Neighborhood Effects on Fruit Removal by Birds: A Field Experiment with Viburnum Dentatum (Caprifoliaceae)

Abstract

The foraging behavior of fruit—eating birds can have important effects on the fitness of plants whose seeds they disperse. An important factor influencing the pattern of fruit consumption by birds may be the amount of friut in the immediate vicinity of a plant. I experimentally examined the effects of neighborhood fruit density and fruit crop size on the rate, proportion, and temporal pattern of removal of northern arrowwood (Viburnum dentatum) fruits by birds in central New York. I manipulated V. dentatum fruit densities in 100 10 X 10 m plots, then monitored fruit removal by birds during 14 wk in the fall of 1985. Each plot had either high or low fruit density in a central 1 m diameter "crop" area, and either high or zero "neighboorhood" fruit density in the rest of the 100—m2 plot. I counted marked fruit in the center of each plot weekly. Fruits in high density neighborhood plots were removed significantly faster than fruits in plots with zero density neighborhoods. A significantly lower proportion of marked friut was removed from plots with both low crop size and zero density negihborhoods. Forty—three percent of all removal of marked friut by birds took place during a 2—wk period early in the friuting season. Birds feeding in flocks were observed feeding in high density neighborhood plots almost exclusively, presumably causing the greater removal from these plots. Fruit abscission accelerated after week 9, and by week 14 no fruits were left on plants. White—tailed deer (Odocoileus virginianus) consumed 400 of the 750 marked infructescences during the 14 wk, but destroyed rather than dispersed seeds. These results indicate that the amount of fruit in the area surrounding a fruiting plant, as well as the size of the plant's own fruit crop, can influence fruit removal by seed dispersers. Thus, disperser—mediated plant fitness is affected by neighboring plants. In contrast to previous studies, I found that high fruit density in the area surrounding a plant enhanced friut removal rather than inhibited it.