Nest building signals male condition rather than age in wrens

An ornament used in mate choice may vary with male age for two reasons. It may be designed to signal age and females could use such an ornament to mate preferentially with older males that may be expected to be of higher quality. Alternatively, if an ornament is condition-dependent it would be expected to vary with age because condition is expected to increase with age. These hypotheses make different predictions about the variation of an ornament with male age between individuals within a year and between years within an individual's lifetime. Female wrens, Troglodytes troglodytes, have been shown to discriminate between males on the basis of the number of cock nests on their territories. An analysis using information from males of known age showed that as males got older they built more nests per season, started building earlier in the year and built for longer. A previous analysis had demonstrated that the number of nests on a territory was influenced by habitat structure and that male age failed to predict any of the inter-male variation in number of nests constructed within a year. This was supported by the present study: male age was not a good predictor of variation in the number of nests constructed between males within a year. As variation within a year between different males is much greater than the variation between years within a male's lifetime it is effectively impossible for a female to mate selectively with older males; rather it suggests that nest construction in male wrens is a condition-dependent trait and not a signal of male age. This analysis suggests that some males are more accomplished nest builders than others and that while males do improve with age, age cannot compensate for a lack of ability. (C) 1997 The Association for the Study of Animal Behaviour.