When a young American redstart makes its first flight south from its breeding grounds in eastern North America, its choice of wintering grounds in Central and South America is a decision that will affect the rest of its life.
One popular area to spend the winter is the island of Jamaica in the Caribbean Sea. Young redstarts have the choice of wintering in either coastal mangrove forests or interior scrub forests.
Unknown to them, the impending dry season will greatly reduce the amount of food and water available in the scrub habitat while the mangrove habitat will remain lush and abundant.
A Good Start for Redstarts
By the time spring migration commences in early April, the young redstarts that wintered in mangrove habitats are fit and loaded with fat to help them migrate to their breeding grounds.
They head north from Jamaica and choose the first suitable habitat they find, one where the leaves are just emerging and insect food is abundant.
In contrast, redstarts that wintered in scrub habitats are not yet ready to migrate and need more time to build up their fat reserves. By the time they do arrive on the southern breeding grounds they find them occupied by other redstarts and must fly further north to nest.
Adult redstarts return to the same breeding and wintering sites year after year, it is only in that first fall that the young redstarts choose their wintering area, which influences their choice of where they breed.
You might wonder how tiny redstarts, which are only about 5 inches long and weigh a paltry 8 grams (about the weight of a quarter and a dime), can be followed throughout their lives?
Although they are too small to carry any sort of transmitter, they can be identified individually by putting a unique combination of colored plastic bands on their legs. This allows researchers to monitor them throughout the winter.
Their breeding location can be deduced by plucking a tail feather on their wintering grounds. That feather grew on the breeding grounds and contains a unique ratio of Hydrogen isotopes that varies by latitude, so researchers can tell how far north or south they spent the summer.
Redstarts in the Future
The Earth is expected to warm in the coming decades and the local prediction for the Caribbean area is that it will get drier. This may tend to favor the redstarts that winter in mangrove habitats as they may be more resistant to droughts.
However, the trees will likely leaf out earlier in the nesting areas so the best areas to nest may be further to the north. Time will tell how the redstarts adapt to a changing world.
This article summarizes the information in this scientific paper:
Studds, C. E., Kyser, T. K. & Marra, P. P. 2008. Natal dispersal drive by environmental conditions interacting across the annual cycle of a migratory songbird. Proceedings of the National Academy of Sciences 105: 2929-2933.
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