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Nägeli, M. (2019)

The quantity and quality of nestlings: how food supplementation and environmental factors influence reproductive traits of Red Kites.

Further information

Master Thesis, University of Zurich

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martin.gruebler@vogelwarte.ch

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Abstract

Environmental factors are often stated as the main regulators of variation in reproductive traits of raptors. But whether food availability or weather are more important is not known for most species. Furthermore, when looking at nestling specific traits, such as body condition and survival, hatching order should be considered. To date, interacting effects of food availability, weather and hatching order on reproductive traits of raptors are poorly understood. By conducting a feeding experiment we were able to uncover how food availability and weather variables influence the nest survival, nestling survival, brood size at fledging and nestling body condition of red kite (Milvus milvus). Brood loss occurred mainly in the incubation period due to rain and adverse food conditions. While food and weather affected survival of last hatched nestlings, the number of fledglings rather depended on natural food conditions probably affecting clutch size. Growth and body condition at fledging depended on food conditions and was reduced by rain and high temperatures. No support for interactions between the two environmental factors was found, meaning that weather influences reproductive traits independent of food availability and vice versa. Environmental factors clearly influenced the quantity and quality of nestlings and therefore affect the productivity of the red kite population. Ecologically improved agricultural land use, leading to higher food availability, might increase the food limited growth of the population. But as food availability cannot counter act adverse weather events, it is not fully clear how the red kite population will react to changing weather conditions caused by climate change.