I have noticed an increase in the frequency and variety of bird songs in the mornings. I have been getting reports that people in other parts of Wisconsin have begun to see and hear some of our early spring migratory birds. Meteorological spring is here, and despite the extended presence of winter, our migratory part-year residents have begun to return.
Migration is a fascinating phenomenon. It has been studied by biologists for hundreds of years, and is still not fully understood. The basic concept of migration is to move between two primary habitats, a summer habitat in a northerly location and a winter habitat in a southerly location. There are some birds that migrate more east and west, but the typical migrant heads south prior to arrival of snow and cold, returning as early as possible the following spring.
You might be wondering why a bird would travel thousands of miles each year to lay eggs in Wisconsin, but spend the winter in Central America. We have more than 230 bird species that breed in Wisconsin, and more than half of them migrate out of the state every winter. It might seem easier to just stay in Costa Rica somewhere year-round, build a nest, lay your eggs and raise your young. Migration is costly, resulting in a large number of bird deaths each fall and spring. Still, it must result in better survival than staying put year round, or they would not have adapted this behavior across countless generations.
For some birds, like many waterfowl species, migration might only mean heading far enough south to find food and open water. Migration is a simple matter of surviving the winter. It seems to me that songbirds traversing the Gulf of Mexico in non-stop flight twice a year have some other more compelling reason than searching out food and open water. Maybe these birds do not migrate south for the winter; rather, they migrate north for the summer. In wintertime, birds are far less territorial, and can pack into small spaces that have a high potential for food production, like a rain forest. For many species, the males need to establish some kind of territory, defend it from invaders, and advertise from a song post. They need to build a nest relatively safe from raiders and predators. Perhaps during that time of year, being in the jungle is less beneficial; heading north to a temperate climate with room to spread out across a great variety of habitats and deal with relatively lower numbers of predatory wildlife species might be more advantageous.
While some birds undoubtedly migrate when they get cues from changes in weather or available food, others are triggered by the length of daylight. There is a distinct advantage to arriving on the nesting grounds early. Birds can find the best locations to build nests, defend territory and find plentiful food for their hatchlings. In a year like this one, that advantage could be lost, with significant strain on the adults who have consumed much of their fat reserves in their travels and now must deal with limited food availability. I think of Canada geese as an example. Every year I see pairs sitting on a frozen marsh, patiently awaiting the retreat of the ice.
Migrating birds find their way with the help of a kind of built-in magnetic compass. Depending on the species, they might also use stars to guide them at night, or familiar terrain (like mountain ranges or rivers) to guide them during the day. They will travel long distances, sometimes dying in midair. A recent effort to improve “stopover” habitat is designed to help migratory birds rest and feed in crucial locations along their migration routes. There are also habitat improvement efforts on their wintering grounds and breeding grounds. Migratory birds are important to our way of life, and continued conservation efforts will help ensure they stay that way.
Jeremy Holtz is a wildlife biologist with the Wisconsin DNR and writes a weekly column in the Star Journal. To contact him, call 715-365-8999.