219 Ecology, 82(1), 2001, pp. 219–231  2001 by the Ecological Society of America FOLIAGE STRUCTURE INFLUENCES FORAGING OF INSECTIVOROUS FOREST BIRDS: AN EXPERIMENTAL STUDY CHRISTOPHER J. WHELAN 1 Illinois Natural History Survey, Midewin National Tallgrass Prairie, 30071 S. Rt. 53, Wilmington, Illinois 60481 USA Abstract. Preferences for foraging in particular tree species have been well documented in a variety of bird species, but underlying reasons remain little investigated. Understanding the causal bases for such patterns of habitat use can help to elucidate mechanisms of habitat selection and, therefore, community organization and structure. I experimentally tested the hypothesis that fine-scale foliage structure of two deciduous tree species influences the foraging behavior of three small, insectivorous bird species. On sugar maple ( Acer sac- charum), with its orbicular leaves elevated above the branch, Black-throated Blue Warblers (Dendroica caerulescens) and American Redstarts (Setophaga ruticilla) captured prey pre- dominantly from lower leaf surfaces, whereas Black-throated Green Warblers (Dendroica virens) captured prey predominantly from upper leaf surfaces. In contrast, on yellow birch (Betula alleghaniensis), with its oblong-ovate leaves held within the plane of the supporting branch, all three bird species captured prey disproportionately from upper leaf surfaces. Overall, aerial maneuvers were used more frequently to capture prey from upper than from lower leaf surfaces on sugar maple, but the opposite occurred on yellow birch, where non- aerial maneuvers were used more frequently to capture prey from upper than from lower leaf surfaces. Those results indicate that the leaf surface from which prey are more easily captured (with less energetically costly non-aerial maneuvers) differs between those tree species. Experimental manipulation of leaf dispersion and distance to prey demonstrated that leaf dispersion is the chief determinant of prey capture location (upper vs. lower leaf surfaces), and that distance to prey is the chief determinant of prey capture maneuver (aerial vs. non-aerial). When foraging on artificial branches in which vertical distance between branches was systematically increased experimentally, Black-throated Green Warblers cap- tured a significantly smaller proportion of prey from lower surfaces of leaves on the upper branch than did Black-throated Blue Warblers. Taken together, these results indicate that different tree species, as well as different locations within tree species, present insectivorous birds with distinct foraging environments and, therefore, constitute distinct foraging mi- crohabitats. Furthermore, even closely related bird species that are generally similar mor- phologically respond in behaviorally unique ways to differences in foliage structure. For those reasons, forest management practices that enhance tree species diversity could con- comitantly enhance foraging opportunities (niche diversity) for forest insectivores and may thus help to promote high bird species diversity and maintain abundant populations. Key words: aviary experiment; bird community structure; deciduous foliage; foliage structure; foraging behavior; insectivorous birds; New Hampshire; northern hardwoods; Parulidae; vegetation structure; warblers. INTRODUCTION Ecologists have long recognized that vegetation physiognomy influences habitat selection (MacArthur and MacArthur 1961, Hilde ´n 1965, Willson 1974, Gor- man and Karr 1978, Klopfer and Ganzhorn 1985, Pribil and Picman 1997), foraging ecology and resource use (Morse 1976, Moermond 1979a, b, 1986, Holmes and Robinson 1981, Huey and Pianka 1981, Paszkowski 1984, Diehl 1993, Parrish 1995a, b, Gunnarsson 1996), and refuge from predation (Crowder and Cooper 1979, 1982, Bro ¨nmark 1985, Kotler and Brown 1988, Hacker and Madin 1991) in a wide variety of taxa. Although most studies have focused on vegetation structure at Manuscript received 15 January 1999; revised 3 January 2000; accepted 3 January 2000. 1 E-mail: virens@attglobal.net the level of the habitat (e.g., MacArthur and MacArthur 1961, MacArthur et al. 1966, Willson 1974, Roth 1976, Gorman and Karr 1978, Moermond 1979a, b, Crowder and Cooper 1982, Brown 1989a, b, Brown et al. 1994), microvegetation structure has also been suggested (Kendeigh 1945, Holmes and Robinson 1981, Bro ¨n- mark 1985), or demonstrated, to affect foraging be- havior (Partridge 1974 1976a, b, Moermond 1979a, b, Moermond and Denslow 1983), microhabitat (foraging site) selection (Klopfer 1963, Diehl 1993, Parrish 1995a, b), or predator avoidance (Hacker and Madin 1991). Understanding how vegetation structure at both the macro- and microscales influences habitat selection and resource exploitation can help to elucidate mech- anistic bases of community organization and structure (Moermond 1986, 1990). Insectivorous birds foraging within the complex