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-We examined temporal variation in abundance of understory birds and fruiting plants in young (5-7 yr) and old (25-35 yr) successional habitats and in intact, lowland rain forest at Estacion Biologica La Selva, Costa Rica, between January 1985 and May 1986. Fruit abundance varied seasonally in each habitat but was consistently greatest in the youngest site. Frugivores and nectarivores accounted for four (forest and older successional) or five (younger successional) of the five most frequently captured bird species in each habitat. Capture rates of arboreal frugivores and arboreal frugivore-insectivores were greatest in the youngest site and were not different between older habitats. Temporal variation in capture rates of frugivores resulted from habitat shifts by resident individuals and from arrival and departure of altitudinal and latitudinal migrants. Capture rates of frugivores correlated with fruit abundance in forest and the older successional habitat but not in the youngest site. Capture rates of nectarivores and insectivores varied over time and among habitats, but rates showed no correlation with capture rates of frugivores. The lack of positive correlations in seasonal capture rates among trophic groups and the correlation between frugivores and fruit abundance support the view that temporal and spatial variation in bird abundance in tropical bird communities is at least partially in response to variation in resource abundance. Received 3 November 1989, accepted 28 July 1990. BIRD populations vary in abundance over time and space in both temperate (e.g. Kendeigh 1982, Holmes et al. 1986, many others) and tropical habitats (e.g. Fogden 1972; Karr 1976; Leighton and Leighton 1983; Martin and Karr 1986a; Loiselle 1987a, 1988; Loiselle and Blake 1991; but see Greenberg and Gradwohl 1986). Fluctuations in abundance arise from variation in population processes (Orell and Ojanen 1983, Faaborg et al. 1984, DeSante and Geupel 1987) and from movement of individuals among habitats (Karr and Freemark 1983, Wheelwright 1983, Recher and Holmes 1985, Loiselle et al. 1989, Loiselle and Blake 1991). Such movements may represent random redistribution of individuals (Wiens and Rotenberry 1978, Wiens 1984), but such hypotheses often have been advanced in the absence of data on food abundance. Individual movements may instead reflect responses to changes in microclimatic conditions (Karr and Freemark 1983, Petit 1989) or to temporal and spatial variation in food (e.g. Leighton and Leighton 1983, Wheelwright 1983, 1 Present address: Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, Missouri 63121 USA. Recher and Holmes 1985, Loiselle and Blake 1991). Most studies lack concurrent data on fluctuations in both bird population levels and food abundance (but see Wheelwright 1983; Stiles 1985a; Levey 1988a, b; Loiselle and Blake 1991). Consequently, a more direct examination of avian responses to variations in food supply is central to resolving the controversial role of food as an influence in organization of species assemblages (Wiens 1984, Martin 1986). To address this controversy, we used data from concurrent studies on temporal and spatial variation in abundance of tropical frugivores and fruit among three habitats in Costa Rica. We focus mainly on frugivores, but for comparison we examined seasonal rhythms of bird groups (nectarivores, insectivores) that rely on different resources. Tropical frugivores are particularly appropriate for the study of the influence of resource availability on consumer populations because their diets can be readily determined (e.g. Wheelwright et al. 1984, Loiselle and Blake 1990) and their resources (fruit) accurately measured (Blake et al. 1990). Fruit-eating birds also are a major component of many tropical communi114 The Auk 108: 114-130. January 1991 This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 05:47:31 UTC All use subject to http://about.jstor.org/terms January 1991] Birds and Fruits in Lowland Tropics 115 ties (e.g. Stiles 1985b, Loiselle 1988, Karr et al. 1990, Loiselle and Blake 1991), and frugivores disperse seeds from a large proportion of tropical shrubs and trees that produce fruits (e.g. Howe and Smallwood 1982, Stiles 1985b). Thus, it is important both from theoretical and practical standpoints to better understand the interactions between frugivorous birds and fruitproducing plants (Stiles 1985b, c). STUDY AREA AND METHODS