Frugivory and North American Migrants in a Neotropical Second-Growth Woodland

Abstract

Bird species visitation to a single fruit source fluctuates over time due to changes in fruit density, presence of behaviorally dominant birds, and availability of alternative fruit sources. As a result, different bird species are major consumers of a fruit source at various periods of the fruiting season. Large dominant birds exploit a small-fruited species mostly at its peak availability. Large dominant birds displaced small birds, so small birds used the fruit less when dominants were abundant. Frugivorous migrants increase in abundance in late spring, due to migration, and they outnumber non-migrants in their representation at fruit sources.

Use of a fruit source by migrant and non-migrant frugivores is influenced by the net energy gain per unit time (profitability). I develop mathematical and conceptual models that describe fruit profitability to birds based on rewards provided by the pericarp (mass and nutrition yields) relative to costs (metabolic requirements, handling time, search time, behavioral interference) associated with finding and eating a fruit. The composition of birds at a fruit source is influenced by factors that determine profitability to individual bird species. Composition can vary over time as profitability changes and birds switch to alternative fruit sources.

The strategy employed by a plant to attract bird dispersers must act within this framework of factors that determine profitability and, hence, attraction of birds. In addition, plants are constrained by resource limitation and life-history considerations. Four factors that are important in determining profitability of a fruit to birds (seed size, absolute yield, nutritional yield, and daily fruit crop size) are directly controlled by the plant. These four factors are regulated by the plants relative to environmental influences, resource limitation, life-history considerations, and attraction of birds.

Frugivores vary in abundance within the second-growth community throughout the dry season. Recapture rates indicate that much of the fluctuation in abundance of frugivores is due to the movements of transients. Non-migrant frugivore abundance is negatively associated with migrant frugivore abundance; when migrants flood the community during spring migration, non-migrant frugivores decrease in abundance. Fruit abundance is high and many of the fruits of second-growth habitats are small, possibly causing selection of second-growth habitats by migrants.