Differences in the sexual cycle of some birds

In a flooded area in Santiago del Estero, northwestern Argentina, several species nest in the fall, while in other parts of the country they breed in spring and summer. This is supposed to depend on the different food conditions caused by the river Juramento (Salado river), which floods the area in January and February. As the water is retiring, fish and batrachians are extremely abundant in May and June, when the birds have nestlings. Some data as to food and nesting habits of herons are added. It is considered interesting that several species appear both as migratory and breeding, and in July when the area is drying, the wintering populations return to their breeding places while the populations which nested in winter migrate to their "summering" areas. It seems furthermore that both populations migrate to the same area, the Argentine littoral and also as far as southeastern Brazil.