Abstract
The latitudinal gradient in clutch size, with birds in tropical areas having smaller clutches than those in temperate areas, was first described by ornithologists in the 1940s. Although many hypotheses to explain this gradient have been proposed they are based on life-history evolution in the temperate region where fewer than 25% of bird species occur. The latitudinal gradient in clutch size may be explained not only by geographical variation in food abundance and nest predation risk, but also by several other constraints, such as ambient temperature directly affecting egg viability, physiological tradeoffs (i.e., immune, metabolic, endocrine systems), and physiological regulatory networks that avoid hyperactivation and ensure autoimmune responses. To develop a more comprehensive understanding of the evolutionary mechanisms of plasticity of clutch size within individuals, and variation in clutch size among populations and species, I review works that: (1) clarify the effects of seasonal change and latitudinal gradient in ambient temperature on egg viability, (2) deal with relationships among life histories, selective factors, and physiological costs (e.g, immunological defense against parasites). Thus, I highlight the importance of understanding the effects of local environmental and physiological factors on clutch size variation.
