Redstart Phoenicurus phoenicurus and black redstart P. ochruros in a mosaic urban environment: neighbours or rivals?

Redstart and black redstart inhabit different types of habitats under natural conditions. However, they come into close contact in a mosaic urban environment. Mechanisms enabling their local coexistence were looked for. We found that: 1) The two species occupied exclusive territories of different habitat compositions. 2) Territories differed in the proportion of trees and buildings. 3) Within their territories, the two species used the same foraging techniques, but with different frequencies. 4) Prey size was similar, but the proportions of taxonomic groups differed. 5) Spontaneous interspecific conflicts were observed. Interspecific aggression was confirmed in playback experiments. The black redstart was dominant in both natural and simulated encounters. We conclude that the main mechanism enabling coexistence of the two redstarts in a mosaic urban environment is different habitat selection with the proportion of trees as a key parameter. This proportion was always below 26% in black redstart territories, and above 27% in redstart territories. This is consistent with the habitat preferences these two species show in non-urban conditions. This pattern is compared to the other cases of coexistence of congeneric bird species such as chickadees Poecile or Sylvia warblers. As the two redstart species are able to hybridise in the wild, the observed interspecific aggression may Aoperate as a pre-copulation barrier.