Stuart Douglas Strahl (1985). The behavior and socio-ecology of the hoatzin, Opisthocomus hoatzin, in the Llanos of Venezuela. A dissertation submitted to the state University of New York at Albany in Partial Fulfillment of the requerements for the degree of Doctor of Philosophy
The behavior and breeding biology of the hoatzin (Opisthocomus hoazin) were studied in the central plains (Llanos) of Venezuela, this species nested in cooperative social units, usually composed of a single breeding pair and a variable number (0-5) of helpers at the nest. Units defended small all-purpose territories during the rainy (breeding) season, but some migrated to areas of permanent water during the dry season. The major displays and behaviors of the hoatzin are described in detail. Helpers were offspring retained from previous years successful nestings, and aided in all reproductive activities of the group except for fertilization and laying of eggs. Breeding females demonstrated reduced effort in parental care when helpers were present. Units with helpers bred earlier in the season, had a reduced inter-nest interval between successive nesting attempts, and produced more nest per season than pairs nesting without helpers. Nesting success and the number of young reared per unit were correlated with unit size. Larger units also reared a higher proportion of their clutches to independence. These differences were associated with faster growth rates and earlier fledging dates for young in units with helpers, which reduced predation on these young.
The level of predation pressure was an accurate indicator of territory quality. Units nesting in areas with low predation were substantially more successful that other units. The results of an experiment indicate: 1) nesting success was significantly augmented by isolation of nesting sites from arboreal predators, and 2) units with helpers rear more young that pairs nesting alone when this major component of territory quality was controlled, demonstrating both territory quality and helper effects on unit reproductive success.
Annual survival of adults and independent young was high (80-90%). The demography of the study population fits the criteria for recent theories regarding the evolution of cooperative breeding. Helping behavior in hoatzins also fits the predictions of a simple inclusive fitness model based on observed direct and indirect fitness gains and losses in the study population.