Nest location, clutch size and nest success in the Scarlet Ibis Eudocimus ruber

Breeding success and nest‐site characteristics were studied during the 1996–1997 breeding season in a colony of Scarlet Ibises Eudocimus ruber in south‐eastern Brazil to test the hypothesis that nest‐site characteristics and clutch size affect nest success. Two nesting pulses produced young, the earlier being more successful. Predation accounted for most failures during the first pulse, wind destruction during the second. A third pulse with few nests produced no young. Adult Ibises abandoned nests when they lost sight of other incubating birds. Logistic regression analysis indicated that nest success during the first pulse was positively related to clutch size, number of nests in the nest tree and in the nearest tree, and negatively to the distance to the nearest neighbour. During the second pulse there were significant negative associations between success, nest height and distance to the fourth nearest nest, and a positive association between success and nest cover. The results agree with the ‘selfish herd’ hypothesis, indicating that nest aggregation may increase breeding success, but the nest‐site characteristics affecting success can differ over the course of one breeding season.