Andereggen, M. (2020)

Prey delivery rate and diet composition of red kites (Milvus milvus) in Switzerland.

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Master Thesis, Universität Basel






The provisioning of nestlings is a tradeoff between parental care and food provisioning, which is shaped by food resources. Food abundance and availability are the key components of the food supply and are limited by environmental factors. Parents may adjust to this limitation of the food supply by altering the rate and the composition of prey delivered to juveniles to lower the cost of foraging during energy-demanding breeding season. Here we investigated how the red kite, a diet generalist and opportunistic scavenger, alters prey delivery rate and the diet composition in response to brood size, age, and environmental factors (elevation, rain, temperature). In 2019, we monitored 11 red kites' nests for 35 days during the breeding season, resulting in a total of 764 hours of analyzable video footage (69 hours ± 17 per nest, mean ± sd). Between 09.00 am and 02.00 pm, red kites delivered on average, 0.782 prey items (sd = ± 1.221) to the nest per hour. The delivery rate increased with nestling age, but only in broods with > 1 chick. On the contrary, the delivery rate decreased with increasing precipitation and towards higher elevations. Rodents, earthworms, and bird nestlings dominated the diet, although the diet composition varied substantially between nests. Importantly, earthworms accounted for 46% of prey deliveries at low elevations, where frequently tilled land dominates. In contrast, rodents and birds were the predominant prey items at a higher elevation, where notill meadows, pastures, and forests are typical. Anthropogenic prey occurred in 82% of all nests and accounted for up to 18% of the diet composition in some nests. Our results suggest that the diet composition in red kites is influenced by the landscape context, as invertebrate prey dominated at lower elevations where most of the managed fields occur. In contrast, the nestling diet was dominated by vertebrate prey at higher elevation where extensive grassland and forest are common.