Nest cavity characteristics, reproductive output and population trend of naturalised Amazon parrots in Germany
In general, non-native bird species may competitively exclude native species from nest sites. The potential resource conflict between a native and an introduced bird species, the native European Starling (Sturnus vulgaris) and the introduced Asian Ring-necked Parakeet (Psittacula krameri), was inferred from hole occupancy in parks of the German Upper Rhine Valley (Wiesbaden, Mannheim, Schwetzingen, Edingen-Neckarhausen), where Ring-necked Parakeets have been known to occur for 15 to 35 years. Only 2.2 % of trees (n = 3487) had cavities with an entrance diameter ≥4 cm. The number of unoccupied tree holes was high, representing 71 % of 639 recorded cavities. A niche separation corresponding to tree size and tree species preferred by both bird species could be detected. Starling preferred trees with a diameter of 0.75 ± 0.30 m, whereas Ring-necked Parakeet favoured ones with 1.16 ± 0.36 m in diameter. Both bird species were the most abundant hole-nesters in the examined cavities. Ring-necked Parakeets preferred Platanus x hispanica, which hosted 57 % of all broods for the parakeet, but did not use Acer pseudoplatanus. Starling favoured Quercus robur with 25 % of all broods, followed by A. pseudoplatanus with 19 %. P. x hispanica showed the highest numbers of cavities with 227 in total and a median of 3 holes per tree, partially produced by Ring-necked Parakeet. The largest colony-in-one-tree in this study was 9 for P. krameri and 5 for S. vulgaris. In this study, Starlings took over two nests from Ring-necked Parakeets, with one case vice versa, and Honeybees took over one active Ring-necked Parakeet nest.