Evolutionary routes to joint-female nesting in birds

Cooperative breeding systems in birds vary widely in terms of group composi tion and degree of reproductive skew among male and female group members. A new classification scheme based on different combinations of male and fema le skew is proposed. A review of cooperative species reveals a fundamental dichotomy between low-skew joint-female species on the one band, and joint- male and high-skew helper species on the other. All cooperative joint-femal e systems are characterized by male-biased incubation, whereas either gende r (but usually females) may perform the majority of incubation in non-joint -female systems. Male incubation is thus a necessary, but not a sufficient, precursor for the evolution of communal laying. Other characteristics of j oint-female systems, such as mating system, group composition, and nestling developmental mode, differ greatly. Three evolutionary scenarios for the e volution of male incubation and joint laying are outlined: (1) large body s ize relative to egg size, which enables the successful incubation of more e ggs than a single female can lay (e.g., ratites and magpie goose); (2) ener getically costly egg laying, incubation, and nestling care, coupled with ad aptations permitting incubation of very large clutches (e.g., anis); and (3 ) cooperative polyandry in Lineages with male-biased incubation (e.g., wood peckers and gallinules). All three scenarios presume that an incubating mal e resource increases the selective pressure on females to lay joint clutche s. Available evidence for the origin and maintenance of the critical precur sors, constraints, and adaptations are summarized and shown to be absent in non-joint-laying species. These factors and conditions affect the critical parameters of the skew models in ways that are predicted to result in low reproductive skew.