Feeding Methods and Efficiencies of Selected Frugivorous Birds
I report on handling methods and efficiencies of 26 species of Paraguayan birds feeding on fruits of Allophyllus edulis (Sapindaceae). A bird may swallow fruits whole (Type I: pluck and swallow feeders), hold a fruit and cut the pulp from the seed with the edge of the bill, swallowing the pulp but not the seed (Type II: cut or mash feeders), or take bites of pulp from a fruit that hangs from the tree or that is held and manipulated against a branch (Type III: push and bite feeders). In terms of
... absolute amount of pulp obtained from a fruit, and amount obtained per unit time, Type I species are far more efficient than Type II and III species. Bill morphology influences feeding methods but is not the only important factor. Diet breadth does not appear to be significant. Consideration of feeding efficiency relative to the needs of the birds indicates that these species need to spend relatively little time feeding to meet their estimated energetic needs, and that handling time has a relatively trivial effect on the time/energy budgets of the bird species observed. orous, eat large quantities of both fruit and insects, or are omnivorous. In this paper, I describe the ways in which these birds feed on Allophyllus fruit, and their feeding efficiency in terms of (1) the rate at which pulp is ingested (exocarp plus mesocarp acquired per unit time), and (2) the absolute amount of pulp ingested, per fruit, regardless of time expended. I also provide data on diets. By comparing the species, it may be possible to identify ways in which body size, bill dimensions, and feeding behavior influence efficiency. The significance of feeding efficiency can then be evaluated through a consideration of the birds' energy requirements and the energy supplied by the fruits. METHODS The study was conducted at El Tirol (ca. 55"47' W, 27"l l' s), Dpto. Itapoa, Paraguay during the A. edulis fruiting seasons of 1980 to 1983, with limited observations also made in 1976, 1978, and 1979. This species, locally known as Cacti, is found in low densities in relatively undisturbed forest, at high densities in second growth forest, and as an occasional shade tree in areas cleared for cultivation (Foster, unpubl. data). Trees may reach heights of 18 m, but rarely exceed 12 m. Allophyllus fruits are drupes borne on racemes throughout the tree. In any given year, the fruit crop may range from 500 to > 30,000 on different trees (Foster, unpubl. data). Fruiting is highly [5661 -FIGURE I. A branch of AIIophyllus edulis with racemes of ripe and ripening fruits. synchronized within and between trees. Ripe fruits usually are available for 4 to 5 weeks between mid-September and the end of October, with a fruiting peak lasting about 3 weeks. Ripe fruits (Fig. 1) are red and range from ca. 6.7 to 9.4 mm long by 6.6 to 9.6 mm in diameter (Foster, unpubl. data). The fruit has a thin exocarp and a sweet, juicy, fleshy mesocarp, and contains a single drop-shaped seed ranging from ca. 5.0 to 7.0 mm long and from 3.9 to 5.4 mm in diameter. Birds eating A. edulis fruits were observed in the wild. Detailed notes were taken on the methods of feeding, and handling times for individual fruits were recorded with a stopwatch. In addition, birds of nine of these species were mistnetted and maintained in fiberglass screen cages (1.0 x 0.75 x 1.0 m high, with one crosswise and one lengthwise horizontal perch) where they were presented with Al1ophyllu.s fruits suspended from fine wires. Feeding behavior was observed, handling time was recorded, and the amount of pulp actually swallowed was determined. The percentage of the available pulp that this represented was calculated as indicated in Table 1 , as a measure of efficiency of pulp removal. Handling times per fruit were recorded from the time the bird first touched a fruit until it either swallowed or dropped the seed. The amount of fruit pulp consumed per unit time, a second measure of efficiency, was calculated using average handling times and average amounts of pulp swallowed per fruit. Birds were held in cloth net cages (28 cm diameter x 38 cm high) in the dark for 1 to 2 hr prior to testing or between trials. If a bird had not begun to eat after 30 min in the test cage, FEEDING EFFICIENCY OF FRUGIVORES 567 517 MITCHELL, M. H. 1957. Observations on birds of southeastern Brazil. Royal Ontario Museum, Univ. Toronto Press, Toronto. MOERMOND, T. C. 1983. Suction-drinking in tanagers Thraupidae and its relation to frugivory. Ibis 125: 545-549. MOERMOND, T. C., AND J. S. DENSLOW. 1985. Neotropical avian fiugivores: patterns of behavior, morphology, and nutrition, with consequences for fruit selection, p.