The role of the white-eyed vireo in the dispersal ofBurserafruit on the Yucatan Peninsula

Abstract

ABSTRACTWhite-eyed vireos (Vireo griseus) winter in the forests and secondary growth of the Yucatan Peninsula whereBursera simaruba(Burseraceae) is an abundant tree. Twenty-five per cent of all white-eyed vireos observed foraging visitedBurseratrees. In addition, presence and abundance of territorial white-eyed vireos in small forest patches were correlated with the size of theBurseracrop. Vireos were the most reliable dispersers ofBurseraseeds. These birds visited 32 of 35 trees observed for at least three hours. They accounted for approximately half of all bird visits, and two-thirds of the seeds dispersed. Most of the other species rarely visited (<5% of visits) or failed to remove seeds from the tree.Peculiarities of phenology and fruit structure may contribute to the tendency ofBurserato be dispersed by relatively few species. The capsules ofBurserafruits do not open when the fruit ripens; birds apparently locate ripe fruit using visual cues, although these are few. In addition, only a small portion of the crop ripens daily over a 7- or 8-month period. The vireo-Bursera simarubarelationship, found regionally on the Yucatan Peninsula, may result from the prolonged fruit ripening period (October-March), the relatively depauperate frugivore community and the relatively high density of smallBurseratrees in the hurricane-disturbed dry forests.Small trees at all times, and all trees from October to February, depend upon territorial vireos for continuous, highly efficient local dispersal of a small number of fruits. In March and April residual fruits ripen rapidly and synchronously, attracting a greater variety of visitors for broad spectrum dispersal during a period of food scarcity. Thus,Burserahas an unusual two-phase phenological pattern, perhaps alternately to take advantage of both specialized and opportunistic dispersers.