Multi-male display sites in serins (Serinus serinus L.)

Multi-male display sites in passerines might be more frequent than commonly expected. Since biparental care is often the case, joined singing may be i mportant for female choice of a breeding partner or of an extra-pair partne r. In this study we examine the existence, frequency and possible function of multi-male display sites in the socially monogamous serin (Serinus serin us). The results show that several males can use the same song post and up to six males display at the same time. Resources like food abundance and ne st site availability are more or less evenly distributed and do not correla te with the number of advertising males at a given song post. Comparing sin gle-male and multi-male song posts, we found that song post characteristics (song post height, vegetation type and exposure) partly differ. This sugge sts that display sites are special landmarks which might be important for d ifferent reasons (e.g., hot spots for female visits, special broadcasting c onditions). The higher number of female visits at multi-male sites might al so indicate a benefit to males from joined singing. According to increased total song output and aggressive interactions in relation to the number of displaying males, our data suggest an optimal group size of two to three ma les. Several males in these groups could be identified as already mated or even as males with an incubating female at the nest, suggesting that males could try to become polygynous or seek extra-pair copulations.