The influence of fruit size and structure on composition of frugivore assemblages in New Guinea

In a mid-elevational rainforest in New Guinea, size and structure of fruits influenced feeding visits by various species of frugivorous birds: large (diam. > 12 mm) structurally unprotected fruits were taken mostly by fruit-pigeons and bowerbirds, structurallyprotected fruits were taken mostly by birds of paradise, and small, structurally-unprotected fruits were taken by nearly all species. Fruit-pigeons as compared with birds of paradise have short, thin bills; small feet; and fewer behaviors for reaching and handling fruits. Birds of paradise use complex food-handling techniques for removing fruits in capsules and other protective structures that are similar to techniques used to capture insects hidden in bark and dried foliage. Absence of large structurally-unprotected fruits in the diets of birds of paradise is not explained entirely as an effect of fruit size. Specialization for frugivory in the entirely frugivorous fruit-pigeons has resulted in exploitation of specific fruit types (large and small structurally-unprotected fruits) rather than all fruit types. Most fruiting plants were visited by a subset of frugivores, as influenced by fruit size and structure. Attracting specific feeding assemblages might be regarded as an adaptation of the plant for enhancing particular patterns of seed dispersal. However, other selective pressures also operate on the evolution of fruit size and structure. BIRD-DISSEMINATED PLANTS HAVE EVOLVED fruits with attributes attractive to birds, such as bright coloration, easy accessibility, convenient size, and nutritional reward (Ridley 1930, Snow 1971, van der Pijl 1972, Howe and Smallwood 1982, Denslow and Moermond 1982, Willson and Thompson 1982, Stiles 1982, Janson 1983). Similarities among fruits of bird-disseminated species have been explained as evolutionary convergence on the narrow range of adaptations competitively effective at attracting birds and dispersing seeds (Snow 1971, McKey 1975, Howe and Estabrook 1977, Stiles 1980). The low diversity of fruit types has itself been invoked as an explanation for the low diversity of frugivores compared with that of insectivores (Snow 1971, Snow and Snow 1971). The question of how fruit characteristics influence usage by frugivorous species has been examined through studies of frugivore autecology (e.g., Snow 1962, Snow 1970, Beehler 1983a, Wheelwright 1983), studies of feeding assemblages at one or a few trees (e.g., Leck 1971; Ricklefs 1977; Howe 1977, 1981; McDiarmid et al. 1977), and theoretical studies. For tropical bird-disseminated species, theoretical studies have emphasized two contrasting situations: large nutritious fruits consumed by large "specialized" frugivores which eat mostly fruit, and small fruits of poor nutritional quality taken by smaller "opportunistic" frugivores which also forage for insects or nectar or both (Snow 1971, McKey 1975, Howe and Estabrook 1977). Studies comparing avian assemblages at fruit-bearing plants are few (Terborgh and Diamond 1970, Ricklefs 1977, Skutch 1980, Howe 1981), and none sampled a broad spectrum of food plants at a single