This question was broadly addressed in the article. Methods section "The sex of the bird (fixed factor) and the number of chicks per nest (covariate) were also added to the models. Body mass and tarsus length (15 d old), body condition (residuals of a regression between body mass and tarsus length at 15 d of age) and initial body mass (5 days old) were also tested as covariates in models". "Only significant terms were maintained in the models (P< 0.05)." In the results section we mentioned: "The body mass measured at the beginning of the experiment (i.e. 5 days old) and the sex of the chick were retained in the model." Signed: Carlos Alonso-Alvarez.