Why Migration Is Important
Risks to Migrants from Predation, Collisions, and Habitat Loss
Migrants may travel thousands of miles between their breeding grounds and wintering grounds. A journey this long takes a great deal of energy and the migrants must make stops to refuel along the way. Habitat loss is reducing the areas that migrants have to stop over and preventing them from refueling as much as they need to. Other risks to migrants include predation, death from poor weather conditions, and collisions with man-made structures such as wind turbines, buildings, communication towers, and power lines. Estimates suggest that half of the birds that start migrating south will not survive to return to their breeding grounds the next spring. With their small size, passerines cannot store much fuel as fat reserves so they must refuel often.
Northern cardinal. Photo by Nate Rathbun
Millions of birds travel through the Great Lakes region each spring and fall. Of the four major flyways (corridors for migrants similar to highways) for migratory birds in North America, two of them (Atlantic and Mississippi flyways) travel through the Great Lakes region. With all of these birds traveling through the area, stopover habitat in the region becomes even more important and reducing the risk of collisions with man-made structures can also have a positive impact on migrant bird and bat populations. Most migrants cannot land on the water of the Great Lakes so they must seek shelter along the shorelines. The shorelines are often where the most stopover habitat is located. Even if the migrants continue inland from over the lake to other habitat, they must travel through the area around the shoreline. Even though an individual may only use the area for a few days or a week while traveling through each season, these areas are critically important to migrants. This website provides more information about stopover habitat around the Great Lakes.
Yellow warbler. Photo by Nate Rathbun
Most bird species migrate at night. This includes the passerines (songbirds) such as warblers, sparrows, robins, and blackbirds. These species migrate at night in part to avoid predators such as hawks that hunt during the day. These migrants can travel hundreds of miles in a night and maintain flight speeds over 30 miles per hour. The main migration season for the spring occurs from March 1st until June 1st with the peak occurring in late April and early may. In the fall, the main migration season for passerines occurs from August 1st until November 1st with the highest numbers occurring in early September. Weather patterns and regional variation may adjust these dates earlier or later, even up until Thanksgiving in the fall. Short cold or warm spells may cause migrants to reverse their direction and travel back through an area they already moved through in search of more food. Birds that get trapped in a downpour or blizzard risk death from starvation or exposure.
Redhead duck flock. Photo by Nate Rathbun
Species like hawks, eagles, vultures, cranes, and ducks migrate during the day. The hawks, vultures, and cranes use thermals to assist their migration and these only are present during the day. Many of the geese and ducks follow along shorelines and waterways as they migrate, often congregating in large flocks. The larger migrant birds are also capable of flying at much faster speeds, with ducks and geese reaching flight speeds up to 50 mph and many migrants can travel hundreds of miles in one flight. The timing of migration for the larger birds also usually starts earlier in the spring and continues later in the fall than the migration of passerines. The larger size of these birds help insulate them from the cold, allows them to store more fuel as fat reserves, and their food sources are often available when the insects and seeds that many passerines eat are unavailable. These reasons allow the larger birds to arrive on their breeding grounds sooner and leave later than many of the small passerines.
Indiana bat. Photo by USFWS
While much is not known about bat migration, what we do know is that bats employ different migration strategies than birds due to their natural history and energy management. One difference is the use of torpor, the ability to suppress their metabolic rate and conserve energy during periods of unfavorable migration conditions (i.e. poor weather). Differences among bats exist also. Long distance migrant bat species such as the eastern red bat (Lasiurus borealis), hoary bat (L. cinereus), and silver-haired bat (Lasionycteris noctivagans) migrate south in the fall and north in the spring just like birds do. Regional migrant bat species like the little brown bat (Myotis lucifugus), northern long-eared bat (M. septentrionalis), and the federally endangered Indiana bat (M. sodalis) travel shorter distances of a few hundred miles to their hibernacula in caves and mines throughout the region. The direction they move may not be south in the fall and north in the winter, but may be opposite of that, or they may move east and west. Additionally, there are local bat species that do not