Spatial and Temporal Variation in Costa Rican Fruit and Fruit‐Eating Bird Abundance

Understory fruit and fruit—eating birds were censused monthly for a year in gaps, intact forest, and second—growth sites of a lowland Costa Rican rain forest. Both fruits and birds displayed significant seasonal variation. Peak fruit abundance corresponded with peak fruit—eating bird abundance. Fruits were most abundant in the mid—to—late rainy season (August—January). Crop sizes were larger on second—growth plants than on either gap or intact forest plants. Also, fruit was much more common in second growth than in gaps and more common in gaps than in intact forest. Fruit—eating birds followed the same general patterns of spatial and temporal variation. They were significantly most abundant in second growth, significantly least abundant in intact forest, and most common from October to January. A large increase in the frugivore population in October was due to an influx of temperate and altitudinal migrants. In addition, populations of some resident frugivore species increased concurrently, suggesting altitudinal migration in some of these species as well. The two most common understory frugivores molted during the period of fruit high and bred during the period of fruit low. I suggest that insect abundance may be more important than fruit abundance in determining breeding cycles of these birds. Given the extent of spatial and temporal variation in fruit and frugivore abundance, and the apparent tracking of fruit resources by birds, large areas of diverse habitats are probably necessary for the long—term maintenance of frugivore populations.