Parental care and adaptive brood sex ratio manipulation in birds

Abstract

Under many circumstances, it might be adaptive for parents to bias the investment in offspring in relation to sex. Recently developed molecular techniques that allow sex determination of newly hatched offspring have caused a surge in studies of avian sex allocation. Whether females bias the primary brood sex ratio in relation to factors such as environmental and parental quality is debated. Progress is hampered because the mechanisms for primary sex ratio manipulation are unknown. Moreover, publication bias against nonsignificant results may distort our view of adaptive sex ratio manipulation. Despite this, there is recent experimental evidence for adaptive brood sex ratio manipulation in birds. Parental care is a particularly likely candidate to affect the brood sex ratio because it can have strong direct effects on the fitness of both parents and their offspring. We investigate and make predictions of factors that can be important for adaptive brood sex ratio manipulation under different patterns of parental care. We encourage correlational studies based on sufficiently large datasets to ensure high statistical power, studies identifying and experimentally altering factors with sex-differential fitness effects that may cause brood sex ratio skew, and studies that experimentally manipulate brood sex ratio and investigate fitness effects.

Breeding ecology and bias in offspring sex ratio in little grassbirds (Megalurus gramineus)

“…Richter 1983;Weatherhead and Teather 1991;Webster 1992;Anderson et al 1993;Sheldon 1999), or species that have unusual mating or social systems, both factors that may cause biased primary sex ratios (e.g. Hasselquist and Kempenaers 2002;Komdeur and Pen 2002). Nevertheless, Gowaty and Droge (1991) provide compelling reasons to investigate the sex ratios of sexually monomorphic species, since they are likely to provide further comparative insights into why birds of some species manipulate the primary sex ratio of their offspring.…”

Maternal influences on brood sex ratios: an experimental study in tree swallows

“…Recent studies suggest that individuals adaptively adjust offspring sex ratios in relation to particular factors associated with the breeding environment of the female (reviewed in Hasselquist & Kempenaers 2002;Komdeur & Pen 2002;Sheldon & West 2004). These factors can include food availability, female body condition and mate quality.…”

“…For example, the benefits of producing high quality sons could be predicted if the female were mated to a high quality male (and male traits were heritable and related to reproductive success) or if maternal condition were related positively to her son's condition and subsequent reproductive success. There is mounting evidence in birds and mammals that certain factors, such as these, are associated with individual biases in offspring sex ratios (reviewed in Hasselquist & Kempenaers 2002;Sheldon & West 2004).…”

Hatchling sex ratio and female mating status in the great reed warbler,Acrocephalus arundinaceus(Aves, Passeriformes): further evidence for offspring sex ratio manipulation

“…The adaptive significance of this behaviour lies in differences in future reproductive value of nestlings of one sex relatively to nestlings of the other sex. In agreement with these expectations, many studies have shown that females may produce adjusted brood sex ratio as a response to their body condition, female mating status, timing of breeding and/or male attractiveness (see Hasselquist & Kempenaers 2002;Komdeur & Pen 2002 for previous reviews; Albrecht & Johnson 2002;Griggio et al 2002;Velando 2002;Gwinner & Schwabl 2005;Husby et al 2006, Freed et al 2009Du & Lu 2010;Philip et al 2010). However, some *Correspondence: Pavol Prokop, Department of Biology, University of Trnava, Priemyselná 4, SK-91843 Trnava, Slovakia.…”

“…Email: atrnka@truni.sk other studies have failed to find such relationships in birds, even in the same species (Lessells et al 1996;Hartley et al 1999;Radford & Blakey 2000;South & Wright 2002;Ewen et al 2004;Johnson et al 2005;Maddox & Weatherhead 2009), questioning the adaptive brood sex ratio manipulation. One problem may lie in the fact that only sex ratios at fledging have been used in many of these studies, ignoring post-hatching sex-biased mortality which can be caused by differential provisioning of male and female offspring or sibling competition over food when sexual size differences are exhibited (Droge et al 1991;Teather 1992;Clotfelter 1996;Hasselquist & Kempenaers 2002;Gonzáles-Solís et al 2005;Verhulst et al 2006). The rationale is that less favoured or smaller sex may be more exposed to starvation during food shortage periods leading to their death than the other sex (Westerdahl et al 2000).…”