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

Abstract

Population‐level estimates of offspring sex ratios in birds typically approximate parity whereas biased ratios within nests are not uncommon. In sexually dimorphic raptors, the costs and relative fitness benefits of rearing male and female progeny vary with changing environmental circumstances. This may lead to substantial deviations from balanced investment in offspring of a particular sex by individual parents. Based on a 13‐year dataset for breeding Merlins Falco columbarius in Saskatoon, Canada, we used a model selection approach to assess the influence of parents, nest‐mates and nesting area on brood sex ratio during the nestling phase. The best model for predicting brood sex ratio included age of the breeding male and brood size for each nest (n = 127); nests with older male breeders and smaller brood sizes had more female young. The population‐level annualized average proportion of male offspring was 0.472 ± 0.017 (mean ± standard error), but tended towards greater production of female young during an initial period of population growth (8 years, 10–21 pairs; proportion male 0.435 ± 0.031) versus a period when the population fluctuated around a presumed carrying capacity (11 years, 24–33 pairs; proportion male 0.500 ± 0.017). Energetics appears to be a finely tuned mechanism driving sex ratio allocation in Merlins at both brood and population levels. Provisioning food for young in the nest represents the male's ability to successfully capture prey, reflecting his age and/or experience, as well as the availability of prey to the male. Confounding this mechanism to determine sex ratio allocation are the pressures created by population dynamics that dictate competition for resources both within the nest (brood size) and external to the nest (population density).

Insularity determines nestling sex ratio variation in Egyptian vulture populations

“…Effects of brood size on offspring sex ratio have been documented in several raptor species, with greater biases in the smallest or largest broods, while intermediate-sized broods tend to show a balanced sex ratio (Dijkstra et al, 1998;Warkentin et al, 2022).…”

“…Long‐term studies are essential for understanding the ecological and evolutionary processes that operate in wild populations (Sheldon et al, 2022 ). Variation in offspring sex ratio has been frequently addressed over the last century (Mayr, 1939 ; Payevsky, 2021 ), although rarely using long‐term monitoring data (Rosenfield et al, 2015 ; Warkentin et al, 2022 ). In birds, offspring sex ratio can be categorized as primary or secondary depending on whether it refers to the proportion of males over females at fertilization or at hatching, respectively (Mayr, 1939 ).…”

“…Offspring sex ratio in raptors is often biased by food availability, with poor years often associated with higher production of males, the less costly sex (Arroyo, 2002 ; Dzus et al, 1996 ; Wiebe & Bortolotti, 1992 ). The age of breeders, which is closely related to their experience and performance (Curio, 1983 ; Forslund & Pärt, 1995 ), also seems to influence offspring sex ratio in raptors, with younger and less experienced breeders tending to raise males more often (Blank & Nolan, 1983 ; Ferrer et al, 2009 ; Warkentin et al, 2022 ). Breeding timing can also affect offspring sex ratio, either by increasing the proportion of females at the beginning of the breeding season and that of males at the end (Mora et al, 2010 ; Ristow & Wink, 2004 ; Tschumi et al, 2019 ), or vice versa (Daan et al, 1996 ; Smallwood & Smallwood, 1998 ; Tella et al, 1996 ), depending on the species.…”

“…Effects of brood size on offspring sex ratio have been documented in several raptor species, with greater biases in the smallest or largest broods, while intermediate‐sized broods tend to show a balanced sex ratio (Dijkstra et al, 1998 ; Warkentin et al, 2022 ). Most raptors also show asynchronous hatching, in which the oldest and thus largest nestling receives more food, irrespective of its sex (Slagsvold, 1990 ).…”