Age of the breeders, but not territory quality, explains hatching sex ratio in booted eagles

Abstract

Natal sex‐ratio variation is a central concept in population demography. When males and females are equally costly to rear, the population sex ratio should be 1:1, but if they differ in cost, more of the cheaper sex should be produced. According the age dependent sex ratio hypothesis, younger breeders should produce more chicks of the cheaper sex. On the other hand, the environmental sex determination hypothesis suggests that parents in poor conditions (scarce food and low‐quality territories) tend to produce more of the cheaper sex. To test hypotheses on adaptive sex ratio, we studied the booted eagle Aquila pennata, a raptor with marked sexual dimorphism, with females larger than males. We sexed 174 nestlings from 121 broods in 1996–2000 to study hatching and fledgling sex ratios. We did not find deviations of the 1:1 sex ratio at the population level during the length of the whole study period or on a yearly basis. Sex ratio was affected by age of the breeders, but not by territory quality (average productivity), quality of year, nearest nest distance or number of breeding pair in the population. Older breeders raised more females (i.e. the expensive sex), supporting the age dependent sex ratio hypothesis. In long‐lived species, older breeders tend to occupy better quality territories than younger ones, confounding some of the commonly used surrogates of territory quality, like frequency of occupation or year of first occupation; results obtained from observational studies should be interpreted with caution. We discuss alternative approaches and different scenarios that could provide new insight into the study of factors that influence sex ratio deviations in vertebrates.

Brood sex ratios in Merlins reflect characteristics of the associated breeding male and population density

“…Although there has been limited examination of how territory quality might influence brood sex ratios, Stauss et al (2005) found that Great Tit Parus major sex ratios shifted towards larger and presumably more expensive males in high-quality habitats, whereas Bell et al (2014) linked higher male quality to a higher proportion of more costly-to-produce male offspring in Blue Tits Cyanistes caeruleus, but noted that this influence only occurred on high-quality territories. In contrast, there was no impact of territory quality on Booted Eagles Aquila pennata, despite an apparent age (quality) of male effect leading to more female offspring when older males were involved (Ferrer et al 2009, Morandini et al 2020. Conversely, more female offspring were produced on higher quality territories for Common Buzzards Buteo buteo (Chakarov et al 2015), but age had no influence.…”

“…2009, Morandini et al . 2020). As male raptors become older, they gain hunting experience (Rutz et al .…”

“…Older male adults may be able to support broods with a higher proportion of energetically expensive female offspring as a result of their greater hunting experience, or possibly also as a result of less consequential impacts on their lifetime reproductive potential (Pianka & Parker 1975, Clutton‐Brock 1984, Morandini et al . 2020). Although we have more information about breeding males because they are more likely to show natal site fidelity than females (James et al .…”

“…2009, Morandini et al . 2020). Where resources for reproduction are limiting, it may be advantageous to produce more of the cheaper‐to‐produce sex (Komdeur & Pen 2002, West 2009).…”

“…Similarly, within-season variation in reproductive value for offspring may lead to male-biased sex allocations early in the season linked to enhanced fitness of the breeders (Dijkstra et al 1990, Daan et al 1996, Krebs et al 2002, Bartlow et al 2021. Parental characteristics such as age have been shown to influence brood sex ratios with young breeders producing more of the cheaper sex (Blank & Nolan Jr. 1983, Ferrer et al 2009, Morandini et al 2020. Where resources for reproduction are limiting, it may be advantageous to produce more of the cheaper-toproduce sex (Komdeur & Pen 2002, West 2009.…”