Seasonal migration refers to the movement of various bird, insect, and mammal species from one habitat to another during different times of the year. The migration is necessary because seasonal fluctuation in factors such as the availability of food, sunlight, and the temperature of the air or water become intolerable in one habitat, usually during the winter, making breeding and even survival difficult.
An example is the migration of various whale species from their summer habitat in Arctic or Antarctic waters to their wintertime breeding grounds in tropical waters near the equator. Some bird species also migrate from their summer home in the Arctic to spend winter at warmer latitudes.
For some species, the seasonal migration cycle involves journeys that exceed tens of thousands of miles or kilometers in length. A migration route can involve resting and refueling stops, although some species complete their migration non-stop.
Efforts to protect migratory species include public and private initiatives, routing of shipping away from known migration routes, and international agreements. The latter includes the Convention on Migratory Species (the Bonn Convention) and the Ramsar Convention on Wetlands.
Historical Background and Scientific Foundations
Seasonal migration likely dates back at least 3,000 years, as records of bird migrations written by the ancient philosophers Homer and Aristotle have been recovered. Biblical descriptions of bird behavior consistent with migration can also be found in the Books of Job and Jeremiah.
Seasonal migration can take place over very long distances. The route of the Arctic tern between the Arctic and Antarctic exceeds 13,670 mi (22,000 km), and the Bar-tailed Godwit travels non-stop over 6,835 mi (11,000 km) from Alaska to New Zealand. Among whale species, the blue whale journeys from Arctic or Antarctic waters to its breeding grounds in equatorial waters in a journey that can take four months, and during this time it does not eat. The route of the Eastern Northern Pacific gray whale between the coastal waters off Baja, California, and the northern Okhotsk Sea between Russia and Japan is over 6,200 mi (10,000 km) in length.
Bat, seal, turtle, and insect species all seasonally migrate. For example, the monarch butterfly migrates from southern Canada to winter in central Mexico.
Stopping en route to rest and refuel with food and water is common among migratory bird species. Wetlands and agricultural land that has been harvested are popular rest stops for species such as the Canada Goose. The need for space and resources such as water along the migration route makes seasonal migration susceptible to climate conditions that adversely affect the land. Drying or pollution of wetlands or conversion of land for urban or industrial use can threaten the migratory journey. As one example, dwindling populations of the monarch butterfly have been traced to overlogging of their wintering grounds in central Mexico. Organizations including Ducks Unlimited work to acquire and preserve wetlands to preserve migratory routes.
Impacts and Issues
Climate change can affect seasonal migration. Bird migration has been affected for species whose route takes them over deserts. Regions such as the Sahel region of Africa have become drier, decreasing the opportunity to stop and refuel. In highly developed areas of the world, light pollution can also pose problems for birds who migrate at night and use the moon and stars for navigation.
Whale species can be threatened when their migration takes them through shipping routes. An example is the northern right whale, whose migration terminates in Canada’s Bay of Fundy. Every year several whales are struck and killed by commercial vessels, depleting a population that has been estimated to be only about 300. As well, the pollution of coastal waters with agricultural and urban runoff is a health threat.
Because migration can be over national boundaries, international cooperation is necessary to help protect migratory species. The Convention on Migratory Species, implemented in Bonn, Germany, in 1979 under the United Nations Environment Programme (UNEP) is concerned with the global conservation of migratory wildlife and habitats. While North America and coastal waters are part of the migratory routes for a variety of bird and mammal migratory species, the United States, Canada, and Mexico are not among the 104 nations to have ratified the convention.
WORDS TO KNOW
HABITAT: The natural location of an organism or a population.
LIGHT POLLUTION: Also known as photopollution and luminous pollution, refers to the presence of excessive amounts of light in the atmosphere.
WETLANDS: Areas that are wet or covered with water for at least part of the year.
Hermes, Patricia. Fly Away Home. New York: Newmarket, 2005.
Journey North.http://www.learner.org/jnorth/ (accessed May 2, 2008).