As the females arrive, the males begin singing to entice a mate. The head start allows the males to set up their breeding territories. The male warblers usually arrive in the spring shortly before the females. On a night with a full moon, a careful observer can sometimes spot the flocks as silhouettes against the surface of the moon. Fall migration flocks can number in the thousands and include many different species of birds. Usually, the birds leave their northern haunts in the fall, long before bitter temperatures settle in. 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. The behavior of the cowbird is known as brood parasitism, because it actually becomes a parasite on the mother warbler's family, or brood. As a result, young warblers starve to death. 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. Cowbird eggs are larger and typically hatch a bit earlier, which gives the young cowbird a distinct advantage over its smaller nest mates. Unfortunately for the warbler, the small birds do not recognize the foreign egg and raise it as their own. 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. Unlike a predator that directly attacks and kills warblers or eats their eggs, the cowbird's threat comes from its breeding habits. Warbler numbers are declining in many areas due to the much larger brown-headed cowbird. 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. By learning their songs, birders can walk into the woods and know which species are there without ever seeing a single bird. Each song, however, is characteristic to a particular species. 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. Only the males sing, except in a few species when the females also join the chorus. 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. No sooner do they land on a tree branch than they are off again to new destination. 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. Most New World warblers scrounge for food on the ground or on plants or trees. A few species add berries and seeds to their diets, and some even rely on those food sources to survive snowy winters. Using their slender beaks, the birds are skillful at plucking small insects and spiders from even very tiny cracks and crevices in tree bark, between leaves, and from other hideaways. These smallish birds spend much of their days on the move and looking for insect meals. Some have more unusual habits, and will live either in deserts or swamps. Primarily forest birds, different species of New World warblers, also known as wood warblers, may be found in everything from thick and dark forests to dry and open woods. New World warblers live in North America, South America, Central America, and the West Indies. The juveniles usually look much like the adult females, and males of some of the cooler-climate species also switch to a female-like, dull coloration in the fall and winter. Many are colorful or boldly patterned, but the females' duller colors make them appear quite drab next to their striking male counterparts. Unlike all other songbirds, New World warblers have only nine primary feathers rather than ten. They stand on thin, delicate-looking legs, typically have short and pointy beaks that are either slender or flat, but a few have heavier-appearing bills. New World warblers are relatively small birds ranging from 4 to 7.5 inches (10 to 19 centimeters) long, although most are 4 to 5.5 inches (10 to 14 centimeters). KIRTLAND'S WARBLER ( Dendroica kirtlandii): SPECIES ACCOUNTS NEW WORLD WARBLERS: Parulidae YELLOW-BREASTED CHAT ( Icteria virens): SPECIES ACCOUNTSÄ«LACK-AND-WHITE WARBLER ( Mniotilta varia): SPECIES ACCOUNTS
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