The use of tracking tunnels to monitor the activity of small mammals in habitats associated with the northern wheatear (Oenanthe oenanthe)

Abstract: Nest predation is the main cause of nest mortality among birds and is thought to be the main reason for breeding failure amongst northern wheatears (Oenanthe oenanthe) in Swedish farmland habitats. Previous studies suggest that small mammalian predators and snakes are important nest predators for wheatears. However, other factors behind nest predation among wheatears have not been thoroughly studied. Here I used ink tracking tunnels (with a piece of meat as bait) to monitor the activity of small mammals in relation to landscape elements (such as linear, forest edge, open area, tall vegetation and stone piles) and land-use types (pastures, crop fields and ungrazed grasslands) in wheatear breeding territories. I also investigated whether the activity of the mammals changed over time using four survey periods (each period represents the time when collecting tracks from tunnels). The two first survey periods took place during the peak of incubation for wheatears and the last two during the peak of nestling provisioning. Footprint tracks from the tunnels revealed that small mammals (shrews, mice, rats, weasels, stoats and cats), birds, lizards, insects and amphibians visited the tunnels. The activity of small mammals increased over time so that the highest tracking rates occurred when wheatears were feeding nestlings. The proportion of tunnels with tracks varied according to landscape features, with the highest percentage of tracks found in forest edges (35.5 %) and the lowest in stone piles (17.6 %). However, in stone piles the proportion of tunnels with tracks of small mammals was dependent on land-use type. Whereas mammal prints were generally rare in stone piles located in pastures (12% of all mammal tracks in pasture) they were much more frequent in crop fields (33 % of all mammal tracks in crop): possibly because stone piles offer the only available predator refuge in crop fields. The increase of mammal activity between the four survey periods differed between land-use categories with a greater increase in grasslands than in pastures and crop fields. Tunnels with tracks of mammals were positively correlated with the amount of local shrub coverage and tall vegetation. No connection was found between proportion of mammal tracks and breeding success for the northern wheatear. This study suggests that there are temporal and spatial variation in small mammal activity and demonstrates the value of using tracking tunnels in a Swedish farmland landscape to increase the knowledge of predator movements. However, further studies with long-term data on small mammal activity are needed for to draw conclusions about mammal activity and breeding success for the northern wheatear.