<P> The population of eastern purple martins (nominate form P. s . subis) is dependent on artificial martin houses of wood or aluminum and fake plastic gourds, supplied by individuals and organizations fond of the bird . This tradition was in place even before the population crash; Native Americans are said to have hollowed out gourds and erected them for this purpose . The situation requires ongoing maintenance, as European starlings and house sparrows compete with martins as cavity - nesters, and will fight with martins over nest sites . Starlings have even been known to kill purple martins, especially nestling young, and house sparrows have been known to evict purple martins from their nests . Thus, unmonitored purple martin houses are often overtaken by more aggressive, non-native species . Purple martin proponents are motivated by the concern that the purple martin would likely vanish from eastern North America were it not for this assistance . </P> <P> Wintering in South America, purple martins migrate to North America in spring to breed . Spring migration is somewhat staggered, with arrivals in southern areas such as Florida and Texas in January, but showing up in the northern United States in April and in Canada as late as May . Males usually arrive at a site before females . </P> <P> Fall migration is also staggered, as birds head south when the breeding season is over . Some birds leave as early as July and others stay as late as October . Martins generally migrate over land, through Mexico and Central America . When not breeding, martins form large flocks and roost together in great numbers . This behavior begins just prior to the southern migration and continues on the wintering grounds . </P> <P> Males arrive in breeding sites before females, and establish their territory . A territory can consist of several potential nest sites . After forming a pair, both the male and female inspect available nest sites . This process is complicated by the fact that artificial nest sites could be houses with many rooms, clustered gourds, or single gourds . The nest is made inside the cavity of such artificial structures and retains a somewhat flat appearance . The nest is a structure of primarily three levels: the first level acts as a foundation and is usually made up of twigs, mud, small pebbles and in at least a few reported cases, small river mollusk shells were used; the second level of the nest is made up of grasses, finer smaller twigs; the third level of construction composing the nest, is a small compression usually lined with fresh green leaves where the eggs are laid . Three to six eggs are laid, and the female is the main incubator, with some help from the male . Purple martins are generally known to raise only a single brood . Fledging, when the young leave the nest, occurs at about one month, after which the parents continue to feed the fledgling young . </P>

When do purple martins leave for south america
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