1. Determining the factors that shape the unequal division of parental effort between the sexes is key to understanding the evolution of mating systems and sexual cooperation and conflict. The range of possible care strategies may be constrained, however, by the duration of the care period. Thus, comparative analyses of parental effort should consider both these dimensions of care; here, we present a method for quantifying parental care that does this.
2. We test three models of the relationship between care duration and the division of parental effort. Using detailed information on parental effort for 330 bird species from 13 families, we use Linear Mixed Effects models and Phylogenetically Weighted Generalized Least Squares models to analyse the relationship between the two dimensions of care. These models provide a starting point for more detailed comparative analyses of the role of covariates, such as sexual size dimorphism on determining the division of parental effort.
3. Within families, although males generally provide less care than females, their relative role often increases with the duration of the care period. Across all species, care strategies vary more in species with short development times.
4. Understanding the evolution of parental care strategies requires consideration of the total duration of care in addition to the division of parental effort. Our method provides a simple means to incorporate both dimensions of care into more extensive comparative analyses.