What is Migration?
Migration refers to the seasonal movement many animals undertake in pursuit of more resources. Although many different types of animals can be migratory, the Neotropical Flyways Program focuses on Neotropical migratory birds: birds that breed in North America during northern winters but spend their nonbreeding time in Mexico, Central, or South America.
But why do some birds travel so far? The two primary resources which drive bird migration are:
These resources are available at a fairly constant rate in the tropics, where many migratory species originated, but competition for these resources is high.
In North America, however, resources can be seasonally much more abundant than in the tropics. Longer daylight hours in summers allow for a higher abundance of food (especially protein-rich food such as flying insects), which in turn allows migratory birds to raise more young (4-6 on average) than species that remain in the tropics (who only have 2-3 young on average).
As the seasons change, however, young birds mature and food becomes scarcer. By returning to the tropics during the northern winter, migratory birds can also take advantage of the consistent resources available in the tropics.
Types of Migration
Each species – and sometimes individual populations – will migrate differently, but there are four main approaches to migration:
Permanent Residents remain in the same area all year long. A large majority of Panamanian bird species are considered resident and occur within the country year-round.
©ryanacandee on Flickr
Short-distance migrants will move seasonally – but only within a small area. These migrations are often elevational, such as that of the Three-wattled Bellbird (Procnias tricarunculatus), which spends its breeding season in Panamá’s western foothills and highlands but moves to the lowlands during the nonbreeding season.
Medium-distance migrants rarely leave North America. Killdeers (Charadrius vociferus) are a good example: populations that breed in Canada will move south in the winter, but only as far as the Gulf Coast and South Atlantic states of the United States.
Long-distance migrants are the birds we usually consider “migratory”. Neotropical migratory birds are almost always long-distance migrants, including about half of all North American bird species.
How Birds Migrate
The factors which spur migration can vary widely by species or population, and are not all understood by science yet. The ones we are currently aware of include:
- Day length
- Food availability
Migration can take anywhere from a few weeks to 4 months on-way, although Spring trips are usually faster. This is primarily because birds often speed up as they get closer to their breeding grounds. As an example, although a Blackpoll Warbler (Setophaga striata) may take as long as a month to cover the first 1,000 miles of their trip, the final 2,500 miles may take only 2 weeks!
Most birds migrate at night, with the exception of soaring birds such as raptors (hawks, vultures, kites, etc.), swallows and swifts. Nocturnal migration allows birds to take advantage of cooler temperatures and calmer air, while daytime migration allows the soaring birds to use air currents more efficiently and catch more flying prey like insects and other birds.
©Gary Leavens on Flickr
An individual’s migratory route tends to stay very consistent throughout their lifetime. Although some birds learn their routes from older birds (particularly waterfowl and short-distance migrants), most Neotropical migrants are driven primarily by genetics. However, even these birds will incorporate a combination of learned behaviors into their navigation, including:
Stopovers and Migration Traps
Because birds typically migrate in a series of flights rather than in a single long trip, Neotropical migratory birds are highly dependent on the availability of special locations known as Stopovers. These sites provide vital food and rest areas that allow birds to rest for anywhere from a day to a few weeks before continuing on their journey. Because food availability changes throughout the year, however, Fall and Spring migration routes are often different to take advantage of different stopover locations.
Migration traps, on the other hand, are locations that experience unusually high concentrations of migratory birds. These spots are usually the result of local topography and weather conditions. Some migration traps are temporary creations, such as in the case of fallout events when birds are caught in storm conditions and make landfall as rapidly as possible, while others are more consistent.
The funnel-shaped geography of Central America and northern Colombia act as a bottleneck, concentrating millions of migratory birds into a small area. Birds migrating through this region may also face major barriers such as the Caribbean Sea and Gulf of Mexico, which magnifies the importance of Neotropical stopover sites and migration traps.
The Great Snipe (Gallinago media) migrates at an average speed of
©Ron Knight on Flickr
How high do birds migrate?
Farthest Annual Migration:
Arctic Tern, (Sterna paradisaea)
©Brian Gratwicke on Flickr
©Becky Matsubara on Flickr
Longest Non-stop Migration:
Bar-tailed Godwit (Limosa lapponica)