| Summary: | I used video monitoring to record prey deliveries at four nests of the northern hawk owl (Surnia ulula) in southern Norway. One nest was monitored in May 2014, while the three other nests were monitored in May-July 2017. Brood size varied from four to eight and no nestlings died during the monitoring. During the net total video monitoring of 960 hours, a total of 680 prey items were recorded delivered at the nest, giving a total gross prey body mass of 23016.8 g. Microtines (Arvicolinae) were the most common prey by number and gross prey body mass, making up 92.8% and 94.5%, respectively of identified prey. Of the microtines, Microtus voles were the most common prey by number and by gross prey body mass. In total 53 prey items were identified as Norway lemming (Lemmus lemmus) and 26 were identified as avian prey. The probability of a prey delivered at the nest being a Norway lemming increased throughout the season, while the probability of an avian prey tended to increase throughout the season. The probability that a prey item was delivered by the female rather than the male increased with nestling age, and was > 0.5 when the oldest nestling in a brood was 15 days old or more. The hawk owl delivered prey at all hours, but the probability of a prey delivery had two marked peaks, one after sunrise and one before sunset, and was low around midnight. There was an increase in number of prey items delivered per nestling per day with increasing nestling age, and the number of prey delivered at the nest per day increased with number of nestlings in the nest. The probability of a prey being decapitated prior to delivery increased with prey body mass. The probability of a nestling feeding unassisted rather than being fed by the female did not exceed 0.50 during the nestling period, and was predicted to exceed 0.50 when the fledglings were 43 days old. Handling time when the female fed the nestlings, tended to decrease with nestling age, probably because the size of the pieces of a prey that the female fed to the young ...
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