Predation impact on threatened spur-thighed tortoises by golden eagles when main prey is scarce

Abstract

A reduction in adult survival in long-living species may compromise population growth rates. The spur-thighed tortoise (Testudo graeca) is a long-lived reptile that is threatened by habitat loss and fragmentation. Golden eagles (Aquila chrysaetos), whose breeding habitats overlap that of tortoises, may predate them by dropping them onto rocks and breaking their carapaces. In SE Spain, the number of golden eagles has increased in the last decades and the abundance of their main prey (i.e., rabbits Oryctolagus cuniculus) has decreased. Our aims were to 1) describe the role of tortoises in golden eagles’ diet, and 2) estimate the predation impact of golden eagles on tortoises in eagles’ territories and in the regional tortoise population. We collected regurgitated pellets and prey remains under eagle nests and roosts, and obtained information on tortoise abundance and population structure and rabbit abundance. We found that tortoises were an alternative prey to rabbits, so that eagles shifted to the former where the latter were scarce. The average predation rate on tortoises was very low at the two studied scales. However, eagles showed a marked selection for adult female tortoises, which led the tortoise sex ratio to be biased towards males in those eagle territories with higher tortoise predation. Whether this may compromise the spur-thighed tortoise long-term population viability locally deserves further attention.

Zooarchaeological investigation of the Hoabinhian exploitation of reptiles and amphibians in Thailand and Cambodia with a focus on the Yellow-headed Tortoise (Indotestudo elongata (Blyth, 1854))

“…This is consistent with the fact that, although some predators, including Monitor lizards, are known to hunt juvenile tortoise individuals, adult tortoises likely have few non-human predators, although some modern specimens bear traces of predation attempts (Ihlow et al, 2016), and predation on other Southeast Asian tortoise species has been reported (Platt et al, 2021). Large felids (Emmons, 1989) and eagles (Gil-Sánchez et al, 2022) are known to be able to hunt adult tortoises, but such predators would undoubtedly leave predation traces on the subfossil bone assemblages studied. Some very rare bones bearing porcupine traces indicate that these animals had a minor impact on some of the assemblages, but not enough to impact the zooarchaeological interpretations.…”

Hyperpredation of Freshwater Turtles and Tortoises by Subsidized Corvids

“…Turtles are reported occasionally and, when present, typically at a low frequency in the diet of predatory birds (e.g., Malan and Branch 1992; Tzankov and Milchev 2014; Walsh and Heinrich 2015). In some apparently exceptional cases, turtles are major prey items for avian predators (e.g., raptors, Clark 1986; gull, Branch and Els 1990; shrike, Barje et al 2005; Georgiev 2009; Gil-Sánchez et al 2022; Milchev 2022). Experimental evidence shows that hatchling turtles can experience size-specific mortality by avian predators (Janzen et al 2000).…”