The marsh itself is the main event. In the dry season it holds thousands of black-bellied whistling ducks, blue-winged teal, jabiru, roseate spoonbills, wood storks, glossy ibis and herons of every Central American species, all crowded onto shrinking water.

The boat trips on the Tempisque river are the standard way to see it, running from the park or from Puerto Humo. They pass American crocodiles on the banks — some very large — and the riverside trees hold howler monkeys and iguanas.

Isla de Pájaros, an island in the river, holds one of the largest waterbird nesting colonies in Central America, best viewed from a boat at a respectful distance.

The dry forest trails and the limestone outcrops behind the marsh hold white-faced capuchins, coatis, and the dry-forest bird specialities — white-throated magpie-jay, turquoise-browed motmot and the black-headed trogon — that are hard to find in the wetter parts of the country.

The Palo Verde lookout above the station gives the overview of the whole floodplain and is worth the short climb at dawn.

January to March is the peak concentration and the reason to come. The wet season from May to November greens the forest but scatters the birds.

Practicalities

Palo Verde is around an hour from Liberia airport, which makes it an easy first or last stop on a Costa Rica trip. The OTS biological station inside the park offers basic accommodation and researcher-led guiding, which is by far the best way to experience it.

Heat and mosquitoes are both serious in the marsh; go early, and take the boat trip in the morning rather than midday.