should be removed. By being active, airports are helping to maintain safe aviation.

Able, K. P. 1990. Comparison of vanishing bearings, orientation directions and ringing recoveries of spring migrant White-throated Sparrows (Zonotrichia albicollis). Journal of Ornithology.  131 (3),  317-323. DOI: 10.1007/BF016410042006.


Cleary, E.C. and R.A. Dolbeer.  2005.  Wildlife hazard management at airports.  FAA Wildlife  Mitigation Manual, Second Edition.

Farner, D. S. and J. R. King. 1963. The Relationship of Fat Deposition to Zugunruhe and Migration. The Condor. 65 (3), 200-223.


Helm, B. Zugunruheof migratory and non-migratory birds in a circannual context. Journal of Avian Biology. 37 (6), 533-540.


Kerlinger, P. 2009. How Birds Migrate. (2nd ed.) Mechanicsburg, PA: Stackpole Books.


By: Mat Natali, Loomacres Wildlife Management © Copyright 2011, All Rights Reserved