DIETARY RELATIONSHIPS OF MIGRANT AND RESIDENT BIRDS FROM A HUMID FOREST IN CENTRAL PANAMA

This study reports on food selection by Nearctic migrants, especially thrushes (Catharus), wood-warblers (Oporornis, Seiurus, Wilsonia), and tyrant flycatchers (Empidonax) in a humid forest of central Panama. W e attempt to determine how these migrants integrate into the resident bird community based on local food abundance and diet comparison betwe en resident and migrant species. At our study site, migrants are most numerous during migration periods, especially in October when abundanc e of arthropods is low and fruits are plentiful. Migrants feed equally on fruits during both migrations, but overall depend more on an inver tebrate diet. Actually, migrants feed extensively on a few invertebrat e taxa that are mostly of two types: (1) small hard-bodied foliage-dwe lling insects such as beetles and ants, which are of low nutritional v alue; and (2) invertebrates well known for producing distasteful or to xic chemicals, such as nonflying termites, millipedes, and centipedes. In contrast, the resident species feed more extensively on invertebra tes of higher nutritional value (spiders, insect pupae, alate ants) an d on large prey that are plentiful at tropical latitudes (orthopterans , lizards). Dietary relationships among species show that diet of migr ants overlap little with that of resident species, even those with whi ch they share a similar foraging substrate. However, unlike residents, migrants belonging to the same foraging guild have a highly similar d iet. These results suggest that past competitive interactions between migrants and residents were more important than the ones among migrato ry species in determining food selection by migrants. Whether the resu lting high potential for competition among migrants is related to the short stay of most species at our study site or is a characteristic of several migrant populations at tropical latitudes needs further inves tigation.