SPATIOTEMPORAL VARIATION IN FRUIT AND FRUGIVOROUS BIRD ABUNDANCE IN OLIVE ORCHARDS

Fruit availability and its relation to bird abundance at different spatio-temporal scales were examined in olive orchards in southern Spain. Because olive abundance in orchards can be manipulated, this system provides an opportunity to examine the link between populations of fruit-eating birds and fruiting plants in much more detail than possible in natural systems. Olive availability varied in space and time as a result of differences in olive yield and in ripening and harvesting rates. The most abundant frugivorous birds in the orchards (Sylvia atricapilla and Turdus philomelos) were frequently able to track olive availability on both local and regional scales. S. atricapilla showed a response to even small-scale harvesting of olives by humans, distributing itself preferentially in unharvested patches. The ability of both species to track fruit availability may explain their high abundance. Other less abundant frugivores (Erithacus rubecula and S. melanocephala) showed less capacity to track olive availability. I suggest that their inability to respond to rapidly changing patterns of olive abundance may partially account for their scarcity in olive orchards. Olive orchards play an important role in maintaining frugivorous bird populations in the Mediterranean area. My data suggest that a key reason for this is great spatial and temporal variability in the availability of olives in orchards, which results in a widely scattered but temporally continuous availability of olives. The capacity of some frugivorous species to track complex spatio-temporal patterns of fruit availability was, I hypothesize, a pre-adaptive feature that allowed them to flourish in the highly modified habitat of southern Spain.