The role of migration in the distribution of the brown rat in the UK agricultural landscape

The aims of this study were to understand the extent and causes of movements between local rat populations in an agricultural environment. This aim was achieved by examining the movement of rats between farm buildings and fields using trapping, tracking plates and video surveillance. There were temporal variations in rat captures; with more rats caught in autumn than in other seasons. Rats in reproductive condition were caught throughout the year. There was a significant difference between the weight of male and female rats, with males heavier than females, and body fat levels were demonstrated to increase with age. Rats were predominantly caught moving from farm buildings towards the fields during the spring and predominantly moved from fields into farm buildings during autumn. More fecund males moved into the farm during autumn accompanied by non-breeding females. Video monitoring of a single trap system provided evidence that rats are active during both day and night. The level of activity was relatively low in this study with an average of 5 sightings per day. Their direction of movement was not consistent and trapping appeared not to change rat behaviour. It appeared that farm buildings provided the most suitable habitat for brown rats all year round and their density remained constant if no control measures were taken. Small mammals dominated the Field habitat and there was little spatial overlap between brown rats and small mammals around farm buildings. In summer brown rats increased in the Field and in autumn small mammals showed their highest abundance in the Farm habitat. However, there was some potential for small mammals to compete for resources with brown rats in open field areas during summer. Farm sites contained more food than agricultural land. Food was available throughout the year at both sites, but was most abundant during winter at farm sites and during autumn in field sites. At the farm site supplemental feeding attracted rats and small mammals. An examination of skull morphology among three rat populations showed significant differences, though there was no evidence of a strong geographical component to variation. There was no significant difference in skull morphology between the sexes, though there was a significant interaction between sex and population. Variation in skull morphology among populations was probably linked to diet type, and food availability as well as genetic drift arising through reproductive isolation of the study populations.