Ecological and Nutritional Effects of Food Scarcity on a Tropical Frugivorous Bird and its Fruit Source

Abstract

The Long—tailed Manakin (Chiroxiphia linearis) was studied from 1971 to 1974 in Guanacaste Province, Costa Rico. From late April until early June 1973 the normal food of this species, i.e., ripe fruits of Ardisia revoluta Humboldt, Bonpland, and Kunth (Myrsinaceae) and Stemmadenia donnell—smithii (Rose) Woodson (Apocynaceae), was unavailable. The birds switched to a diet of green Ardisia fruits upon which they normally do not feed. The primary components of the normal and alternate diets (fruits and varying ripeness) were analyzed to determine differences in nutrient and energy contents in an attempt to quantify the impact of the food shortage. Birds needed to eat 6.6x as many green fruits of A. revoluta to obtain the same metabolizable energy contained in a ripe fruit and 4.5, 8.5, and 18.6x as many to obtain comparable amounts of H2), protein, and trichloroacetic acid—soluble carbohydrates, respectively. The birds had to eat 1.4, 8.3, 4.8, and 5.8 x as many green A. revoluta fruits to obtain the same amounts of water, lipids, protein, and metabolizable energy contained in one feeding unit of ripe S. donnell—smithii. Feeding on green fruit probably is the best way for Long—tailed Manakin to cope with a period of food scarcity for several reasons. The fact that the birds maintained their normal weights during this period supports this idea. Experiments showed that seeds from green and ripe fruits germinated with almost equal frequency. Maturation appears to influence primarily the speed of germination. It also enhances dispersal by making the fruits more attractive to dispersal agents.