Measuring productivity of Ardeidae is often problematic, challenged by difficulties in site access and the need to avoid disturbing nesting birds. Drones are increasingly used for ecological applications and their use seems to at least partially overcome these problems. The present study aimed to assess the effectiveness and safety of drone-conducted monitoring of productivity in Purple Herons Ardea purpurea breeding in NE Italy wetlands. The accuracy of drone counts using two drone approaches (overflight vs close-up) was compared using their combination as assumed true counts. The 14 colonies studied hosted 615 nests, the majority of which were located in reedbeds. Colony size did not differ in relation to habitat type. Colonies showed significant differences in productivity between habitats, with more young fledged in reedbed-located colonies than in bushes. Overall the accuracy of both overflight and close-up drone counts remained high in serial revisits throughout the breeding period. Drone surveys caused a low degree of disturbance. No apparent negative effects of drone flights on nesting pairs, clutches or young were observed. The study shows that drone use allowed an effective measurement of Purple Heron productivity that was rapid and safe for the birds.
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