Evidence for a maternal effect benefiting extra‐pair offspring in a songbird, the house wren Troglodytes aedon

Abstract

Female birds are hypothesized to mate outside the pairbond to secure alleles that enhance the fitness potential of their offspring. To test this, researchers typically compare fitness‐related attributes of extra‐pair (EP) and within‐pair (WP) half‐siblings. Often neglected, however, is the possibility that females may provide EP offspring with certain non‐genetic advantages. For example, in species in which eggs hatch asynchronously, females may place EP offspring amongst earlier‐laid eggs in the clutch. Because they tend to hatch first, chicks from earlier‐laid eggs are often larger and more developed than their nestmates and thus have a competitive advantage. We tested for an association between offspring paternity and position in the laying/hatching sequence in a Wyoming population of the house wren Troglodytes aedon. Eggs in this population always hatch asynchronously, usually in the order laid, over 24–48 h, setting up a stair‐step‐like size hierarchy within broods early in the nestling stage. This suggested that we could thus use a chick's mass relative to that of its nestmates as an index of the chick's position in the laying sequence. We first confirmed this, showing that position in the laying sequence explains 70% of the variance in chick size relative to nestmates. We then compared masses of EP and WP offspring shortly after hatching in 27 broods with mixed paternity. On average, EP offspring weighed 14% more than their WP counterparts, a highly significant difference. Our results therefore suggest that EP offspring are more likely than WP offspring to occur in earlier‐laid eggs, thus gaining a size‐based competitive advantage. We recommend that, when comparing EP and WP half‐siblings in species in which eggs hatch asynchronously, researchers test for this potential maternally derived effect on offspring performance.