Food‐supplementation does not override the effect of egg mass on fitness‐related traits of nestling house wrens

Abstract

1. Cross‐fostering experiments have shown that egg mass per se positively affects post‐hatching growth and development in passerine birds. In most studies, however, the initial influence of egg mass was not sustained, leading to questions about the importance of egg mass relative to the cumulative effects of environmental factors during the period of post‐hatching care. 2. In house wrens, Troglodytes aedon Vieillot, evidence from cross‐fostering experiments suggests that food availability mitigates the influence of egg mass on posthatching growth: early in the season, when food was putatively abundant, the initial effect of egg mass was lost as nestlings aged; however, later, when food was putatively scarcer, the effect of egg mass persisted until nestlings left the nest. 3. We tested the hypothesis that abundant food for provisioning nestlings can decouple the relationship between fitness‐related traits of nestlings and the mass of the eggs from which they hatch (‘egg‐mass override hypothesis’) by providing daily mealworm supplements to broods of house wrens. 4. The food supplements did not increase final nestling mass, final nestling tarsus length, growth rate, or survival until nest‐leaving in late‐season broods, when the hypothesis of food limitation predicts that there should be such an effect. Instead, the food supplements significantly, but only slightly, increased final nestling mass in early‐season broods, when food is putatively abundant. 5. The effect of the food supplements on final nestling mass was independent of egg mass. Nestling survival in early‐season broods, and final nestling mass and tarsus length in both early and late‐season broods significantly increased with increasing egg mass in both experimental and control broods. 6. We propose that food was not limiting during the course of the experiments and that final nestling mass and tarsus length remained positively related to egg mass because nestling growth and development in house wrens is ‘predetermined’ by some component or property of eggs that covaries with egg mass.