African Grass Owl Nest Site Use and Nest Survival in Fragmented Grassland Landscapes

Studies of nest site use in owls rarely involve species utilizing dynamic nest substrates such as grasses or sedges. We studied nest site use and nest survival in African Grass Owls (Tyto capensis) in the Highveld Grasslands of South Africa by monitoring 41 nests found in Gauteng and Mpumalanga between 2009 and 2017. We also placed radio- and GPS-tracking devices on seven nesting females to collect information about habitat use within their home ranges. Nest site use was investigated at the home range scale under three hypotheses related to how they select nest patches with respect to predator avoidance, namely the total foliage, potential-prey-site, and edge-effect hypotheses. Nest patch metrics and land-cover characteristics were modelled for differences between African Grass Owl nest patches and 41 random patches, using matched-pairs logistic regression. African Grass Owls used nest patches that were larger (x̄ = 3.6 ha vs x̄ = 1.9 ha) and older (x̄ = 2.9 years vs x̄ = 2.3 years) than random patches, and that had more Imperata cylindrica in closer proximity to the focal patch within the home range. They also used patches with a greater percentage wetland (x̄ = 7.4% vs x̄ = 4.3%) and a lower percentage developed land-cover (x̄ = 0.5% vs x̄ = 2.1%) within the home range than random patches. Multivariate models supported the total foliage hypothesis over potential-prey-site and edge-effect alternatives. Percentage developed land-cover was the most significant driver of nest survival, with daily nest survival decreasing with an increase in developed land-cover. The results of this study confirm that African Grass Owls are highly selective in terms of nesting habitat and emphasize their vulnerability with respect to habitat loss and fragmentation.