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Both physical and biological processes shape species assemblages (communities). For birds, vegetation structure has long been assumed to be the dominant factor in habitat selection, especially along successional gradients. While vegetation may be important as a proximate factor, detailed knowledge of ultimate factors governing habitat selection is required. Gradients of microclimate, especially temperature and moisture, may be such an ultimate factor through direct physiological pressures on birds or indirectly through distribution and availability of food resources. We documented the existence of gradients of both vegetation structure and microclimate in the undergrowth of seasonally humid forest in central Panama. To assess the relative importance of these gradients in shaping local avian distribution, birds were netted in undergrowth (up to 3 m above ground) during 2—wk periods in dry (March) and wet (July) season for 4 yr (1979—1982). A total of 3037 captures of 95 species was recorded during that period. Although patterns of species richness and capture rate are relatively simple, species composition and other assemblage attributes are complex and difficult to interpret without careful evaluation of the dynamics of individual species. The variety of activity patterns exhibited by birds on moisture and vegetation gradients clearly demonstrates the complexity of dynamics that affect assemblage attributes. Overall, birds are more active at dry sites and at sites with intermediate shrub density. However, activity levels change with time, suggesting that avian activity reflects a synamic process of habitat selection. Though no food resource data are available at present, the general pattern of habitat use on diurnal time scales for several guilds suggests that birds track microclimatic optima for physiological reasons. Habitat selection processes are both extremely complex and variable in time and space for the avifauna of tropical forest undergrowth. Indeed, the scale of study in space and time is important in determining the conclusion of a study. The recent perception of a dichotomy between equilibrium (deterministic) and nonequilibrium (stochastic) assemblages is undermined by these results. Habitat selection in birds produces dynamic (nonequilibrium) assemblages in space and time. However, these are not stochastic assemblages. Each species seeks habitat optima in the context of current environmental conditions on diurnal, seasonal, and between—year time scales. Thus, instead of nonequilibrium and stochastic assemblages, the avifaunas of tropical forest undergrowth are nonequilibrium but relatively predictable from knowledge of current environmental conditions.

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