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 earlierlaid 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 24Á48 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.

Superiority of extra-pair offspring: maternal but not genetic effects as revealed by a mixed cross-fostering design

“…2010). If EPY were laid in early eggs, they would have a maternally derived competitive advantage compared to WPY, and this pattern was recently found in three passerine species (Johnson et al. 2009; Magrath et al.…”

Individual variation and environmental context in avian life-history evolution: An experimental approach using mountain bluebirds (Sialia currucoides).

“…In some bird species, EPO are disproportionately male ), or nonrandomly positioned within the egg laying and hatching sequence (Krist et al 2005;Johnson et al 2009b;Magrath et al 2009), and these non-genetic factors can have important effects on nestling phenotype. As a result, these effects may either enhance or obscure phenotypic differences between maternal half-siblings that are due to paternal genetic contribution, and so may limit our understanding of the true fitness benefits of female mating decisions.…”

Eggshell porosity covaries with egg size among female House Wrens (Troglodytes aedon), but is unrelated to incubation onset and egg-laying order within clutches

“…In altricial birds, the eggs within a clutch often hatch asynchronously over several days, causing offspring to differ considerably in size shortly after hatching, with last-hatched nestlings usually half the size of their older siblings (e.g., Clotfelter et al 2000; Maddox and Weatherhead 2008; Johnson et al 2009). This initial hierarchy is typically maintained throughout the time offspring are in the nest, and often results in the death of the younger, smaller nestlings (Slagsvold 1986; Mock and Parker 1997; Lago et al 2000; Johnson et al 2003, 2009; Maddox and Weatherhead 2008).…”

“…This initial hierarchy is typically maintained throughout the time offspring are in the nest, and often results in the death of the younger, smaller nestlings (Slagsvold 1986; Mock and Parker 1997; Lago et al 2000; Johnson et al 2003, 2009; Maddox and Weatherhead 2008). Synchronous hatching of eggs also occurs in a wide variety of avian taxa (Clark and Wilson 1981; Slagsvold and Lifjeld 1989; Hébert and Sealy 1992; Stoleson and Beissinger 1995; Hébert 2002) and occurs when all eggs hatch over a short period of time, typically within a day, which results in nestlings being similar in size throughout their time in the nest (Bowers et al 2011).…”