Breeding Biology and Brood Reduction of Herons and Ibis in a Northern Brazilian Mangrove Swamp: Eggs Do Not Starve

Abstract

Few studies on pelecaniform breeding have been conducted in tropical regions. Most species in the Order Pelecaniformes (Family Ardeidae and Threskiornithidae) are facultative brood reduction strategists, laying extra-eggs that might not fledge. In obligate brood reduction in birds, extra-eggs are insurance-eggs, but in facultative brood reduction birds, extra-eggs have historically been interpreted as additional offspring in unpredictable environments. However, this study on six species in Brazil suggests that the insurance-value might be a major factor selecting for extra-egg laying, even in facultative strategists. While Egretta thula, E. caerulea, E. tricolor, Nycticorax nycticorax, and Nyctanassa violacea showed facultative brood reduction strategies, Eudocimus ruber showed a brood survival strategy and was used to compare brood reduction parameters between strategies. Mean brood size in different species ranged from 1.94-2.52 eggs, 1.88-2.25 hatchlings, and 1.65-2.05 fledglings. Clutch sizes were between two and three in brood reduction species, and almost always two in E. ruber, which either raised the entire brood or totally failed. Individual mortality events in non-failed nests were lower in E. ruber. This difference occurred only during the egg stage, when starvation does not occur, so differences between strategies resulted from an insurance-value of extra-eggs. Nest failures were higher in E. ruber. Insurance could be the main force driving evolution of extra-egg laying in facultative species, with survival of additional offspring a collateral benefit.