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Fruit-Size, Gape Width, and the Diets of Fruit-Eating Birds


  • Nathaniel T. Wheelwright


In most animals, especially those that must swallow food items whole, prey size is related to predator size. This paper examines gape limitation and the influence of fruit size on diet in fruit—eating birds, drawing on data gathered over a 5—yr period on 70 bird species and 171 plant species in the lower montane forests of Monteverde, Costa Rica. The results suggest that fruit—eating birds face many of the constraints imposed on other gape—limited foragers, but have an unusual minimum—size relationship with their food because of the unique characteristics of fruits. Fruit—eating birds with broad gapes consumed more lauraceous fruit species and a larger mean and maximum size of fruits overall than narrow—gaped birds. However, the size of the smallest fruits eaten was not correlated with gape width; large—gaped species commonly fed on diminutive fruits. Birds effectively selected among individual fruits within a tree on the basis of fruit size, dropping bulky fruits beneath the tree. Effective size selectivity also occurred among trees of different species in the same family and among plant species in various families. The diet of broad—gaped birds was not comprised differentially of large fruit species. For example, Three—wattled Bellbirds favored medium—sized fruits, whereas Long—tailed Manakins took individual fruits in the same proportions as they took fruit species of different mean fruit diameters. Gape limitations and effective size selectivity have obvious consequences for seed dispersal patterns: Plants with large fruits attracted fewer species of birds than plants with small fruits. Moreover, the broad—gaped bird species on which large—fruited plants specialized were those with the most generalized diets.