<P> Purple martins suffered a severe population crash in the 20th century widely linked to the release and spread of European starlings in North America . Starlings and house sparrows compete with martins for nest cavities . Where purple martins once gathered by the thousands, by the 1980s they had all but disappeared . </P> <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 continual 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 . The wide - spread adoption of a semi-eliptical entrance in purple martin houses has been effective in reducing starling predation . This entrance style was first patented in the 1990s by Frank Prekep, an avid Purple Martin enthusiast from Milton, Delaware . 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>

Where do purple martins go for the winter