Birds bias offspring sex ratio in response to livestock grazing

Abstract

Livestock grazing, which has a large influence on habitat structure, is associated with the widespread decline of various bird species across the world, yet there are few experimental studies that investigate how grazing pressure influences avian reproduction. We manipulated grazing pressure using a replicated field experiment, and found that the offspring sex ratio of a common upland passerine, the meadow pipit Anthus pratensis , varied significantly between grazing treatments. The proportion of sons was lowest in the ungrazed and intensively grazed treatments and highest in treatments grazed at low intensity (by sheep, or a mixture of sheep and cattle). This response was not related to maternal body condition. These results demonstrate the sensitivity of avian reproductive biology to variation in local conditions, and support growing evidence that too much grazing, or the complete removal of livestock from upland areas, is detrimental for common breeding birds.

Grazing pressure affects offspring sex ratio in a socially monogamous passerine on the Tibet Plateau

“…The impact of livestock grazing on bird abundances in grasslands has long been recognized (Knopf 1994, Henderson et al 2004), but the impact of grazing intensity on the sex ratios within broods for local grassland birds have received little attention (Prior et al 2011). We found that a quadric term (GI 2 ) significantly correlated with the offspring sex ratio at fledging of an alpine-steppe passerine.…”

“…The proportion of sons was lower in ungrazed and intensively grazed treatments but higher in areas with low grazing intensity. This study adds evidence of adjusted offspring sex ratio in response to habitat conditions (Suorsa et al 2003, Komdeur 2004, Prior et al 2011. To better understand the pattern we observed in sex ratios, large manipulative experiments with respect to the effect of livestock grazing will be necessary.…”

“…The direct and indirect effects of grazing intensity on avian species are needed to create management guidelines for grassland ecosystems under accelerated anthropogenic disturbances (Lindsey et al 2013). Studies have shown that livestock grazing altered breeding environment with variable quality (Evans et al 2005) and affected primary sex ratio allocation of a grassland bird species (Prior et al 2011), but the grazing effect on the secondary sex ratio has been rarely studied.…”

“…Offspring sex ratio is an important trait of population demography for sexually breeding species. This demographic trait has been shown to vary with many factors, such as habitat quality (Nager et al 1999, Suorsa et al 2003, Prior et al 2011) and parental condition (Delmore et al 2008, Cox et al 2011, Bell et al 2014. It was generally thought that females in high-quality habitat or in good condition should have more resources for breeding and is more likely mated with high-quality males, and ultimately, produce higher quality offspring (Trivers and Willard 1973, Burley 1981, Komdeur 2004.…”

“…We predicted that females with good condition or paired with good-conditioned males will produce male-biased brood. Given that too much or too little grazing could lead to reduced food availability (Henderson et al 2004, Evans et al 2006, Loe et al 2007, Prior et al 2011, we predict that females in habitat with moderate grazing intensity should produce male-biased broods to maximize the fitness return.…”