Abstract
Bird nest selection in forests can be influenced by the composition of key structural elements and resources. This has important consequences in terms of species population dynamics since it can determine reproduction success. Here, we assessed Common raven nest-site selection and reproductive success, and how these might be determined by foraging behavior and habitat structure. A previously documented breeding raven population that exerts high predation pressure on young Spur-thighed tortoises (Testudo graeca) in a Mediterranean forest was monitored. Generalized linear mixed models were performed to determine the singularities of the trees with nests and the drivers of reproductive success of breeding pairs of ravens. The results showed a high density of breeding pairs in the study area (0.8 pairs/km2), which selected taller trees in areas with higher bare ground cover and a high density of tortoises for nesting. Nests were spatially aggregated; breeding pairs occupied smaller territories and intraspecific competition seemed relaxed, reflecting the abundance of food resources. Most breeding pairs occasionally predated on young tortoises. Tortoises seem to play a part in raven reproductive success in our study area, which might be associated with the availability/catchability of young tortoises. The study illustrates that Spur-thighed tortoise distribution and abundance plays a role in the breeding behavior of ravens and is mediated by habitat structure. Understanding the drivers of nest-site selection and the breeding behavior of ravens is pivotal to implementing appropriate habitat management and conservation strategies across their distribution range, particularly in areas where ravens potentially affect threatened species.
Highlights
The trees with and without nests were characterized by heights of 12.2 ± (SD) 1.7 m and 9 ± 1.7 m, respectively, and bare ground cover of 10 ± 7.3 and 3.9 ± 5.6%, respectively
Maamora forest is not an exception, and breeding pairs selected taller trees and trees located in areas with more bare ground cover and a higher density of tortoises, the latter possibly offering a high number of detectable tortoise juveniles [29]
Other studies have documented the distance to the road as a factor related to nest-site selection; shorter distances being preferred for nesting [48] because they offer a higher availability of food resources, which may be related to higher reproductive rates [49]
Summary
The Common raven Corvus corax is considered both as an opportunistic species, which appears to have a generalist diet at the population level [9], and as a forest specialist well adapted to exploiting forest food resources (e.g., predation on forest birds [10]). Their feeding habits may gather at local food resources showing different degrees of site fidelity for varying lengths of time depending on whether they are single birds (highly vagrant and with lower site fidelity) or territorial breeding pairs [11,12]. In natural ecosystems, ravens naturally breed in trees [15,21,22] and prefer high trees, which suggests that forest structure may be a key feature in raven breeding behavior [8,14,15]
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