Nest Concealment and Nest Defence by Two Passerines: Effect of Protective Nesting Association.

Selection of favourable micro-habitat conditions at nest-sites and nest defence can be important anti- predatory strategies in open-cup nesting birds. In response to nest predation risks, some species of birds appear to form protective nesting associations in which both may gain benefits due to mutual warning and nest defence. Despite the many studies assessing the impact of various factors on nest defence and nest placement, how interactions between species while breeding can modify these strategies is still poorly understood. Here I evaluate whether nesting associations in two species influence nest defence intensity and nest-site selection. An observational approach was used to analyse the defensive behaviours of the Barred Warbler Sylvia nisoria and Red-backed Shrike Lanius collurio in an agricultural landscape in eastern Poland. Association was determined by the position of the nest with respect to that of the associated species (by the nest of one species being within the breeding territory of the other). Nest defence behaviour of these two passerines was assessed as their response to a human intruder near an active brood. This study showed that the nest size and visibility were similar in nest-sites of pairs nesting in association and in spatial isolation. Barred Warblers nesting within shrike breeding territories strongly defended their nests. Warblers breeding alone displayed a lower level of defence than birds nested in association. Shrikes not nesting with warblers were significantly more aggressive than those breeding in the protective nesting association. I suggest that shrikes tried to compensate for the lack of assistance by warblers in joint nest defence and were forced to invest more into defending their own nests. This research suggests that positive interactions within the heterospecific network of relations in ecosystems may be one of the factors responsible for diversifying the intensity of avian nest defence.