Timothy C. Moermond

Fruit Choice in Neotropical Birds: The Effect of Distance Between Fruits on Preference Patterns

The influence of distance between fruiting plants on choice patterns of four species of tanagers and two species of manakins was experimentally tested in a lowland rain forest in Costa Rica. Paired clusters of two fruit species (Miconia affinis and Urera caracasana) were presented on each of two perches separated by selected distances ranging from 0.3 to 6.4 m. When the two perches were closest together, all the birds ate the preferred berries, M. affinis, on both perches before eating the less preferred U. caracasana. At the extreme perch separation distance, all but one of the birds ate both species of fruits on the first perch before flying to the second perch. The distance at which each bird switched between the two feeding sequences was correlated with the strength of their preference for one species of fruit over another. Manakins switched at greater separation distances than did tanagers. Since manakins take most of their fruit on the wing, the difference in switch distance is likely related to their flight pattern, but no consistent correlation with simple morphological characteristics (wing-loading or aspect ratio) was found. The results suggest that, in the field, less preferred items may often be taken when more preferred fruits are spaced even moderately far apart. The sequence of feeding and, by implication, diet breadth appears to be a consequence of the abundance and spacing of available foods; hence, degree of generalization is a dynamic feature of a birds foraging pattern. This result supports the prediction that traveling time is an important determinant of prey and patch choice.

The effect of accessibility on rates of fruit removal from tropical shrubs: An experimental study

SummaryIn a series of field experiments using Costa Rican rain forest plants, we examined the effect of accessibility on fruit removal rates. We compared the effects of fruit placement in terminal and axillary infructescences on diurnal and nocturnal removal rates, visitation rates, and incidence of fruit damage. We used three different species of berries (Phytolacca rivinoides, Psychotria brachiata, and Psychotria pitteri) and worked in three different habitats (fallow fields, treefall gaps, and forest understory) and in two different seasons (July–September, a season of fruit abundance and December–January, a season of fruit scarcity.)We found that in oldfields especially, diurnal removal rates by birds were significantly greater from axillary than from terminal infructescences. Nocturnal removal from axillary infructescences-presumably by rodents-is also occassionally significant. From these data, from observations on climbing ability and fruit use in captive rodents, and from reports in the literature, we suggest that rodents are significant sources of fruit and seed loss in tropical shrubs. We hypothesize that placement of the infructescence on the plant affects fruit removal by both seed-dispersing birds and by the less agile, often seed-destroying rodents. The balance between the two rates is an important component of a plants dispersal success.Diurnal fruit removal rates were higher during the season of fruit scarcity than during the season of fruit abundance and higher in old fields than in forest gaps or understory. Fruit damage rates-probably due to orthopterans-were slightly greater in gaps and understory than in old fields.

Effectiveness of three turacos as seed dispersers in a tropical montane forest

Abstract We studied the effectiveness of three species of turacos, the Great Blue Turaco, the Ruwenzori Turaco, and the Black-billed Turaco, as seed dispersers in a tropical forest in Rwanda. For each species of turaco, we examined two factors affecting the effectiveness of seed dispersal: (1) the proportion of ingested seeds dispersed away from the parent tree and (2) the distances seeds were dispersed. To estimate these measures of effectiveness, we systematically observed the foraging activities of focal birds over a 14-month period. We also fed six species of tree fruits to captive Ruwenzori Turacos to determine the gut retention times for seeds of various sizes. Based on these measures, we calculated the seed shadows generated by each turaco species for tree seeds with either long or short gut retention times. The gut retention time of seeds was not correlated with seed size, but was positively correlated with the time interval over which seeds were defecated. Seeds with a long gut retention time were not only dispersed farther away from the parent tree, but were also deposited over a longer time period and were thus likely to be dispersed over a greater variety of habitats. Of the three turacos, the Ruwenzori Turaco deposited the highest percentage of ingested seeds away from the parent tree because it had the shortest residence time in feeding trees. However, the Ruwenzori Turaco also dispersed seeds for the shortest distance, partly due to its short flight distances. The Great Blue Turaco on average dispersed seeds the farthest due to its long flight distance and long gut retention time. The Black-billed Turaco, which fed least frequently, deposited seeds more evenly among perching sites than did the other two species. All three species of turacos dispersed over 80% of ingested seeds away from the parent tree.

Spatial components of fruit display in understory trees and shrubs

Bird-dispersed plants of the tropical rain forest understory are widely spaced and produce sparse crops (most with fewer than 10 ripe fruits available at any one time). Such plants probably disperse the greatest proportion of their lifetime output of seeds in the short periods during which they occur in treefall gaps. There, environmental conditions stimulate the production of large crops of fruit. At the same time, however, competition for dispersers is also likely to be high because of the proximity of other fruiting plants in the same clearings. For birddispersed shrubs and treelets, treefall clearings thus represent both critical opportunities for fruit production and seed dispersal and effective bottlenecks of increased competitive pressure from other fruiting plants.

The influence of morphology on fruit choice in neotropical birds

The morphology of fruit-eating birds has a strong and complex influence on their selection of fruits. Through a series of aviary experiments, we examined the fruit-selection behavior of frugivorous birds. Among the species we tested, there were two fairly distinct groups, separated on the basis of their morphology and foraging behavior. One group, which includes the Cotingidae, Pipridae, Trogonidae, and Tyrannidae, have fairly short, broad wings, a wide gape with a short, flat bill and comparatively little leg musculature. The second group (e.g., Rhamphastidae, Emberizinae, Thraupinae) are characterized by longer, deeper bills and stronger legs. These differences in morphology are correlated with how the birds feed and thus, their selection of fruits. Fruit placement, for example, is differentially important to the two groups. Birds of the first group will readily fly for fruits that are difficult to reach from a sturdy perch whereas birds of the second group rarely do so, preferring to reach. Flying for fruits requires a greater energy expenditure than reaching; birds of the first group display a correspondingly higher selectivity of fruits than members of the second group whose low-cost reaching apparently allows them to include lower quality items in their diets.

Competition in Umbra-Perca fish assemblages: experimental and field evidence

SummaryOne potentially important effect of interspecific competition in freshwater fish communities is to increase predation intensity from gape-limited piscivores by lowering growth rates of prey species. We investigated the operation and consequences of competition between central mudminnows (Umbra limi) and yearling yellow perch (Perca flavescens) in a system where size-limited predation on mudminnows by larger perch is a principle structuring mechanism. During laboratory experiments in which mudminnows foraged for patchily-presented food in the presence and absence of yearling perch, the food intake of mudminnows decreased at both the population and individual-fish levels when perch were present. Mudminnows were neither less active nor did they occupy lower-quality food patches in the presence of perch; exploitation competition, rather than interference or predator avoidance, appeared responsible for reduced feeding success. To assess effects of competition in the field, we examined size distributions and condition factors of yearling mudminnows and perch in small Wisconsin lakes having mudminnow-only and mudminnow-perch assemblages. Yearling mudminnows were smaller and in poorer condition in an assemblage composed (by mass) of 45% yearling perch than in two mudminnow-only assemblages. Conversely, yearling perch were larger and in better condition when the mudminnow-perch assemblage contained only 45% perch then when it contained 92% perch. Our experimental and field results indicated that an asymmetrical competitive relationship could contribute directly to the interspecific population dominance of perch over mudminnows in Umbra-Perca assemblages through reduced food intake, growth, and condition of mudminnows, and indirectly through increased vulnerability of mudminnows to size-limited predation.