INSECTIVOROUS MIGRATORY BIRDS IN TROPICAL ECOSYSTEMS - THE BREEDING CURRENCY HYPOTHESIS

In many tropical habitats, the number of birds can more than double,vith the arrival of migratory birds in the autumn. This influx often occurs at a time when the total biomass of potential food resources is at an annual low. Previous food-related hypotheses have usually focused on migratory species that consume fruit or other plant products, and are more difficult to extend to insectivorous birds, which make up the bulk of tropical migrants. I hypothesize that inter-habitat and inter-seasonal variation in the size distribution of arthropods may underlie the use of habitats by migratory birds. Most habitats have a few taxa of large arthropods that serve as ''breeding'' currency. The abundance of these arthropods may not be coupled with the abundance of small arthropods during the non-breeding season. Where small arthropods are abundant during the non-breeding season and large arthropods rare during the breeding season, the surplus non-breeding food is available to the large number of small insectivorous birds that comprise most of the migratory species.