The Bobolink breeds in old field habitats throughout North America. After breeding they congregate in marshes and other food rich and safe habitats to molt before a long migration. In fact is it among the longest migrations of New World passerines. Bobolinks appear to largely fly over water from the United States to northern South America. From there they proceed southward to winter in north-central Argentina, parts of Paraguay and Brazil. There they move about in monospecific flocks and inhabit moist grassland or larger marshes. Their primary food appears to be seeds during the non-breeding season. Flock sizes tend to be under 400 birds, although historically in northern Argentina flocks up to five thousands were known. Similarly in Argentina, their wintering distribution is now more restricted than it was historically. It does seem that Bobolinks in South America are nomadic, and in Argentina their presence is correlated with wet El Niño years, or other years of river flooding in the east of the country. In dry years it is assumed that Bobolinks winter farther to the north, being less common then in Argentina. Before migration northwards Bobolinks perform a complete molt, being the only passerine in the New World that has two complete molts a year. This is certainly an adaptation to the long distances it travels, particularly over water.