Fall is a season of change, and for many species one of those changes is migration. Migration is the seasonal movement of a population of animals from one area to another. Many fall migrants are also spring migrants, but migration has numerous patterns and varies by species. It may occur as a response to changes in temperature, food supply or the amount of daylight but can be accomplished for reproductive purposes, too. Mammals, birds, fishes and insects are groups of animals in Illinois that include migrant species.
Migration has been studied extensively in birds. More than one-third of the world's birds migrate. Birds migrate during the day or night. Daytime, or diurnal, migrators are generally large (geese) or are predators (hawks). These birds navigate by sight and have few, if any, predators. Songbirds migrate in darkness (nocturnal). Their daylight hours are spent searching for food and resting for the next leg of their trip. Some birds are short-distance migrants. Birds that live in one type of habitat during the nesting season may not require the same habitat when they are in their winter habitat.
The ability of birds to migrate great distances and return to the same general area year after year is a subject that has fascinated people for centuries. Diurnal migrators fly along broad air routes established by physical features such as major rivers, coastlines, mountains and lakes. The position of the stars and moon and the earth's magnetic field are used by nocturnal migrators.
Illinois bird species show several types of migratory patterns. While we cannot discuss all of the migratory bird species here, the following examples will help you to understand the variety of migratory styles.
Summer bird residents that migrate south for the winter include the mourning dove (Zenaida macroura), blue-winged-teal (Anas discors), ruby-throated hummingbird (Archilochus colubris), killdeer (Charadius vociferus) and turkey vulture (Cathartes aura).
Many mourning doves seen in Illinois in late summer are migrants from farther north. Illinois doves begin migrating in September and spend the winter in Georgia, Florida, Texas, Louisiana and Central America. Some doves overwinter in Illinois, too. Migrant mourning doves begin returning to Illinois in late April.
Blue-winged teal may begin arriving in Illinois in mid- to late-July, although mid-August is more common. Migration continues through November. Some of these birds overwinter in the state. Most travel to the southern United States and Central and South America for winter. They return in spring after the ice melts, usually starting in late February.
Ruby-throated hummingbirds from farther north in the United States and Canada start arriving in Illinois in August. Migration of Illinois' summer hummingbirds occurs from late September through early November. They overwinter from southern Texas to Costa Rica and start returning to Illinois in April.
The killdeer's post-breeding migration may start as early as June, although most occurs from mid-August through fall. Some overwinter in Illinois. Others travel to Mexico or the southern United States. These birds start returning to Illinois in February and March.
Turkey vultures begin migrating in August and continue through October. Some of these birds overwinter in southern Illinois while others migrate to the southern United States. Spring migrants begin returning in February.
Other bird species, such as the snow goose (Chen caerulescens), yellow-rumped warbler (Setophaga coronata) and rough-legged hawk (Buteo lagopus) migrate into the state to spend the winter months.
Many bird species, like the greater yellowlegs, migrate through the state but do not live here.
Some insect species migrate, too. Our state insect, the monarch butterfly (Danaus plexippus), is one of them. Monarchs from east of the Rocky Mountains in the United States and southern Canada travel as much as 2,500 miles from their summer habitat to their overwintering grounds in Mexico. Monarchs are the only butterfly species with a regular, predictable, round-trip migration. The monarchs from the eastern North American population spend the winter in oyamel fir (Abies religiosa) forests at high altitudes in the mountains of central Mexico where they form dense clusters on the trees.
The spring migration from the mountains to eastern North America is completed over the course of two generations of monarchs. They move northward as milkweeds start growing on the migratory route. Milkweed plants are necessary for the survival of monarch larvae. These monarchs live a few (two to six) weeks with the females laying eggs along the way. The return trip to Mexico is composed of monarchs from the last generation of the year. They fly the entire route to Mexico. They are biologically and behaviorally different from the other monarchs that developed in spring and summer. They live for about eight to nine months. Unlike the monarchs that develop in spring and summer and can reproduce about four to six days after emerging, the last generation of adult monarchs each year does not have fully developed reproductive organs, an energy-saving feature. This generation of monarchs feeds voraciously on nectar and builds up fat reserves. They benefit by feeding on nectar, if available, along the migratory route, to save the fat reserves for winter survival. It has been shown that decreasing day length, cooler night temperatures and decreasing milkweed plant quality lead them to start migration in the fall. Ultraviolet light, an internal time-compensated sun compass and possibly other factors, like mountain ranges or bodies of water, may all play a role in navigation.
Why do monarchs migrate? Scientists believe that monarchs originated in the tropics and moved north each spring to find more milkweed plants. They have not evolved ways to survive the harsh winters in eastern North America so they return to more favorable areas to wait until milkweed is available again.
Other butterfly migrants in Illinois include the painted lady (Vanessa cardui) and buckeye (Junonia coenia). These butterflies leave Illinois in the fall for more southern states. They must repopulate the state each year, and their populations often fluctuate depending on how successful the return migration is in the spring.
Many fish species migrate. Their migration may occur daily, annually or over a period of several years. Usually fish migrations are for feeding or reproductive purposes.
Potamodromous species migrate from one freshwater habitat to another, usually for food or reproduction, for instance to lay their eggs where more oxygen is present in the water. For example, the shovelnose sturgeon (Scaphirhynchus platorynchus) lives on the bottom of large rivers and streams. However, it migrates upstream and to the surface of the water for spawning where its eggs have a better chance of surviving.
Catadromous fishes migrate from fresh water into the sea to spawn. American eel (Anguilla rostrata) adults migrate from fresh water to the Atlantic Ocean (near Bermuda and the West Indies) where they reproduce and die. Their eggs hatch to a larval form that takes one year to reach the coast of North America. The larva transforms two more times before becoming an adult. Males remain in streams and brackish areas along the coast. Females move upstream and may stay there for 15 years before returning to the sea.
Illinois' migrating mammals are bats. Not all bat species in the state migrate, however. Some bats have adapted to the cold winters by hibernating in caves and other locations. Other bat species leave the state for locations further south. These actions are related to the lack of their food source, flying insects, in the winter.
The gray bat (Myotis grisescens) often migrates in large groups. The females leave first, starting in August. The males and juveniles follow later. The species winters in Alabama, Missouri and Tennessee. The red bat (Lasiurus borealis) migrates throughout autumn, moving at least as far south as the Ohio River. The hoary bat (Lasiurus cinereus) migrates from August through October to its winter habitat in Mexico, Central America and the southeastern United States.
Many hazards are present for all migrants. Nocturnal and low-flying migrants risk flying into humanmade objects like tall buildings, power lines and towers, windows and aircraft. Other constructed objects, such as dams, can block migratory routes. Songbirds may encounter predators (hawks) migrating at the same time. Habitat destruction and pollution are also migrational hazards. Storms can kill migrants. Hunting seasons are established for some species (ducks, geese, mourning doves) during the fall migration. Even though birds are harvested, hunting is only allowed within limits that a population can withstand.