The evolution of clutch size in Australian songbirds in relation to climate, predation, and nestling development

The study of clutch size has been a productive approach to gaining better understanding of life-history evolution, especially in Northern Hemisphere birds. Factors driving life-history evolution in Southern Hemisphere species are less well understood. Moreover, studies often focus on single hypotheses for clutch size evolution and rarely test several hypotheses simultaneously. This severely limits more general conclusions of life-history evolution. We assembled an extensive dataset on 313 species (ca.98%) of Australian songbirds (Passeriformes) and tested three hypotheses for the evolution of clutch size in birds: (1) resource availability and their seasonality (Lack’s and Ashmole’s hypotheses), (2) nestling mortality (age-specific mortality and Skutch’s hypotheses), and (3) fledgling developmental gradient (Martin’s hypothesis). The mean clutch size of Australian songbirds was 2.69 eggs and increased in higher latitudes. Clutch size was positively related to the length of the nestling period and in species with short nestling periods offspring left the nest with lower body mass, consistent with Martin’s hypothesis. In contrast to many Northern Hemisphere studies we did not detect any direct effect of the productivity of environment, its seasonality or nest predation rate. Our work provides one of only a handful of comprehensive tests of clutch size evolution in Southern Hemisphere birds. Its findings stress the importance of breaking the Northern temperate bias of life-history studies.