Black vultures (Coragyps atratus) foraging on Olive ridley (Lepidochelys olivacea) sea turtle eggs and hatchlings

The incubation period, emergence, and dash of freshly emerged hatchlings to the sea is a critical period in the life cycle of sea turtles, although it is short compared to their life span. Predators can claim up to 100% of sea turtle eggs or hatchlings, depending upon the site, time of year, or location. We examined the foraging behavior of black vultures (Coragyps atratus) on an arribada beach of the olive ridley (Lepidochelys olivacea) sea turtle at ustionai, Guanacaste, Costa Rica. We address the following: 1) What methods of foraging do vultures use? 2) How successful are they in obtaining food? and 3) What factors affect their foraging, and we speculate on the possible effects of vultures on olive ridley reproductive success. The number of black vultures feeding on a 1500 m beach averaged 643 + 104 (max of 1243). Vultures foraged by hunting (nests with eggs or embryos, emerging or crawling hatchlings), social parasitism (using other birds or dogs to locate foraging opportunities), parasitism (eating eggs or hatchlings at nests other vultures found first), piracy, and scavenging of washed up adults, eggs, and dead hatchlings. Foraging opportunities included finding nests at eroded stream banks, nests uncovered by tides or wind, nests dug up by dogs, eggs thrown up by digging female turtles, and hatchlings emerging on their own. Overall, almost half the vultures gathered around exposed nests ate eggs/embryos (mean number of vultures was 10.6), while 64% obtained hatchlings at nests with emerging hatchlings. Both the number of vultures, and the percent feeding, varied by foraging opportunity, with the highest percent feeding on and below vertical banks of eroded streams, and the lowest percent feeding with dogs. Interference competition with wood storks (Mycteria americana) resulted in significant decreases in the number of foraging vultures within 5 m of an exposed nest, and a significantly lower percent were able to feed, when storks were present at nests exposed at stream banks and by tidal action, and when hatchlings were emerging. 82% of the variance in the number of vultures feeding was explained by feeding location (foraging opportunity), number of storks, number of vultures present, and time of day. Although the turtles are supporting a healthy population of black vultures, most of the vulture food source on the beach are eggs that would not otherwise hatch, including inviable eggs, developing embryos and nearly-developed hatchlings that were exposed by stream bank collapse, tidal erosion, dogs or Wood Storks, digging female turtles during arribadas, and human egg-harvesters.