Palo Verde National Park, (Parque Nacional Palo Verde, $6 admission) 28 kilometers south of Bagaces, protects 13,058 hectares of floodplain, marshes, and seasonal pools in the heart of the driest region of Costa Rica—the Tempisque basin, at the mouth of the Río Tempisque in the Gulf of Nicoya.

The park, which derives its name from the palo verde (green tree) shrub that retains a bright green coloration year-round, is contiguous to the north with the remote 7,354-hectare Dr. Rafael Lucas Rodríguez Caballero Wildlife Refuge and, beyond that, the Lomas Barbudal Biological Reserve.

For half the year, from November to March, no rain relieves the heat of the Tempisque basin, leaving plants and trees parched and withered. Fires started by local farmers eviscerate the tinder-dry forests, opening holes quickly filled by ecological opportunists such as African jaraguá, an exotic grass brought to Costa Rica in the late 19th century to grow pastures.

Unlike Costa Rica’s moist forests, the tropical dry forests undergo a dramatic seasonal transformation. In the midst of drought, vibrant yellow and pink flowers synchronically burst onto bare branches. Myriad bees moths, bats, and wasps pollinate the flowers. And moist fruits ripen throughout the dry season, feeding monkeys, squirrels, peccaries, and other mammalian frugivores.

In all, there are 15 different habitats and a corresponding diversity of fauna. Plump crocodiles wallow on the muddy riverbanks, salivating, no doubt, at the sight of coatis, white-tailed deer, and other mammals come down to the water to drink. Birds abound, including such endangered species as the great curassow, yellow-naped parrot, and king vulture.

Palo Verde National Park is best known as a bird-watchers’ paradise. More than 300 bird species have been recorded, not least great curassows and the only permanent colony of scarlet macaws in the dry tropics. At least a quarter of a million wading birds and waterfowl flock here in fall and winter, when much of the arid alluvial plain swells into a lake. Isla de Pájaros, in the middle of the Río Tempisque, is replete with white ibis, roseate spoonbills, anhingas, wood storks, jabiru storks, and the nation’s largest colony of black-crowned night herons.

Three well-maintained trails lead to lookout points over the lagoons; to limestone caves; and to waterholes such as Laguna Bocana, which are gathering places for a diversity of birds and animals. Limestone cliffs rise behind the old Hacienda Palo Verde, now the park headquarters (tel./fax 506/200-0125), eight kilometers south of the park entrance.

Dry season is by far the best time to visit, although the Tempisque basin can get dizzyingly hot. Access is easier, and deciduous trees lose their leaves, making bird-watching easier. Wildlife gathers by the waterholes. And there are far fewer mosquitoes and bugs. When the rains come, mosquitoes burst into action—bring bug spray.