Abstract
A community of 22 species of hummingbirds and over 45 species of omithophilous plants was studied over a seven-year period in the humid Caribbean lowlands of Costa Rica. There are two approximately equal peaks of blooming of hummingbird flowers in the year, during the dry season (March) and early in the rainy season (July-August); with a severe scarcity of flowers between November and January, at the end of the rainy season. The epiphytes of the forest canopy flower mainly in the dry season, understory plants mostly in the wet season, and at light gaps, edges, and in old second growth flowers are present year-round. Habitat shifts of hummingbirds are closely associated with flowering of preferred food plants, as is the differential habitat use by the sexes of some species. Migration of some species into and out of La Selva also reflects the cycle of flower abundance.
In general, the breeding seasons of the hummingbirds coincide with the first peak, and molting seasons With the second peak of flowering. Deviations from this pattern are few, and generally reflect the dependence of a given hummingbird upon a given flower species, or physiological mechanisms permitting molt-breeding overlap. The maximum body weight and fat reserves also occur during the second flowering peak, and both decline rapidly during the following season of flower scarcity. The low point in body weight occurs during this lean season, while the annual low of fat reserves comes in the following breeding season; another body component, probably muscle protein, must therefore be used as well as fat by the birds to survive the season of flower scarcity, when most of the annual mortality probably occurs.
Year-to-year variations in rainfall are considerable, but the hummingbirds are normally buffered from these by a series of compensatory phenological reactions among the plants that tend to maintain a regular, staggered sequence of flowering peaks. Nevertheless, a severe moisture stress, such as the drought of early 1973, may disrupt these patterns and have drastic effects on hummingbird populations.
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