Factors influencing diet, reproductive success and road mortality in the Barn Owl (Tyto Alba)

Wright, Rebecca and Hartley, Ian (2019) Factors influencing diet, reproductive success and road mortality in the Barn Owl (Tyto Alba). PhD thesis, Lancaster University.

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Abstract

In the UK, a decline in the Barn Owl population has been attributed to increased agricultural intensity and urbanization. Shifts in farming practices have resulted in a reduction of habitat diversity and homogenization of the UK's landscape, causing a reduction in the number and diversity of prey animals for predatory species. One species that has been affected by these ecological shifts is the Barn Owl. The expansion of road networks, to accommodate more vehicles on the road, has led to habitat fragmentation and ecological traps. The Barn Owl has shifted its feeding patterns due to the pressure of ecological traps caused by the expansion of road networks to accommodate more vehicles and are now the most frequent bird species encountered on road casualty surveys, with over half of known Barn Owl deaths being a result of wildlife-vehicle collisions. The objectives of this study were to investigate factors affecting diet and reproductive success in the Barn Owl, as well as to identify characteristics of Barn Owl road casualty hotspots. The study was conducted in Anglesey, north Wales. Diet was investigated through the morphological analysis of owl pellets; 377 pellets were collected from 26 nest/roost box locations during the Barn Owl breeding season and winter roosting season. Reproductive success was analyzed using data provided by the British Trust for Ornithology (BTO), and nest/roost box locations allowed habitat analysis to be performed using Arcmap GIS. Barn Owl road mortality hotspots were investigated using data provided from the North and Mid Trunk Road Agency in correspondence to Arcmap GIS and Google Earth, which allowed for habitat analysis. The date that the road mortality was reported to the North and Mid Trunk Road Agency was used for the seasonal analysis. Data provided by the BTO allowed the effects of population in relation to road casualties to be studied, using reproductive success as a proxy for population success. The results of the study found, 12 species of vertebrates form regurgitated Barn Owl pellets, with the three most abundant being the Field Vole (60.52%), Common Shrew (16.31%) and Wood Mouse (8.65%). The results of the study found that the mean field vole weight per pellet was higher outside the Barn Owl breeding season (64.65g) than during it (53.53g). The mean Wood Mouse weight per pellet was lower outside the breeding season (4.06g) when compared to inside (7.16g). The home range of the Barn Owl is typically within the 1km-4km radius around the nest site, which is compromised of a variety of habitats depending on location. The Barn Owl home range habitat composition on Anglesey varied from 14-99% agriculturally improved grassland, 0-43% arable horticulture and 0-9% fen marsh swamp. These habitats were found to have a negative association with the number of successful fledglings per nest, which could reflect prey availability and abundance within these habitats, as an owl which can provide more food will be able to raise more young. However, small mammal trapping would be needed to confirm the abundance and availability of prey in these habitats. There were 117 Barn Owl road casualties on the A55 Anglesey recorded by the North Trunk Road Agent between 2001 and 2017, this equates to 0.196 Barn Owls/year/km. The month of April incurred the most deaths (18) and the least August, with only 1 death being reported. No relationship was identified between the time of year and the number of Barn Owl road casualties. Additionally, no relationship was found between habitat and owl road mortalities. However, a relationship was found between the presence of grass slope verges at the side of the A55 and bi-monthly road mortality. For instance, the number of deaths of barn owls in areas with grass sloping verges were reported as 29 and those without grass slopes recorded as 87. In conclusion, the results from this study highlights that in areas of intense agriculture, maintaining species rich diverse habitats is important for the success of Barn Owls. The results of this study suggest conservation efforts should be focused on the restoration of varied habitats in order to provide rich biodiversity through ecological management. The conservation management of habitats of different levels - which contain a wider variety of vegetation - should allow predatory birds, such as the barn owl, to exploit habitats at different times of the year depending on food abundance. Additionally, measures have been suggested t prevent the occurrence of wildlife-vehicle collisions such as low flight barriers which would target low flying animals such as the Barn Owl. Many studies have outlined the importance of grass verges as foraging grounds for Barn Owls, (Taylor, 1994 and Bolger et al., 2001), which suggest instead of removing these key foraging grounds and introducing manmade structures, conservation efforts could focus on making these foraging grounds safer. For instance, the introduction of grass lope verges could be used as a wildlife-vehicle collision preventative measure in the future.