Living on the edge: assessing the extinction risk of critically endangered Bonelli's eagle in Italy

Long-term changes in the breeding period diet of Bonelli

Context Dietary analyses are essential to achieve a better understanding of animal ecology. In the case of endangered species, assessing dietary requirements is crucial to improve their management and conservation. The Bonelli’s eagle (Aquila fasciata) has experienced a severe decline throughout its breeding range in Europe and, in Italy, fewer than 50 pairs remain, and only in Sicily. This species is subject to major threats, including changes in landscape composition and, consequently, prey availability, which is further aggravated by the occurrence of viral diseases in the case of rabbits. Aims To provide current data on the diet of the Bonelli’s eagle in Sicily during the breeding period and to examine dietary shifts with regard to previous studies conducted in the same study area. To discuss possible implications for conservation of the Italian population of this endangered species. Methods We used a combination of three methods, including pellet analysis, collection of prey remains, and imagery from camera-traps installed at nests, to examine the diet of 12 breeding pairs of Bonelli’s eagle from 2011 to 2017. We compared this information with data collected between 1993 and 1998 in the same study area. Key results In number, birds were the most frequently predated items (61.6%), followed by mammals (36.88%) and reptiles (1.52%). However, in terms of biomass, mammals were the main prey (65.71%), followed by birds (34.12%) and reptiles (0.17%). There was a decrease over the course of the current decade in the consumption of European wild rabbit (Oryctolagus cuniculus), which was compensated for with an increase in both dietary diversity and breadth in bird consumption, a trend not observed in the earlier study in the same region. Conclusions Here, we provide an updated assessment of diet composition of Bonelli’s eagle during the breeding period. Interestingly, we found significant differences within the study period (2011–2017) in terms of frequency of occurrence, percentage of biomass, dietary diversity and dietary breadth in a species at risk. Furthermore, we found significant differences between the two study periods in both frequency and percentage of biomass, with significant changes in the consumption of lagomorphs and birds. Implications Our results indicated that shifts in the diet are linked to changes in prey abundance, which may be contributing to population declines in the Bonelli’s eagle population in Sicily. Overall, measures aimed at increasing main dietary prey should be promoted to favour occupation of new territories and enhance vital demographic parameters (i.e. breeding success and survival rate) of Bonelli’s eagle across the species range. This would be particularly important for small isolated populations such as the Sicilian one.

Population Viability and Vital Rate Sensitivity of an Endangered Avian Cooperative Breeder, the White-Breasted Thrasher (Ramphocinclus brachyurus)

Social behaviors can significantly affect population viability, and some behaviors might reduce extinction risk. We used population viability analysis to evaluate effects of past and proposed habitat loss on the White-breasted Thrasher (Ramphocinclus brachyurus), a cooperatively breeding songbird with a global population size of <2000 individuals. We used an individual-based approach to build the first demographic population projection model for this endangered species, parameterizing the model with data from eight years of field study before and after habitat loss within the stronghold of the species' distribution. The recent habitat loss resulted in an approximately 18% predicted decline in population size; this estimate was mirrored by a separate assessment using occupancy data. When mortality rates remained close to the pre-habitat loss estimate, quasi-extinction probability was low under extant habitat area, but increased with habitat loss expected after current plans for resort construction are completed. Post-habitat loss mortality rate estimates were too high for projected populations to persist. Vital rate sensitivity analyses indicated that population growth rate and population persistence were most sensitive to juvenile mortality. However, observed values for adult mortality were closest to the threshold value above which populations would crash. Adult mortality, already relatively low, may have the least capacity to change compared to other vital rates, whereas juvenile mortality may have the most capacity for improvement. Results suggest that improving mortality estimates and determining the cause(s) of juvenile mortality should be research priorities. Despite predictions that aspects of cooperative systems may result in variation in reproduction or juvenile mortality being the most sensitive vital rates, adult mortality was the most sensitive in half of the demographic models of other avian cooperative breeders. Interestingly, vital rate sensitivity differed by model type. However, studies that explicitly modeled the species' cooperative breeding system found reproduction to be the most sensitive rate.