Life Cycles of Migratory Birds
By Pete Marra. Pete Marra is a Research Scientist with the Smithsonian Migratory Bird Center. He studies the birds throughout their annual cycle.
Cycles are everywhere in nature. They can occur over the length of a day, a season, a year, or longer. Physical cycles as important as the rotation of the tilted earth around the sun determine patterns of light and dark, as well as changes in the seasons. The rotation of the earth around the sun influences human sleep and activity patterns and drives the behaviors of trillions of other living organisms. In fact, the timing of much of what organisms do on earth, such as breeding and migrating, happens because of the power of physical cycles. The life cycles of migratory birds are also driven by the earth's changing seasons and daylight hours, the latter being a primary cue that impels them to begin their migrations.
Each year, billions of migratory birds move across the Western Hemisphere to take advantage of flushes of food abundance to the north in spring and summer and to the south in fall and winter. Extra daylight hours during the nesting season offer more time to capture prey and feed nestlings. This repeated seasonal movement is defined as migration and makes up a small, but critical, part of the annual cycle of a migratory bird.
The American Redstart (Setophaga ruticella) is a model species for migratory birds that travel between most of North America (the Nearctic) and the Neotropics, including Central and South America, the Caribbean islands, and southern Florida. From late April until late May, males and females journey northward from the Neotropics to their breeding areas.
Older males arrive first, establish and defend their territories, and begin singing in earnest. They spread their tails, flashing the bright orange patches, most likely for the benefit of female redstarts. Once males and females establish pair bonds, the birds become architects and construction workers to build their nests. Nest building varies by species. Sometimes both the male and female are involved, but in the case of the redstart, it is the female that gathers tree bark, lichens, and animal hair and designs a tightly woven nest in the crotch of a tree. When the nest is ready, the pair mates and the female lays eggs. In roughly a dozen days, the nest is filled with ravenous nestlings constantly demanding that their parents search the surrounding habitat for insects. If all goes well and the family escapes severe weather, human disturbance, being eaten by a predator, and other factors that threaten their survival, the nestlings will leave the nest. Even then, the parents will continue to feed them for a few weeks.
As the nesting season draws to a close in June and July, the next phase of the cycle begins - migration to the nonbreeding grounds. This time, the order of departure is reversed, and the young redstarts are often the first to leave. The adults must first replace their feathers in a process called "molting". Once donned in fresh feathers that will help them make their long-distance flight, they begin their migrations. Exactly where their journeys take them is unknown, but eventually they make it to Mexico, the Caribbean, Central America, and as far south as Colombia. Often, the adults return to the same territories they occupied the previous year.
American Redstarts spend up to 7 or 8 months on their non-breeding grounds. Though males and females form tight bonds while nesting, they are independent of one another the rest of the year. No nests are constructed and no eggs are laid. They may even be in very different habitats and geographic locations. As April approaches, birds begin to prepare for another northward journey, fattening up to help them survive the long flight. As American Redstarts return to their same breeding sites, the annual cycle is completed.
Life cycles such as this are similar for hundreds and thousands of species of migratory birds worldwide. Although we understand the generalities of annual cycles, our interpretations of bird biology and bird conservation have not been explored in the context of the complete life cycles. What happens to birds in one period of their cycles influences these same individuals in subsequent periods. If an American Redstart starts its journey to breeding grounds with insufficient food and fat stores, it may not complete the migratory flight. Likewise, impacts on the breeding grounds, such as habitat loss, may affect a bird's ability gain a feeding territory on the wintering grounds.
The beauty of life cycles is not only the fascinating and intricate biology that proceeds like clockwork year in and year out. It is also that the migratory cycles of birds provide a connection between people, species and their cultures from far away lands.