Different reproductive tactics in house sparrows signalled by badge size:: Is there a benefit to being average?

Sexual selection models usually predict directional selection for ornamental traits because of intra- as well as inter-sexual selection. Animals frequently face reproductive trade-offs, such as between mating and parental effort. Provided that both are essential and have opposite effects on ornament expression, we may however not necessarily expect directional selection for ornament size. The house sparrow is an ideal species to study such a trade-off, as the size of the male ornament, the black throat badge, seems to be inversely related to mating and parental effort. It has been suggested that large-badged males invest more in female attraction and territory defence, while small-badged males may invest more in parental care. In a nest-box study, we show that females started to breed earliest and produced the largest clutches when mated to males with average-sized badges that invested in paternal care more than other males. These results are discussed in view of inter- as well intra-sexual selection. Overall, average-badged males experienced the highest hatching failures, their chicks were in the poorest physical condition and they did not fledge more chicks than other males. It is therefore unlikely that the mating advantages that we observed could by themselves lead to stabilizing selection for badge size. Our results rather suggest that badge size in male house sparrows signals different reproductive tactics, which are adapted flexibly according to their physical condition and socio-ecological situations.