Prunus mahaleb and Birds: The High-Efficiency Seed Dispersal System of a Temperate Fruiting Tree

Abstract

Seed dispersal of Prunus mahaleb (Rosaceae), a tree producing large fruit crops, was studied in southeastern Spain to establish the degree of reciprocal dependence between the plant and the birds which disperse it. P. mahaleb drupes contain a relatively large seed (pulp: stone ratio 0.62). The water content of pulp is 82.9% and dry flesh contains 3.2% crude fat and 2.8% crude protein, being largely made up of carbohydrates. Four bird species were the main seed dispersers. Visitation rates, feeding efficiency, and degree of dependence on P. mahaleb fruits for food varied substantially among species. Turdus merula and Sylvia atricapilla showed the highest visitation rates, were behaviorally the most efficient, removed the bulk of seeds and, after feeding, tended to fly preferentially towards the apparently safest sites for growth and survival of saplings. They were also most heavily dependent on P. mahaleb fruits for food, whereas the other disperser species relied largely on insects. Individual plant location and dispersers' habitat preferences produced a differential seeding pattern over the patchy habitat surrounding study trees. The "key" dispersers T. merula and S. atricapilla are specialized frugivores, efficiently dispersing seeds of a plant producing large numbers of extremely low—reward fruits. These results conflict with theoretical expectations and suggest that both the nature of the correlation between bird— and plant—related coevolutionary gradients and their amplitudes, as well as the nature of bird—plant coevolutionary interactions, may differ between tropical and temperate habitats.