Song and sperm competition in the reed bunting

Why and how females choose their extra-pair partners is a critical question in the discussion of the evolution of social monogamous, but genetic promiscuous avian mating systems. The role of signals in extra-pair copulations (EPC`s) is still little known. In our study we will focus on the function of bird song in relation to extra-pair behaviour. Therefore we have chosen a species with an extraordinary high level of sperm competition. We will investigate more than correlational evidence; we will carry out field and lab experiments to reveal causal effects. Among nine different hypotheses, we will test for the first time in the field, if female `deception` by male songs results in extra-pair copulation. We will also investigate if song characteristics are honest indicators of male quality and we will carry out experiments to test female preferences to a variety of different song traits. We will examine several questions related to song output in different phases of the breeding cycle: Is song output before pair bonding positively related to male quality? Is song during the fertile period an indicator for paternity losses? Do males sing more during incubation of their mate to attract extra-pair partners or do they sing to retain their mate for the next breeding attempt? Is there a non-sexual selected function for bird song after pair bond establishment and do males address their song to their mates to avoid predation? To answer these questions we will combine results from observations, field and lab experiments and paternity analyses. Field work will be done in the reed beds of Lake Neusiedl, while most of the mate choice experiments will be carried out in aviaries at the Konrad Lorenz Institute for Comparative Ethology in Vienna. We will get results about the function of song in the reed bunting and we will contribute to a further insight into the evolution of avian communication in social and genetic mating systems.

Mate choice and competition over mating partners are central for the biology of all organisms. In the course of evolution a great variety of adaptations developed to optimally endow individuals for this race. When females mate with several males for the production of a clutch, scientists call this sperm competition in the broadest sense of the word. Females are rather choosy when picking a male to mate with. They employ various criteria for that choice. What exactly they go for depends on the ecological context. So, in one case they may value the potential contribution of the male to care for the brood, in other circumstances heritable male attractiveness or its resistance against parasites may be the main criterion. Another important aspect is the potentially detrimental effects of inbreeding. Males advertise their qualities with, for instance, singing. This is why this project tried to combine bio- acoustic with genetic investigations. The study showed that indeed females exhibit a great deal of marital infidelity: 76% of the nests found by the researchers contained chicks not sired by the social father. Under such conditions one would expect that that song is mainly optimized to deter other males and to attract females willing to engage in extra-pair matings. Data obtained in the study showed, however, that males obviously also sing to signal the incubating female "all is clear!". That allows the female to leave the nest for a break safely. The genetic data revealed that inbreeding may pose a problem: chicks of related partners were more likely not to hatch than in broods of distantly related partners. Further analyses will show whether females are able to determine their relatedness to a male by listening to its song.