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Steen R., Løw L.M., Sonerud G.A., Selås V. & Slagsvold T. 2011. Prey delivery rates as estimates of prey consumption by Eurasian Kestrel Falco tinnunculus nestlings. Ardea 99: 1–8. In altricial birds the type of prey selected by parents for their nestlings may affect the allocation of time and energy spent on hunting, preparing prey and feeding the nestlings, which in turn may affect the rate of provisioning. Raptors take relatively large prey items, which facilitates the quantification of rates of prey items and prey mass delivered to nestlings. Estimates of rates of prey delivery in raptors are nevertheless few and have been based on direct observations from a hide in combination with analyses of prey remnants and regurgitated pellets. To obtain better estimates we video monitored prey deliveries at 55 nests of Eurasian Kestrels Falco tinnunculus. Of the 2282 prey items recorded, voles were most abundant by number, followed by birds, shrews and lizards, while insects and frogs were rare. An average brood of 4.3 nestlings was estimated to consume 18.3 g/h, hence a nestling consumed on average 4.2 g/h. This is equivalent to 67.8 g/d, given an average daily activity period of 16.1 h. The estimated delivery rate of prey items required to feed an average brood in our study was 91 per h if the kestrels had provided only insects, and 3.4, 1.9, 0.83 and 0.52 if they had provided only lizards, shrews, voles or birds, - respectively. This corresponds to one prey delivery per 40 s if feeding solely on insects and one per 18, 32, 75 and 120 min if feeding solely on lizards, shrews, voles or birds, respectively. We argue that kestrels in the boreal forest would be unable to raise an average brood solely on insects or lizards, unlikely to do so solely on shrews, but able to do so solely on voles in a vole peak year.

Prey Delivery Rates as Estimates of Prey Consumption by Eurasian KestrelFalco tinnunculusNestlings