When the opportunist becomes specialist: Comparison of the small mammal diet of eagle owl and barn owl in southwest Spain

Small vertebrate assemblages are usually the result of predator accumulations. Predatory activity and predator preferences may have an influence in the general view of the total animal community from which they hunt their prey. Specialist predators only eat certain types of prey, while opportunistic predators take all prey species available within a given size range. The European eagle owl (Bubo bubo) is usually considered a complete opportunist predator due to the varied nature of its diet. However, in the Iberian Peninsula, this large raptor shows a specialist behaviour, not observed in other countries from Europe. Its diet mainly consists of rabbits (Oryctolagus cuniculus), while brown rats (Rattus norvegicus), hedgehogs (Erinaceus europaeus) and hares (Lepus spp.) are alternatively consumed in areas of low rabbit abundance (Penteriani and Delgado, 2019). Considering these dietary preferences, small mammal assemblages accumulated by eagle owls in the Iberian Peninsula could be extremely biased, causing distorting effects in palaeoecological inferences. In this work, we analysed how the diet of eagle owls considerably varies in comparison with other nocturnal predators and how it may affect palaeoecological inferences from Iberian sites. A bibliographic review of the taxonomic diversity of small mammals from 28 eagle owl nests was compared with 23 barn owl nests (Tyto alba), an overall opportunist predator, in the southwest Iberia. Results indicated important differences between both predator assemblages. In general terms, the body mass of the prey spectra hunted by eagle owls is usually between 200 g and 2 kg, when lagomorphs are considered, and between 200-800 g considering only small mammals (< 1kg). Barn owls usually accumulated prey of less than 200 g, including incidentally some young rabbits. These results indicate that an important bias related to prey size intake is present and this fact may affect to palaeoecological inferences. Differences were also observed in the representation of the different taxonomic groups, with murids being the most frequent prey consumed by barn owls and arvicolines and glirids the best represented taxa in eagle owl assemblages when lagomorphs are not considered. These differences in prey assemblages from different predators were pointed out by Andrews (1990), which demonstrated the presence of differences between the prey spectra for three different predators hunting over the same area within one-time period. The application of the Habitat Weighting Method indicates different environmental conditions depending on the predator evaluated. Barn owl assemblages are dominated by open dry/humid environments and open woodland and open humid areas are best represented in eagle owl accumulations when lagomorphs are included in these analyses. On the 28 4th Meeting of the ICAZ Microvertebrate Working Group Tübingen (Germany) 8th 10th September 2022 contrary, in eagle owl prey assemblages in which only small mammals are considered wetland and woodland areas constituted more than the 50% of the habitats represented. These preliminary results highlight the necessity of applying taphonomic analyses prior to any palaeoecological inference. Likewise, knowing the feeding ecology of the predators in the different areas inhabited also plays a key role once a predator has been identified as the main accumulation agent of a small mammal assemblage