Birds are the biggest attraction at Palo Verde National Park. In the wet season the floodwaters of the Río Tempisque spread to form extensive marshes that welcome migrant birds from the Americas at the onset of winter. As the December through May dry season (called summer in Costa Rica) progresses the marshes shrink to isolated patches and pools, and the migrants depart for their summer nesting grounds in temperate climates. Many of the resident tropical bird species nest on a small mangrove island in the Río Tempisque. Isla de Pajaros (bird island), as it is known provides a safe place for Cattle Egrets, Roseate Spoonbills, Wood Storks, Green-backed Herons, Anhingas, Great Egrets, and Black-crowned Night-Herons to breed. Other birds you can expect to see in the park include White Ibis, Glossy Ibis, Black-necked Stilt, Black-bellied Whistling-Duck, Snail Kite, American Widgeon, Northern Shoveler, and Blue-winged Teal.