Increased hunting effort buffers against vole scarcity in an urban Kestrel Falco tinnunculus population

Capsule In years with low vole abundance birds visited hunting grounds more frequently and for longer.

Aims To describe diet composition, hunting behaviour, habitat choice and reproductive success of urban Kestrels during changing vole abundance.

Methods For five years, we studied the hunting effort of Kestrels in a medium-sized city during the breeding season. Pitfall traps were used for determining vole abundance. Kestrel diet composition was determined from pellet analyses. The number of eggs and offspring was recorded during at least two consecutive visits for each nest and each breeding stage.

Results In contrast to larger European cities, the Common Vole Microtus arvalis was a key part of the Kestrels' diet and did not fluctuate significantly according to vole availability. Reproductive success was quite high and stable throughout years with different vole abundance. In years of low vole abundance, the arrival frequency at hunting grounds and time spent there increased. During years with a low vole population, Kestrels had less hunting success and the rate of successful visits decreased. Therefore, Kestrels probably had to change hunting grounds more frequently. During low vole years Kestrels used less demanding techniques, e.g. perching, despite the lower success of these hunting techniques, to avoid extremely high energetic costs.

Conclusions An increase in hunting helps to maintain a proper diet and consequently reproductive success. Vole abundance did not change dramatically during the study period, as reported by studies from western and northern Europe. The proportion of ruderal habitats on the city periphery is higher than in more monotonous farmland habitats. Ruderal habitats can be important when Kestrels look for mammals other than voles, especially during vole scarcity.