TERRITORIAL POLYGYNY IN-HOUSE WRENS - ARE FEMALES SUFFICIENTLY COMPENSATED FOR THE COST OF MATE SHARING

We tested two hypotheses to explain the occurrence of polygyny in a bo x-nesting population of the house wren (Troglodytes aedon), a small, i nsectivorous songbird. Some proportion of females in this population r outinely settle with already-mated males even though unmated males hol d territories relatively short distances away. The ''polygyny-threshol d'' hypothesis proposes that mated males possess territorial resources that compensate females for the cost of mate sharing (i.e., reduced a id in feeding young). Contrary to a key prediction of this hypothesis, however, we found that secondary females produced fewer offspring tha n females who chose nearby unmated males. The ''sexy son'' hypothesis proposes that mated males father attractive, prolific sons, which resu lts in secondary females obtaining as many grandoffspring as expected had they chosen available unmated males. Our data suggest that if male mating success is at least moderately heritable, secondary females ma y produce enough fledglings per breeding attempt relative to their mon ogamously mating counterparts to recoup fitness losses in the next gen eration. However, full acceptance of this hypothesis must await confir mation that male mating success is heritable. We suggest a third hypot hesis for why females readily mate polygynously when better, monogamou s breeding options are clearly available. We argue that females may ch oose mated males because these males possess high-quality nest sites ( i.e., nest-boxes), and that access to such nest sites would provide fe males with sufficient compensation for the costs of polygyny under nor mal conditions when all available unmated males would have poorer-qual ity, natural nest sites. This ''expected compensation'' hypothesis ass umes that polygynously mating females terminate mate search before the y discover that available unmated males also possess nest-boxes. A rec ent theoretical exploration of mate search strategy suggests that this assumption is reasonable.