Adjustment of incubation according to the threat posed: a further signal of enemy recognition in the Blackcap Sylvia atricapilla?

Nest predation and brood parasitism are costly for nest owners, and natural selection should therefore favour the evolution of parental counterdefences. We addressed the question of whether Blackcaps Sylvia atricapilla change their incubation behaviour in response to various nest intruders and whether this adjustment matches the intensity of mobbing exhibited towards these intruders. Near focal nests, we successively exposed a dummy of a brood parasite, nest predator and an innocuous species. After the parents had responded, we removed the dummy and filmed their incubation. The most aggressive response towards the Common Cuckoo Cuculus canorus and high nest attendance after its disappearance indicated recognition of the brood parasite. Low-intensity response to the Jay Garrulus glandarius, together with reduced subsequent parental care, suggested that Blackcaps perceived it either as less deleterious at the egg stage than the Cuckoo or as a danger to themselves. Almost no aggression towards the Turtle Dove Streptopelia turtur, along with the resumption of incubation after its removal, implied that Blackcaps recognised it as harmless. In addition, we found that the level of aggression positively correlated with nest attendance, suggesting a link between the intensity of mobbing and subsequent parental care. Altogether, our results demonstrate that the issue of enemy recognition may be viewed as a complex of both aggressive and post-presentation behaviours.