Non-sequential fruit tracking by birds along an altitudinal gradient

Abstract

Despite landscape-level changes in fruit quantity, food resources may be constantly available to frugivorous animals if they track asynchronous fruit peaks. To investigate fruit availability patterns and their consequences on bird abundance, we tested for the occurrence of seasonal patterns in fruit production at three elevations (range 500–1000 m asl) in the Brazilian Atlantic Forest over three years and compared these patterns among the elevations and years. Fruit production occurred throughout the year and showed only slight seasonality, and only at the two higher elevation sites. Contrary to expectation, fruiting peaks were neither predictable nor complementary. A few plant species (about 5%) contributed disproportionately to the high annual variation of fruit production in plant communities. We found no relationship between fruit production and bird capture rates of both generalist and specialist fruit eating species, suggesting that birds do not track fruit resources along the gradient, at least in some parts of the Atlantic Forest, or in some years. The apparent constancy of fruit availability probably does not stimulate birds to track to other elevations. In addition, plants with high annual variation in fruit production could be influencing and biasing our assessment of this relationship.