Adaptability of a specialist predator: the effects of land use on diet diversification and breeding performance of Verreaux's eagles

Abstract

Specialist predators are generally negatively impacted by habitat change. Predators that inhabit transformed areas are usually forced to diversify their diet and this departure away from traditional resources can have negative consequences for fitness and demographic parameters. We consider this relationship as it applies to Verreaux's eagles Aquila verreauxii, which is typically considered to be a highly specialised predator of hyraxes (Procavia and Heterohyrax spp.). We investigate diet in relation to land cover in two adjacent areas of South Africa and explore the links between diet diversity, the percentage of hyrax consumed, and the breeding performance of eagles. We also examine these same patterns using data from other studies. We found that diet diversity was greater in the agriculturally developed Sandveld region compared to the natural Cederberg region. Proportions of the three main prey types were correlated with the proportion of agriculturally developed land around the nest site. Breeding performance was correlated with the diet, but not in the manner expected, with breeding productivity being greater in regions with large diet diversity and a small proportion of hyrax in the diet. We found similar patterns when placing our results into a broader geographical context using other dietary studies of Verreaux's eagles, suggesting our results were not unique to our study system. Thus, our results suggest that diet diversification does not necessarily impinge on breeding performance in the presence of adequate alternative prey resources. This research adds to the growing number of studies suggesting that some predators may be adaptable up to a threshold level of habitat transformation. These results have implications for predicting changes on such species by anthropogenic habitat transformation and highlight the potential for agriculturally developed areas to maintain a conservation value when habitat heterogeneity is maintained.

Regional and temporal variation in diet and provisioning rates suggest weather limits prey availability for an endangered raptor

“…A possible explanation for this seasonal shift in diet could be that inland breeders have more opportunities for hunting birds later in the season. Inland nests were often surrounded by a mosaic of natural vegetation, pastures and occasionally wheat fields, which may increase the availability of alternative food resources such as reptiles or grassland birds such as Common Quail Coturnix coturnix (Ogada 2009, Murgatroyd et al 2016). However, on inspection of provisioning rates, we found that, overall, the frequency of birds or reptiles delivered to inland nests was lower compared with coastal ones, suggesting a constraint rather than an adaptive or opportunistic shift.…”

Deforestation May Trigger Black-and-Chestnut Eagle (Spizaetus isidori) Predation on Domestic Fowl

“…The trophic niche breadth of S. isidori in Ciudad Bol ıvar and Jard ın-the localities with the largest proportion of forest on their landscapes and where the eagle hunted a wider variety of prey-compared with values calculated for Gachala ( Table 1), suggest plasticity on the part of this eagle that allows it to feed on different types of prey as a function of what is available in environments modified by humans. This could be related to changes in the composition of the landscape induced by anthropic processes that can lead to variations in the availability of prey eaten by raptors (i.e., Garcia-Heras, Mougeot, Simmons, & Arroyo, 2017;Murgatroyd, Avery, Underhill, & Amar, 2016;Rebollo et al, 2017). In Spain and Azerbaijan, in the face of changes in food availability induced by humans, a species with a mainly carrion-feeding habit such as the Griffon Vulture (Gyps fulvus) ate mainly domestic animals (Karimov & Guliyev, 2017;Margalida et al, 2014).…”

Nestling White-backed Vulture (<i>Gyps africanus</i>) eaten by a Verreaux’s Eagle (<i>Aquila verreauxii</i>) at a nest occupied for a record 21 years.

“…The diet of Verreaux's Eagles generally comprises Rock Hyrax (Procavia capensis) and Bush Hyrax (Heterohyrax brucei) as the predominant prey species (Gargett 1990;Barry and Barry 1996;Chiweshe 2007), but in certain areas the diet of these eagles may be more catholic (e.g. Murgatroyd et al 2016). At least 47 species of mammal, 43 species of bird, 10 species of reptile and some fish (Brown et al 1982;Boshoff et al 1991;Davies 1999;Chiweshe 2007;Scholte 2010;Armstrong and Avery 2014) have been recorded in their diet (Table 1), but this is the first record of a Verreaux's Eagle eating a White-backed Vulture.…”