Evolutionary routes to joint-female nesting in birds

Cooperative breeding systems in birds vary widely in terms of group composition and degree of reproductive skew among male and female group members. A new classification scheme based on different combinations of male and female 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-female systems are characterized by male-biased incubation, whereas either gender (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 joint-female systems, such as mating system, group composition, and nestling developmental mode, differ greatly. Three evolutionary scenarios for the evolution of male incubation and joint laying are outlined: (1) large body size relative to egg size, which enables the successful incubation of more eggs than a single female can lay (e.g., ratites and magpie goose); (2) energetically costly egg laying, incubation, and nestling care, coupled with adaptations permitting incubation of very large clutches (e.g., anis); and (3) cooperative polyandry in Lineages with male-biased incubation (e.g., woodpeckers and gallinules). All three scenarios presume that an incubating male resource increases the selective pressure on females to lay joint clutches. Available evidence for the origin and maintenance of the critical precursors, 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.