Dear-enemy effect between two sympatric bird species

Abstract

Although interspecific interactions appear to be less competitive than intraspecific ones, interspecific territoriality is much more frequent and more stable than is commonly believed. When territorial interactions between competing species are repeated, discrimination between heterospecifics can allow for a more precise allocation of resources and risk. An example of such a strategic approach is the dear-enemy effect, according to which individuals are less aggressive to familiar than unfamiliar individuals. We investigated the occurrence of the dear-enemy effect in territorial interactions between two nonpasserine bird species: water rail, Rallus aquaticus , and little crake, Zapornia parva , which compete for food and nest site resources. We used a cross-species playback experiment where water rails received calls of strangers and neighbouring little crakes, while little crakes received calls of water rails in the same manner. In addition, each focal pair was exposed to control calls of little grebes, Tachybaptus ruficollis . In both species, territory holders produced more calls in response to heterospecific strangers than heterospecific neighbours, and the response to control stimuli was the weakest. Furthermore, little crakes often approached and attacked the loudspeakers and mostly when they heard the calls of stranger water rails. Our results indicate that both species were able to distinguish not only each other from the control species, but also specific individuals of the second species. In line with the dear-enemy effect, water rails and little crakes adjusted their territorial response according to the intruder's status, being most aggressive to heterospecific strangers. These findings suggest an adaptive territorial response to resource competition that is commonly confirmed among conspecifics but rarely between individuals of different species. • We studied the dear-enemy effect between two competing bird species. • Water rails and little crakes recognized specific individuals of the other species. • Both species responded with lower vocal intensity to heterospecific neighbours. • Stronger nonvocal response to stranger calls was observed only in little crakes.