Interhabitat differences in the diet of the Eagle Owl (Bubo bubo) in the mid Ebro river valley (NE Spain): effect of European rabbit (Oryctolagus cuniculus) availability

Full article

Published:

Volume 45(1), June 1998. Pages 35-46.

Language:

Spanish

Original title:

Diferencias interhábitat en la alimentación del Búho Real (Bubo bubo) en el Valle medio del Ebro (NE de España): efecto de la disponibilidad de conejo (Oryctolagus cuniculus)

Keywords:

Bubo bubo, diet, Ebro basin, interhabitat differences, rabbit availability.

Abstract:

In this paper, we examine the trophic differences between two nearby subpopulations of Eagle Owl that inhabit a climatic and geographical homogeneous area (mid Ebro valley, Aragón, NE Spain) as related to small-scale diferences in habitat types and rabbit availability. This fine-grained approach has been scarcely considered in Mediterranean habitats of southern Europe. The climate of the study area is Mediterranean-semiarid (rainfall around 300 mm). Between 1994 and 1998 we examined 3670 prey items (Table 1) found in pellets of 14 pairs (6 in northern area and 8 in the southern one). The proportion of habitats in a circular surface of 2.4 km radius around each nest was determined. "Humid habitats"included irrigable lands, groves and river banks while "dry habitats"included dry lands, Mediterranean scrub and pine woods of Pinus halepensis. On the other hand, the density of european rabbits, the main prey of Eagle Owls in Mediterranean areas of the Western Paleartic, was estimated by means of 25 and 30 km nocturnal transects driven in winter and spring of 1995. Both subpopulations differed in the proportion of habitats available and in the availability of European rabbits found appropriate habitats (North: 25-50% of "humid habitats" and 10.4 rabbits/100 km; South: 0-10% and 56 rabbits/100 km). The nucleus of both subpopulations were 30 km away from each other. We carried out two principal components analysis (PCA) (Tables 2 and 3) that showed the replacement of Lagomorpha, Alectoris rufa and Invertebrata by little Muridae and Pisces in the frequency of prey items captured between the southern and the northern area and that of Lagomorpha by small Muridae, Rattus and Columbidae in the biomass contribution of prey between areas. In the northern area, Mus spretus was numerically the most important prey, while Oryctolagus cuniculus was the most captured species in the southern territories (Table 4). In terms of biomass, the rabbit was the main prey for both areas, although a lesser importance in the northern area was detected. Secondary prey showed different frequencies of appearance between both subpopulations: fish were captured only in northern territories, while reptiles and invertebrates were significantly more taken in the southern ones. Moreover, results of the PCAs showed that the importance of other prey linked to "dry habitats"such as Alectoris and Invertebrata in the diet of Bubo, was significantly related to the frequency of rabbits in such diets, suggesting a change in the use of the habitat in northern territories. Likewise, the differences in the average size of prey (Fig. 1) and trophic diversity (Table 4) between both subpopulations was determined. The mean weight of prey was 87 g in the northern area and 266.7 g in southern territories. Only trophic diversity expressed as biomass frequencies showed highly significant variations between both areas, revealing that Bubo diversifies its energy sources in northern area due to the lesser abundance of an optimal species like the rabbit.