Nestling sex ratio is unaffected by individual and population traits in the griffon vulture

Abstract

Variation in offspring sex ratios is a central topic in animal demography and population dynamics. Most studies have focused on bird species with marked sexual dimorphism and multiple-nestling broods, where the offspring sex ratio is often biased due to different individual or environmental variables. However, biases in offspring sex ratios have been far less investigated in monomorphic and single-egg laying species, and few studies have evaluated long-term and large-scale variations in the sex ratio of nestling vultures. Here, we explore individual and environmental factors potentially affecting the secondary sex ratio of the monomorphic griffon vulture Gyps fulvus. We used information collected at three breeding nuclei from central Spain over a 30-year period (1990–2020) to analyse the effects of nestling age, parental age, breeding phenology, conspecific density, population reproductive parameters, and spatial and temporal variability on nestling sex. Sex ratio did not differ from parity either at the population or the nuclei level. No significant between-year differences were detected, even under highly changing conditions of food availability associated with the mad-cow crisis. We found that tree nesting breeders tend to have more sons than daughters, but as this nesting behavior is rare and we consequently have a small sample size, this issue would require additional examination. Whereas further research is needed to assess the potential effect of breeder identity on nestling sex ratio, this study contributes to understanding the basic ecology and population dynamics of Griffon Vultures, a long-lived species with deferred maturity and low fecundity, whose minor deviations in the offspring sex ratio might imply major changes at the population level.

Insularity determines nestling sex ratio variation in Egyptian vulture populations

“…30 years, is consistent with that found for this species using subsamples of this dataset (Grande, 2006;Sanz-Aguilar et al, 2017) and in other species of vultures from Europe (Bosé et al, 2007;Davidovic et al, 2022;Gómez-López et al, 2022;López-López et al, 2011;Villegas et al, 2004) and Asia (Arshad et al, 2009). We also found that the offspring sex ratio of the Egyptian vulture remained stable at 1:1 over the years, similar to studies with the Griffon Vulture (Gómez-López et al, 2022) and the Cinereous Vulture (Villegas et al, 2004) in regions of peninsular Spain. However, offspring sex ratio was male-biased in the Canary Islands, while a slight trend toward females was observed in peninsular Spain.…”

“…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). Following Fisher's sex allocation theory (Fisher, 1930), numerous studies have confirmed that offspring sex ratio tends to be 1:1 in different species and populations (e.g., Clutton-Brock, 1986;Donald, 2007;Ellegren et al, 1996;Gómez-López et al, 2022;Gowaty, 1993). Although Fisher's statement of sex ratio parity is based on the idea that the cost of rearing male and female offspring is the same (Fisher, 1930), this is not true in species that exhibit sexual size dimorphism (Komdeur & Pen, 2002;Navara, 2018;Szász et al, 2012), which would cause differential parental investment in male and female offspring and, therefore, a biased sex ratio (Szász et al, 2012).…”

Interspecific differences in plasma carotenoid profiles in nestlings of three sympatric vulture species

“…Blood was conserved with dry heparin and transported in a cooler, and centrifuged at 13,000 g for 10 min in the laboratory to obtain plasma, which was frozen at −20 °C until analysis. A small quantity of blood was used for sexing individuals through molecular procedures after DNA extraction ( Gómez-López et al 2022 ). The nestlings analyzed for carotenoid levels were sampled in 2013 (griffon vulture, n = 35, cinereous vulture, n = 5, Egyptian vulture, n = 11) and 2004 (cinereous vulture, n = 5).…”

“…The variation in total plasma concentration (mg/mL) of xanthophylls, carotenes, and total carotenoids according to age and sex were evaluated for each vulture species by using generalized linear models (GLMs) with Gaussian error distribution and identity link function. Wing length was used as a surrogate of age in the growing nestlings ( Elósegui 1989 ; Gómez-López et al 2022 ).…”

A long‐lasting, distant journey of a male griffon vulture informs on the success of differential parental investment

“…During June and September 2021, 15 adult griffon vultures ( Gyps fulvus ; seven males, eight females) were captured with a large cage baited with livestock carcasses in the colony located in Hoces del Río Riaza, Segovia, central Spain (Almaraz et al, 2022). We selected adult individuals previously ringed as nestlings (Gómez‐López et al, 2023) or aged by their plumage characteristics. Individuals were marked with PVC alphanumeric rings (when they were not previously ringed) that allowed their identification at distance, bled (<0.1 mL from the brachial vein) to determine their sex by molecular methods (details in López‐Rull et al, 2015) and fitted with telemetric tracking devices (50 g solar‐powered GPS‐GSM transmitters; Ornitela, http://www.ornitela.com) programmed to record fixes (i.e., GPS positions) on a flexible schedule of 5–15 min depending on battery charge.…”