Interpretation of fossil material using comparative anatomy often relies on relationships predicted from data collected from extant species. Some years ago, it was suggested that femur length of birds could be predicted from egg mass but this relationship was counter-intuitive because egg mass is usually related to a measure of body size. The original analysis was also not phylogenetically controlled. This study used the same data to determine phylogenetically controlled relationships for body mass versus egg mass, and egg mass versus femur length. Further analysis showed that order was important in the prediction of egg mass from either body mass or femur length. For some orders, the single regression estimate through all data significantly over-, or under-estimated egg mass. This problem was more pronounced for femur length compared with body mass. Extrapolation of the relationship between femur length and egg mass for large extinct birds seemed to be provide useful data for the Gastornithidae but under-estimated egg mass for other large bird species of a variety of families. Use of equations derived from extant birds to gain insight into the reproductive biology of extinct species needs to be undertaken with great care.