Sex manipulation within broods of house wrens? A second look

Most reports of adaptive manipulation of offspring sex in birds have come from single-year studies with relatively small sample sizes. Calls have thus gone out for repeat, multiple-year studies to confirm positive results. In a study of a Wyoming population of house wrens, Troglodytes aedon, Albrecht (2000, Animal Behaviour, 59, 1227) asked whether embryos from last-laid eggs of clutches were predominantly female. Last-laid eggs typically hatch 36-48 h after most other eggs in the clutch. This puts offspring from last eggs at a competitive disadvantage and these offspring are often in poor physical condition at fledging. Albrecht argued that being in poor condition should negatively affect male fitness more than female fitness because males must compete for limited nest sites. Moreover, males that secure multiple sites have the potential to pair polygynously. Albrecht found a strong female bias among offspring from last-laid eggs (27 of 34 individuals) during the 1997 breeding season. We repeated Albrecht's study using the same population over three seasons, 2002-2004. We found no sex bias among offspring from last-laid eggs in any year; overall, 42 of 86 individuals (48.8%) were female. We also found no sex bias among offspring from second-to-last eggs, which also hatch late, or among offspring from the first two eggs. Why our findings differ from those of the original study is unclear. Nevertheless, we argue that whether (or when) house wrens adaptively manipulate the sex of offspring within broods must be considered an open question.