BARKEMA-DROST L.P. & E. VAN DER VELDE (2016) Nestling diet of Marsh Harrier Circus aeruginosus as studied by automated camera traps. LIMOSA 89 (2): 79-83.

Little is known about the diet of the Marsh Harrier. It is normally reconstructed from prey remains found at the nest at the time nestlings are being ringed, but these may not constitute a representative sample. We gained experience using automated camera traps to study the nestling diet at two Marsh Harrier nests in the province of Fryslân in 2013. Motion sensor camera traps were placed at the edges of the nests during the late nestling phase, and footage was obtained for five days and nights at each nest. Prey remains were collected at the time the young were ringed. Cameras observed more and a higher diversity of prey than were found at the nests, even though cameras might not have recorded all prey delivered to the nest. Not all prey could be identified from the pictures as young sometimes shielded prey or swallowed prey immediately. At nest 1, only the male seemed to deliver prey, whereas at nest 2 both the male and the female brought prey to the nest. Less prey were delivered at nest 1 than at nest 2. Correspondingly, young at nest 1 were in a poorer condition upon ringing compared to the young at nest 2. Remarkably, three days after the camera was placed at nest 1, the young moved to a replacement nest that was built 5 m away. The absence of the female at nest 1 and the construction of the replacement nest could have been caused by the presence of the camera, suggesting that disturbance is an issue. We conclude that camera observations give a better impression of the diet of Marsh Harriers, but care should be taken not to disturb the birds.

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