Animal migration

Animal migration is the relatively long-distance movement of individual animals. This behavior usually occurs on a seasonal basis. It is the most common form of migration found in ecology. You can find this phenomenon in all major animal groups. This includes birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. Migration is not just a local dispersal or a sudden movement called an irruption. To be a true migration, the movement must be an annual or seasonal occurrence. It can also be a major habitat change that is part of an animal's life.

Zoologists study migration through several different concepts. One common definition comes from the zoologist J. S. Kennedy. He suggested that migration includes four related ideas. First, it involves persistent straight movement. Second, it is the relocation of an individual on a scale larger than daily activities. Third, it involves seasonal to-and-fro movement of a population between two areas. Finally, it includes movement that leads to the redistribution of individuals within a population. Scientists also distinguish between obligate and facultative migration. Obligate migration means the individuals must migrate to survive. Facultative migration means individuals can choose whether or not to migrate.

Different types of migration are driven by different natural cycles. Seasonal migration happens when species move between habitats during the year. This is often caused by changes in resource availability. Temperature is another major driving factor for seasonal movement. Many birds migrate to warmer locations to escape poor winter conditions. Some species, like Pacific salmon, migrate specifically to reproduce. They swim upstream to mate and then return to the ocean. Other animals use circadian migration. This relies on circadian rhythms, which are internal biological clocks. Birds use these daily and annual rhythms to regulate their orientation in time and space.

Other animals use the movement of water to guide their journeys. Tidal migration involves using tides to move between habitats. This helps organisms find food or mates. Tides can carry organisms horizontally or vertically. This movement can span from a few nanometres to thousands of kilometres. Many species move in and out of the intertidal zone during twelve-hour tidal cycles. Crabs, nematodes, and small fish often use these zones because they are rich in nutrients. During low tide, smaller or younger species might emerge to forage in shallow water. During high tide, larger species may be found in the deeper water.

Each animal group has unique migratory strategies and distances. About 1,800 of the 10,000 bird species migrate long distances every year. The Arctic tern holds the record for the longest bird migration. It flies from the Arctic to the Antarctic and back each year. This journey covers at least 70,000 kilometres. In the insect world, the globe skimmer dragonfly makes the longest ocean crossing. It travels between India and Africa. Some butterflies, like the monarch, use a generational strategy. No single individual completes the entire migration. Instead, successive generations continue the journey through mating and reproduction.

Mammals also participate in massive, coordinated movements. The Serengeti features a "great migration" in Africa. This is an annual circular pattern of movement. It includes approximately 1.7 million wildebeest and hundreds of thousands of other animals. These include gazelles and zebra. Some mammals, like reindeer, have very long terrestrial migrations. In North America, they can travel up to 5,000 kilometres per year. Grey wolves also move great distances over a year. One recorded grey wolf covered a cumulative annual distance of 11,200 kilometres.

Scientists use many tools to track these complex movements. In the past, they used traditional identification tags like bird rings. These were recovered later, but they did not show the actual route. Modern researchers now use electronic tracking devices. Radio-tracking collars can be followed by hand, in vehicles, or in aircraft. GPS tracking allows for very accurate position broadcasts at regular intervals. Another option is the Argos Doppler tag, also called a Platform Transmitter Terminal. This sends data to polar-orbiting satellites. By using the Doppler shift, scientists can estimate an animal's location at a lower cost and weight.