Habitat preference models for nesting Eagle Owls Bubo bubo: How much can be inferred from changes with spatial scale?

Aims: To analyze whether habitat preference patterns of the Eagle Owl Bubo bubo change with spatial scale in an area of very high rabbit Oryctolagus cuniculus density as compared to an area of lower prey availability (Martinez et al., 2003a).Location: An area of over 2,100 km2 located in the province of Toledo, central Spain.Methods: 17 habitat variables were measured around 100 nests that were occupied between 1999 and 2003 and around 100 random points at four spatial scales (circular areas of 250, 500, 1000 and 1,500 m of radius). The range of spatial scales was established on the basis of the observed high density of Eagle Owl nests in the study area, the second highest reported to date. Habitat features of occupied and random areas were compared by means of logistic regressions for each spatial scale. The possible effect of the spatial autocorrelation was assessed using as additional predictors all the terms of a cubic equation defined by the coordinates of the sampling points. Results: Topographic irregularity and distance to the nearest stream were included into the models at all scales as the main predictors of the presence of Eagle Owl nests, classifying a high percentage of both random and occupied points. Percent correct classification of the models did not change across scales. Positive selection of areas with irregular topography and close to streams can be interpreted as due either to a choice of protected areas for nest location and/or of areas with high prey availability. At the 500 meters of radius scale the model included marginally the positive selection of areas with high covers of dehesa, a variable that may be interpreted in the same way that the selection for the two main predictors. Two terms (X and Y2) of the cubic equation witch defined the spatial distribution of the nest and random points entered into all the models as relevant factors.Conclusion: No hierarchical patterns of habitat preference were detected, contrasting with results from a previous study carried out in an area of lower rabbit abundance (Martinez et al., 2003a). This result may be related to the high abundance of rabbits in central Spain, witch would have lead to a preference for good nesting places rather than for areas with higher than average prey abundance. Methodological effects cannot be ruled out, however, in either this comparison or in multiscale habitat preference studies in general. Independent data on the foraging behavior of the involved species and/or on the fitness consequences of habitat selection would be necessary to ascertain whether results from multiscale studies truly reflect underlying biological processes (and what processes) or are biased by the parameter values of the modeling approach.

Joaquin Ortego & Mario Diaz, Ardeola 51(2), 2004, 385-394