Growth benefit to house wren nestlings of having an asynchronously late-hatching nestmate is greater for extra-pair offspring

Abstract: Because nestling growth influences survival and future reproductive success, early-life conditions that impact growth have potentially large effects. One notable source of variation in early-life conditions for birds is the relative timing of egg hatching: hatching asynchrony leads to size hierarchies among nestlings and may affect competition for, or parental allocation of, resources. Paternity is another potentially important factor, as extra-pair offspring have been shown in some systems to be of higher quality and therefore might be better competitors. We tested whether the presence of an asynchronously late-hatched nestmate affected nestling growth in a population of house wrens (Troglodytes aedon) and whether paternity status (within-pair vs. extra-pair) affected growth either individually or in interaction with hatching synchrony. We measured mass and wing chord of nestlings repeatedly between the ages of 0 and 10 days and determined the paternity status of each nestling. Both mass and wing chord were significantly greater in nestlings who shared the nest with at least one late-hatched nestling. This effect was significantly greater for extra-pair chicks for wing chord, but not mass. We found no effect on nestling mass or wing chord of paternity status alone, hatch timing (early or late), hatch date, brood size, sex, or social father mass. Considered together, these results suggest that hatching synchrony affects nestling growth and that extra-pair nestlings may be more responsive to early-life conditions in some aspects of growth. We discuss possible mechanisms for these effects. Significance statement: Bird eggs can hatch synchronously or asynchronously. In nests with asynchronous hatching, nestlings are different sizes, which may affect competition among them and therefore their growth rates. If hatching asynchrony does influence nestling growth, it may have far-reaching impacts on the fitness of the nestlings. We measured mass and wing length in house wren nestlings. Nestlings with an asynchronously late-hatching nestmate were larger in both measures, demonstrating an advantage to having a younger nestmate. This advantage was greater for extra-pair nestlings for wing length. This suggests a difference between extra-pair and within-pair nestlings in their responsiveness to early-life conditions, which might be due to the genetics of the extra-pair chicks or to differences in parental investment. This study documents a previously unknown potential benefit of hatching asynchrony, as well as a context-dependent difference between extra-pair and within-pair nestlings.