Recognizing interspecific dominance signals? Blue tits adjust nest defence based on great tit's black bib size

Animals use colours, morphological structures and behaviour to advertise their dominance status and ability to obtain resources (Resource Holding Potential, RHP) in agonistic contexts with conspecifics. Dominance relationships are also known between species, although the interactions and the information used to assess the other species' RHP remain obscure and empirical evidence of interspecific recognition of status is hard to obtain. Using behavioural inference is particularly important when a species needs to assess the potential loss of fitness in terms of reproduction or survival (e.g., risk of damage or predation) that could be inflicted by another species acting as a predator, parasite or competitor. Our study shows for the first time the existence of interspecific recognition of status signals: Blue tits Cyanistes caeruleus (subordinate species) perceive Great tits Parus major (dominant species) as threats as hole-nesting competitors, and respond differently based on the size of the great tit's black tie stripe. Blue tits seemed to recognize the meaning of great tits' black ties because they were bolder when defending their nests against small-tie (less dominant) great tits than when repelling attacks by large-tie (more dominant) great tit males. Our results stress that behavioural inference can potentially be used to assess the meaning of status signals that presumably evolved in an intraspecific signalling context.