Waiting for the mate? Spatial behaviour of common terns, Sterna hirundo, during courtship

In monogamous and migratory birds that do not have continuous pair bonds throughout the year, an important cost of mate fidelity is waiting for the previous mate at the breeding site. Assessment of new mates, however, also requires time and energy investment. We analysed the behaviour of common terns during the mating period in the light of this conflict. We predicted that mating behaviour should effect spatial and temporal patterns of movements within the colony site. Therefore, we analysed attendance of transponder-marked individuals at resting places during courtship, supplemented by courtship observations. We found clear differences in the spatial behaviour depending on sex, breeding experience and mate fidelity. Females used more sites than males did, first-time breeders more than experienced breeders and unfaithful birds more than faithful ones. These results indicate a stronger affinity of males, experienced and faithful birds to their territory. Furthermore, we detected seasonal changes in attendance during courtship. The number of courtship partners was independent of sex or experience, but tended to decrease with increasing pair bond duration. Courtship behaviour differed between temporary partners and final pair mates. Our results support the hypothesis that common terns search for a mate immediately after arriving at the colony site without waiting for their, possibly delayed, previous mate, but with the option to switch back if it returns within a given period. Consequently, the time costs seem to outweigh the energy costs for a potentially unnecessary mate assessment.