On The Wing
An estimated five million birds migrate through Costa Rica every year. The magnitude of this phenomenon is barely noticeable because most travel by night, at cruising speed powered by the wind, soaring within the clouds or taking advantage of air currents formed by the mountains. Others skirt the coastline, landing at sundown to catch insects and rest for a few hours before continuing on their voyage.
Over Costa Rica, birds converge from the Arctic Circle, Alaska, the Canadian tundra, the U.S. Great Lakes and the east and west coasts of the United States. Spanning 8 to 11 degrees north of the equator, Costa Rica acts as a funnel point for migrating birds. The country’s mountain ranges and vast lowlands bordering the Atlantic and Pacific oceans create many different habitats and ecosystems.
Above our heads, as we sleep or go about our daily routines, legions of birds are passing overhead. Thanks to their excellent vision, migrating birds use groups of stars to orient themselves in relation to the horizon. Some sense the Earth’s magnetic field. Others sense low frequency sound produced by the breaking surf or the prevailing winds over the topography. Pelagic birds travel solely over the ocean, making brief stops on islands and ocean-side cliffs.
Night flying migrants include forest dwelling birds as well as shorebirds, from warblers, tanagers, orioles and grosbeaks, to sandpipers, plovers and gulls. Other birds, such as hummingbirds, swallows and raptors migrate during the day. Larger diurnal species such as egrets, herons and storks form large groups and fly in a “V” formation using their broad wings to gain lift from rising air currents, reaching great heights without flapping.
Because most birds migrate at such high altitudes, they aren’t visible to the naked eye and scientists use radars can detect them. About half of migratory birds migrate below 1,000 meters above sea level (m.a.s.l), 30 percent do so between 1,000 and 2,000 m.a.s.l. and the remainder at higher altitudes. Ornithologists who study migratory species usually observe these birds when they are feeding in a particular area of the country, but they rarely see them arrive.
Evolutionary theory postulates that the majority of birds evolved in the tropics and subtropics, then these species began migrating to the temperate and northern zones to breed. For birds, migration is not only an escape from winter and the scarcity of food, rather a return to their point of origin.
Migratory birds spend time at several different latitudes which adds to the difficulty of preserving their habitats. We are speaking of creatures with an accelerated metabolism which at some point in their evolution, began making the long journey north or south to take advantage of longer daylight hours, abundance of resources and better conditions for reproduction. Of the 910 bird species in Costa Rica, roughly 220 are migratory, and according to Birdlife International it is one of the countries with greatest bird diversity per square meter in the whole American continent. That is almost nine percent of all the known bird species in the world, and if one visits a “hotspot” it is possible to observe more than one hundred species in a day.
Depending on the species, some birds will spend on average six months here before heading back to their breeding grounds in the United States and Canada where they literally race the clock. They arrive on the breeding grounds in a hurry to obtain a good territory, build nests, lay and incubate eggs, raise the hatchlings and as soon as the young are ready to fly they are ready to head out once again on their trans-continental trip. In some species the young learn their way to the tropics following the rest of the flock. In the case of some of the larger species such as Great-blue Herons and Osprey, younger individuals stay in Costa Rica and do not head back north until they reach sexual maturity.
Starting in July, in the height of northern summer, one may begin observing the very first migratory species moving through Costa Rica. The most obvious ones are the shorebirds, such as plovers and sandpipers which gather at river mouths, plowed fields, salt flats and shrimp farms around the Gulf of Nicoya. In August some of the smaller species begin arriving: vireos, flycatchers and warblers such as the threatened Cerulean Warbler (Setophaga cerulea) and the abundant Red-eyed Vireo (Vireo olivaceus) which are mainly visible on the Caribbean. The first migratory raptors also start making their appearance in August when the Swallow-tailed Kites (Elanoides forficatus) and Plumbeous Kites (Ictinia plumbea) begin gliding through the Caribbean skies.
By the time September and October move in, migration is at its peak and the skies along both coasts and the Central Valley become filled with swallows and the Peregrine Falcons that hunt and eat them without even stopping. The rest of the habitats in the country, from the giant forests of the mountains and rainforest to the city squares and back yards are inundated with warblers, flycatchers, orioles, grosbeaks and other songbirds. Some of these species will stay for months, but the vast majority will continue their voyage to other areas. When October ends and November kicks in the fresh water wetlands along the Pacific become filled with a diversity of ducks, while large groups of Wood Storks cruise the sky along the Caribbean in perfect formation.
Resident birds live in Costa Rica. Some will migrate internally, moving from the higher part of the mountains to the lowlands, or from one ecosystem to another, in synchronization with the fruiting of trees they use for feeding or nesting. Every species has evolved different strategies to meet these needs.
The Long-tailed Manakin varies its diet depending on the availability of resources. During the breeding season when males need a lot of energy for their spectacular courtship dance, they feed primarily on sugar-rich fruits which provide energy. After the breeding season when fruits are not as abundant, this manakin switches its diet to mostly insects. High in protein, this diet allows birds to produce beautiful feathers and be in optimal conditions for the following breeding season. Other birds migrate within Costa Rica depending on the availability of water. The Jabiru needs shallow water to catch its food, but as bodies of water dry up, this stork is forced to fly to other sites where water depth is appropriate for feeding.
Other species with interesting internal movement strategies are the Bellbird and the Quetzal, who are highly dependent on the fruits of wild avocados. The trees only produce fruit during a short period of time, so these birds must move to other areas, either at higher or lower elevation, to find different species of avocado that will be fruiting at different times of the year. Scarlet Macaws also move around within specific areas of Costa Rica, but instead of being a seasonal occurrence, they move daily. Macaws tend to spend the night in the same place every day, but at daybreak they fly towards their feeding grounds which change during the year depending on the species of tree from which they’re eating. As birds move around the country, they carry out an extremely important task for the ecosystem by becoming important seed dispersers. This role is very important because it helps provide forests with a diversity of species.