FRUGIVORY ON BURSERA-MICROPHYLLA (BURSERACEAE) BY WINTERING GRAY VIREOS (VIREO-VICINIOR, VIREONIDAE) IN THE COASTAL DESERTS OF SONORA, MEXICO

In the coastal desert of Sonora, Mexico, two species of elephant trees (genus Bursera, Burseraceae) produce bird-dispersed fruit in contrasting patterns. Bursera hindsiana produces fruits that are eaten by a variety of birds. In contrast, B. microphylla produces fruits that are primarily eaten by only two species of bird. One of these species, the gray vireo (Vireo vicinior), is unusual among migratory birds that become primarily frugivorous in the winter because individual vireos arc territorial within and between winters and the vireos' frugivorous diet consists almost entirely of fruit of B. microphylla. A possible mutualistic relationship between gray vireos and B. microphylla may explain the high degree of overlap between the winter range of the bird and the geographic distribution of the plant.