SFB-TRR 212

Effects of niche choice on host-parasite interactions of common buzzards

Common buzzards occur in three distinct plumage morphs: light, intermediate and dark. One of the main hypotheses how this polymorphism is maintained suggests that different morphs are more cryptic in different habitats and environmental conditions and preferentially exploit these. However, this has never been shown in terms of habitat use, and it is unknown when the preference may be established. Buzzards have a long altricial period of 6-7 weeks during which they could imprint on the habitat directly surrounding their open nest. After fledging, juvenile buzzards spend a few weeks to several months in their natal territory when they might imprint on the wider nest vicinity. After leaving their natal territory, subadults wander over a period of 1-2 years. During this time, individuals can flexibly choose their whereabouts, thus expressing or modulating a current preference. Most buzzards that survive to maturity eventually return and establish a territory close to their natal one, where they subsequently reproduce. Over the course of their average lifetime of 3-5 years, buzzards may build many nests for breeding and signalling. This offers the possibility to get repeated measures of environmental niche choice, elucidate the cues shared by consecutively preferred locations and assess the stability of choice or its change over different life stages. Importantly, the establishment of a territory and choice of nesting sites have an additional social component. Common buzzards are solitary breeders but in our study population in Eastern Westphalia, the distance between neighbouring active nests ranges from 50 m (semi-social) to 800 m (strictly isolated), with potentially strong repercussions for parasite transmission. During incubation or resting around their nest, buzzards get bitten by blackflies Simulium, which contract Leucocytozoon buteonis from infected adults, and later transmit them to their newly-hatched offspring. Usually, the donors appear to be the own parents, but transmission from close neighbours is possible. Such quasi-vertical transmission, potentially leading to coadaptation and suppression of parasite virulence, can only be maintained if unrelated donor hosts, such as breeding neighbours, are kept at a distance, e.g. through optimal nest site choice. This suggests that nest sites too close to conspecifics and transmission from unrelated hosts can lead to more detrimental health and fitness consequences in nestlings compared with typical infections with family-specific Leucocytozoon strains.

This project will therefore continue the individual-based long-term monitoring of buzzard habitat use, breeding and infection. We will conduct field experiments and combine cutting-edge techniques from movement ecology to cytology to address key questions about individualised niches. We will address five main hypotheses:

1. Niche choice is determined by imprinting on the early environment and is maintained over life stages.
2. Niche choice is optimised to the individual phenotype over consecutive life stages.
3. Niche choice determines the intensity of social interactions and potential of quasi-vertical parent-to-offspring transmission. Pro-social niche choice and experimental mixing of family groups facilitate horizontal transmission of parasites.
4. Horizontal transmission and mismatching of parasitic lineages lead to more antagonistic interactions between hosts and parasites on a cellular and molecular level, lead to higher stress hormone levels and lower fitness of infected host individuals.
5. Social niche choice and behaviour of both sexes during reproduction are affected differently by infection status to facilitate or hamper transmission to offspring.

Altogether, we will further elucidate causes of niche choice and its pervasive but cryptic consequences for host-parasite coevolution.