Schwitter, P. (2020)

Determinants of stopover sites of migrating Swiss red kites (Milvus milvus).

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Master's Thesis, University of Zurich.






Stopover sites are important places along the migration route of birds to rest, sleep and search for food before they move on, and can therefore be crucial for conservation measures. The aim of this thesis was the identification of regions with high densities of stopover sites of migrating Swiss red kites (Milvus milvus) along their migration path to Spain.
Based on the movement data of 400 red kites equipped with GPS transmitters from the Sense district (Switzerland), 271 autumn migration trajectories were identified, which were subsequently examined for stops, using a time- and distance-based clustering algorithm. In total, 792 stops of 202 individual birds were found, for which the 95% kernel density estimation polygon was generated in order to subsequently investigate characteristics of stopover site areas. French stopover sites and randomly placed sites of equal size within France were compared using a generalised linear model. A total of nine stopover site determinants were investigated. Five of them were related to the question of which areas are preferably selected as stopover sites in terms of habitat or resource variables, namely the proportion of forest and pastures, elevation and the occurrence of conservation areas, landfills or composting sites within the stopover site. The other four determinants, shortest distance to roads, railway lines, wind turbines and the occurrence of power lines within the stopover site, were related to the question of whether red kites are exposed to greater mortality threats at stopover sites than at random sites.
The results indicate that there are mainly three hotspot regions with high densities of stopover sites: (1) at the northwestern edge of the Alps 20km west of Chamb´ery, south of the narrows between the Alps and the Jura, (2) in the Massif Central with especially high densities north of the C´evennes and south of the Aubrac high plateau and (3) at the northern foot of the Pyrenees, mainly in the southwest of Toulouse. Furthermore, stopover habitat was similar to the typical breeding area in Switzerland and includes pastures, highly structured arable land, minor settlements and small forest patches used for the overnight stays. The mortality threats at stopover sites were not larger than elsewhere. Only the higher presence of power lines indicated a slightly higher risk in the area of stopover sites than at random sites, while the threat from wind turbines was even significantly lower at stopover sites.
At stopover sites, red kites seem to prefer heterogeneous landscape compositions with high proportions of pastures, similar to the landscapes in the breeding areas. These preferred landscapes can be located at different elevations, but seem to be higher than other agricultural landscapes in France. Finally, the clear identification of high or low densities of stopover sites within France allows an efficient implementation of conservation measures and should therefore be taken into account in future action plans and conservation decisions.