The mass of a bird’s egg is a critical attribute of the species’ life history and represents a fundamental component of reproductive effort.  Indeed, the trade-off between the number of eggs in a clutch and clutch mass lies at the heart of understanding how environmental attributes such as nest predation or adult mortality influence reproductive investment.  However, egg masses have not been reported for the majority of avian species.  We capitalized on the strong allometric relationship between avian body mass and egg mass to produce egg mass estimates for over 5,500 species previously lacking such information.  These estimates are accompanied by measures of the robustness of the regressions used to produce them (e.g., sample size, root mean square error of estimation, coefficient of determination, degree of extrapolation), thus allowing independent evaluation of the suitability of any estimate to address a particular research question relating to avian life history.  Most estimates (~5,000) were based on family level egg mass-body mass regressions, with the remainder derived from other relationships such as ordinal regressions.  We compared estimating regressions based on adult vs. female body masses, and after finding little difference between the two based our final estimates on adult masses as those were more numerous in the literature.  What small differences between adult- and female-based regressions that did occur were not related to sexual size dimorphism across families.  These new estimates, coupled with ~5,000 egg masses reported in the literature, provide a foundation of over 10,000 species for wider investigations assessing variation in reproductive effort in birds over a broad array of ecological and evolutionary contexts. 

Avian egg mass and body mass data downloaded from various published sources as indicated in Methods.  For the large majority of species for which egg masses had not been reported we estimated them based on the ordinary least squares regression relationship derived from other species with known values in the same family.

See ReadMe file for contents of each dataset.