New Guinean passerines have globally small clutch-sizes

Abstract

Tropical birds have small clutches. The mechanisms responsible for generating this pattern remain debated, and are typically examined by comparing tropical species, with small clutches, to their temperate counterparts, with large clutches. However, variation in clutch-size among tropical regions is seldom considered. We show that New Guinean forest passerines lay markedly smaller clutches (n = 102 species; mean ± s.d. = 1.52 ± 0.48) than other tropical avifaunas. Whereas tropical species commonly lay two-egg clutches, a substantial number of New Guinean passerines appear to solely (38%) or frequently (24%) lay single-egg clutches. We used phylogenetic comparative methods to demonstrate that New Guinean passerines lay significantly smaller clutches than congeneric South-East Asian species. We also show that reductions in clutch-size have occurred multiple times among New Guinean passerines, suggesting phylogenetic constraint does not explain this pattern. Instead, current environmental factors, including high levels of parasitism or predation, may explain why New Guinean passerines lay small clutches. We conclude that variation in clutch-size between tropical regions offers a valuable opportunity to test drivers of this variation, such as parasitism and predation, originally developed within a tropical-temperate framework.