Martin, T. E., J. C. Oteyza, A. J. Boyce, P. Lloyd, and R. Ton. 2015. Adult mortality probability and nest predation rates explain parental effort in warming eggs with consequences for embryonic development time. American Naturalist 186: 223-236.
The time that embryos take to develop is thought to affect mortality in later life because of physiological trade-offs associated with development time. Yet, an unexplored alternative hypothesis suggests that mortality may be the cause rather than consequence of embryo development time in birds; adult and offspring mortality may exert selection on parental effort in warming eggs to thereby determine embryo development times. We examined these possibilities based on field studies of 64 songbird species on four continents. Structural equation modeling showed that an association of age-specific mortality and embryonic development time was best explained by mortality as a cause of average embryonic temperature. Influence of age-specific mortality on parental effort in warming eggs is a unique result that follows from life history theory and provides a causal mechanism for explaining variation in embryonic development time across diverse species.