THE SOCIAL ORGANISATION OF BEE‐EATERS (MEROPIDAE) AND CO‐OPERATIVE BREEDING IN HOT‐CLIMATE BIRDS

Summary In the colony-nesting bee-eater Merops bulocki of the northern savannas of Africa males preponderate in the proportion 1 -5 to 1 female, initial breeding occurs at varying ages, and adults of both sexes and all age classes, particularly one-year-old males, assist breeding pairs with nest excavation, incubation, and provisioning the pulli and fledglings. Up to three helpers can occur at one nest, and they may be siblings of a previous year's brood. Pair-bonds are for life, and bonds formed between the breeding pair and their helpers may endure until next season. Some evidence suggests that other gregarious open-country bee-eater species are similar in these respects but that forest and solitary-breeding savanna species have the sex ratio at parity and do not breed co-operatively. Breeding dispersion of savanna species is related to specific size, the largest (which forage most widely) nesting in the densest aggregates. Other instances of birds with comparable social organisations are briefly reviewed. The 80 species involved comprise broad systematic and ecological spectra and about 60 of them inhabit hot climates in America, Africa and Australia. It is suggested that co-operative breeding provides a reserve of experienced individuals whose recruitment to breeding permits (1) fine control of stable populations in non-seasonal habitats (tropical forest) within the capacity of the food resources; (2) crude adjustment of fluctuating populations in habitats with unpredictable rainfall (Australia), enabling them rapidly to recuperate from depression and to achieve the highest population level the improved food resources can sustain; and (3) regulation of populations in intermediate habitats (tropical savannas) by either means according to the dictates of immediate ecological circumstances.