Perhaps the most characteristic behavior, and the most frustrating for birders who are trying to spy one of the birds through their binoculars, is the nearly constant motion of New World warblers. No sooner do they land on a tree branch than they are off again to new destination. For this reason, plus the sometimes-dense woods that hide them from view, most birders in the field recognize different species of warblers not by sight but by their songs. Only the males sing, except in a few species when the females also join the chorus. Many species have lovely, bright songs, but others are merely loud, and some have quite quiet, scratchy voices that sound more like insects than birds. Each song, however, is characteristic to a particular species. By learning their songs, birders can walk into the woods and know which species are there without ever seeing a single bird. As in other birds, both males and females also communicate through various quick cheeps and chips, some of which may also be very distinctive to a particular species.
Most of the species that summer in North America migrate far south for the winter, sometimes flying 3,000 miles (4,800 kilometers) or more to a warm, sunny location. Usually, the birds leave their northern haunts in the fall, long before bitter temperatures settle in. Fall migration flocks can number in the thousands and include many different species of birds. The flocks travel from sunset to sunup. On a night with a full moon, a careful observer can sometimes spot the flocks as silhouettes against the surface of the moon. The birds return the following spring.
Warbler numbers are declining in many areas due to the much larger brown-headed cowbird. Unlike a predator that directly attacks and kills warblers or eats their eggs, the cowbird's threat comes from its breeding habits. Instead of laying eggs in its own nest and raising its own young, the cowbird lays its eggs in other birds' nests and leaves the parenting to the adoptive parents. Unfortunately for the warbler, the small birds do not recognize the foreign egg and raise it as their own. Cowbird eggs are larger and typically hatch a bit earlier, which gives the young cowbird a distinct advantage over its smaller nest mates. Sometimes, the cowbird pushes the others out of the nest and to their death, but even the baby warblers that remain often miss out on feedings from their mother as the larger cowbird can push its beak to the front for meals. As a result, young warblers starve to death. The behavior of the cowbird is known as brood parasitism, because it actually becomes a parasite on the mother warbler's family, or brood.
The male warblers usually arrive in the spring shortly before the females. The head start allows the males to set up their breeding territories. As the females arrive, the males begin singing to entice a mate. Once a pair forms, the warblers go about making a nest and preparing for egg laying. Some species make their typically cup-shaped nests on the ground, others in shrubs, and some high in the trees. A typical clutch is three or four eggs, which are usually white with irregular spots. The female sits on the eggs while the male dashes about finding food and bringing it back to her. When the eggs hatch nearly two weeks later, the mother helps the father find and deliver food to the babies. The young grow quickly, and are nearly adult weight by the time they are ten days old. At that point, they test their wings and leave the nest, but they don't go far. The parents continue to feed them, but since the young are no longer together in the nest, the mother generally takes care half the offspring, and the male feeds the other half. After a few weeks, the parents stop their care and the young birds are on their own.
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