Variation in the abundance of the Barn Owl Tyto alba in the Tagus Estuary: effect of land use, grazing and soil interventions

Abstract

The South Floodplain of Vila Franca de Xira is of high importance for the Barn Owl Tyto alba during the post-natal dispersal period, a phase of the life cycle when the condition of individuals is crucial for survival. The objective of this study was to evaluate the temporal and spatial variation in Barn Owl abundance in the floodplain relative to land use, cattle grazing (stocking rate and length of stay), and soil interventions (surface tillage, deep tillage and seeding). From 2007 to 2014, between August and December, we repeated a 22.2 km long nocturnal owl survey transect by car 97 times. The transect was divided into four sections, representative of land use in the area: a rice field section and three sections with different levels livestock management (stocking density and temporal use). Within these sections, plots (areas with one "constant" land use) were identified and noted. The variables were associated with the plot and grouped by section and by season. Statistical analysis was based on the comparison of linear mixed effect models using the Akaike information criterion (AIC), with the section as random factor. Barn owl abundance is 1.21±2.60 ind/km in grazed pastures, 1.13±1.48 ind/km in ungrazed pastures, and 0.97±1.48 ind/km in rice fields. Both land use and the presence of cattle contributed to explain the variation in owl abundance; the former assumed greater importance in summer (Akaike weight wi=0.81; fall: wi=0.51) whereas the latter assumed greater importance in the fall (wi=0.78; summer: wi=0.67). In areas with cattle, the effect of grazing season (ΔAIC=0, wi=0.67) overcomes the effect of stocking density (ΔAIC> 4; wi=0.07) and temporal use (ΔAIC>10; wi=0). Soil interventions appear not to influence the general pattern of Barn Owl abundance in the area. The results suggest the need to explore the effect of other variables related to the spatial distribution of cattle in the plots.