Factors affecting population regulation of a colonial vulture

Abstract

Two hypotheses have been proposed to link population regulation to density‐dependent changes in demographical parameters: the habitat heterogeneity hypothesis (HHH) states that, as population density rises, an increasing proportion of individuals are forced to occupy low‐quality territories, which provokes a decline in average per‐capita survival and/or productivity although some individuals show no decline in fecundity; and the individual adjustment hypothesis (IAH), which suggests that increased densities lead to reductions in survival and/or fecundity by enhancing agonistic interactions, which affect all individuals to a similar extent. However, density‐dependent effects can be affected by density‐independent factors (DIF), such as weather. We test the effects of density dependence on annual reproductive success in Griffon Vultures Gyps fulvus at four spatial scales, nest‐site, cliff, colony and metacolony, in northern Spain from 2008 to 2015. Our results showed most support for the HHH at all scales. At the colony and cliff scale, IAH and DIF had similar importance, whereas there was little evidence of IAH at the metacolony and the nest scale. The best protected eyries (caves, potholes and sheltered ledges) produced the most fledglings and were used preferentially, whereas low‐quality eyries (exposed ledges or open crevices) were used only when the number of breeders increased. The significant interaction between breeding failure and density found for the more exposed eyries suggests that at higher densities, breeding pairs are forced to use poorer nesting areas, and the negative effect of density at the cliff scale could be due to the combined effect of a higher proportion of pairs using low‐quality eyries and the negative effect of rainfall.

Contrasting Trends in Abundance of Indian Vultures (Gyps indicus) Between Two Study Sites in Neighboring Indian States

“…Other potential threats to vultures across India include kite festivals, nest disturbance, habitat destruction, and drought (Chhangani, 2004;Kushwaha and Kanaujia, 2010;Hall et al, 2012;Roy and Shastri, 2013). Inclement weather has been shown to affect population trends of cliff nesting vultures (Zuberogoitia et al, 2019). India is also the fourth largest producer of wind energy globally with a number of wind farms in the state of Gujarat (bordering Rajasthan) and at least two in Rajasthan.…”

Environmental conditions modulate compensatory effects of site dependence in a food‐caching passerine

“…It could also exacerbate differences between territories, resulting in more pronounced density‐dependent relationships over time. For example, in high rainfall years, only the best protected nest sites of Griffon Vultures ( Gyps fulvus ) produced young (Zuberogoitia et al 2019). Our results suggest that a similar process is likely taking place with Canada Jays in Algonquin Park because, in years with an increased number of freeze‐thaw events, we saw a positive effect of the proportion of occupied territories that were of high quality on fecundity.…”

“…One of the challenges of describing how populations are regulated is being able to distinguish between site dependence and interference competition (Ferrer et al 2006, Beja and Palma 2008). To distinguish between these mechanisms, previous studies have examined how the coefficient of variance in fecundity (Ferrer and Donazar 1996), skewness of fecundity (Ferrer et al 2006), and territory‐quality‐specific estimates of fecundity (Kokko et al 2004, Zuberogoitia et al 2019) vary with density. However, simulation studies suggest that these metrics are not able to distinguish between site‐dependent regulation and interference competition (Beja and Palma 2008, but see Ferrer et al 2008).…”

“…Many studies investigating site‐dependent regulation have reported preferential occupation of high‐quality sites (Kokko et al 2004, Ferrer et al 2014, Flesch et al 2015, Zuberogoitia et al 2019) and that site dependence regulates fecundity (Krüger and Lindström 2001, Kokko et al 2004, Nevoux et al 2011, Zuberogoitia et al 2019). Additionally, when site‐dependent regulation is acting on a population, high‐quality territories can contribute a disproportionate amount of the overall number of young produced in a given year (Ferrer and Donazar 1996, Zuberogoitia et al 2019). A relatively small number of studies have investigated how extrinsic conditions, such as weather, can interact with territory quality and modulate the effects of density on fecundity, and results are equivocal, with some studies suggesting that high‐quality territories may buffer individuals from severe weather events (Franklin et al 2000, Ferrer et al 2014, Zuberogoitia et al 2019), while others suggest that weather events may influence all territories equally (Reynolds et al 2017).…”

“…Despite the fact that territoriality and variation in habitat quality are common features of natural systems, there is only limited evidence for site‐dependent regulation acting on wild populations, with most work focusing on birds (Rodenhouse et al 1997, Penteriani et al 2003, Kokko et al 2004, Nevoux et al 2011, Ferrer et al 2014), in particular birds of prey (Sergio and Newton 2003). Many studies investigating site‐dependent regulation have reported preferential occupation of high‐quality sites (Kokko et al 2004, Ferrer et al 2014, Flesch et al 2015, Zuberogoitia et al 2019) and that site dependence regulates fecundity (Krüger and Lindström 2001, Kokko et al 2004, Nevoux et al 2011, Zuberogoitia et al 2019). Additionally, when site‐dependent regulation is acting on a population, high‐quality territories can contribute a disproportionate amount of the overall number of young produced in a given year (Ferrer and Donazar 1996, Zuberogoitia et al 2019).…”