"Ecological Release," Seasonal Variation in Food Supply, and the Hummingbird Amazilia Tobaci on Trinidad and Tobago

Abstract

Resident populations of the Coppery—rumped Hummingbird, Amazilia tobaci, occupy the West Indian Islands of Trinidad and Tobago. Trinidad has 15 other hummingbird species, Tobago only 4 others. We predicted that A. tobaci on Tobago would have a broader feeding niche and a denser population than would A. tobaci on Trinidad. We monitored populations of Amazilia tobaci for 13 mo at study sites in the Arima Valley of northern Trinidad and in central Tobago. The two sites had nearly identical climates, and similar assemblages of flowers visited by hummingbirds. At both sites, food available to nectar—feeding birds varied > 100 fold over the year. For 9 mo (December—August), neither A. tobaci population faced a severe food shortage. From September through November, nectar was scarce at both sites. During this period, Tobago A. tobaci faced only two other, uncommon species of nectar—feeding birds, which had very different feeding habits. The Trinidad population endured the food shortage along with six other hummingbird species. At this time Tobago A. tobaci utilized variously sized and shaped flowers of numerous plant species. On Trinidad, where many flowers were visited by the other hummingbird species, A. tobaci primarily used flowers that matched their bill size, which resulted in narrower feeding niche than on Tobago. These effects of "ecological release" on Tobago were apparent for only 3 mo, however, and when data for an entire year are compared there were few statistically significant contrasts between Tobago and Trinidad populations. At both sites, niche (diet) breadth responded in density—dependent fashion to changes in food density, broadening with high densities of A. tobaci relative to food, narrowing when food was abundant. Median values for niche breadth, population density, and population density per unit resource were not significantly greater on Tobago than on Trinidad. Ecological differences between the two populations during the food shortage may be linked to a phenotypic difference. We found no increase on Tobago in the intrapopulation variance of any morphological parameter important to feeding. However, Tobago, A. tobaci had longer wings than Trinidad conspecifics, resulting in lower wing disc loadings (ratio of body mass to area swept out by the wings). Among hummingbirds in general, low wing disc loading indicates a capacity to forage at a wide variety of flowers, which suggests that the Tobago population has undergone "within—phenotype nich expansion." Two experiments showed that individuals from the Tobago population of A. tobaci used a wide variety of artificial flowers, and a wide range of flower densities in field manipulations, than did conspecifics from Trinidad. The results suggest that occasional food shortages play a crucial role in the ecology and evolution of island populations, and that this might be undetected in short—term studies.