Builder Wren and counting hens

Wrens start singing early. They have been doing it occasionally since the first days of January, their powerful quick song cheering up the dark winter days. Today they seem to have gone for it on earnest: I heard four different males singing on my way to work. They have lots to do and there is so little, precious time. They have to start building their nests before females are ready to lay. Yes, I said nests, not just one, but many, lots, as many as he can possible make before the females start visiting. And also, I said females, as wrens are polygynous, with one male mating with between one to nine females per season. Each spring, male Wrens, Troglodytes troglodytes, build several nests in their territory in quick succession. They are called "cock nests" and are spherical, with a side entrance, placed in suitable locations and offer the structure of the finished nest (the female will line it with hair and feathers). It takes between half a day to five days to make a nest. Although Wren's songs are incredible powerful for their minute size it is the nests the females will judge before settling for a male, in particular, the number of nests. The male will display to entice the female to each vacant nest, singing and excitedly guiding her to the nests, to advertise what an accomplished builder he is. The more empty nests a female is shown the better. The next graph shows the positive relationship between the number of nests built by a male and the number of females making breeding attempts - and therefore the number of fledglings that the male will sire.

(from Evans and Burn, 1996)

 Males vary in their ability to make nests - or in their ability to defend a territory where the nests can be built. Age makes a difference, with older males better at making nests. Habitat structure is also important, with denser vegetation indicating a better territory, where nests are less likely to be predated. But the most important explanatory factor is male condition: heavier males at the beginning of the season will be able to make more nests, so the number of nests is a measure of male quality. Females benefit from mating with males demonstrating their good condition, so it pays them to carry out a nest count before settling for a male. Experimental manipulating of number of nests present on each male's territory carried out by Mathew Evans and Joe Burn showed that the actual number of nests is the mate-choice cue that the female uses to assess male quality. The making of multiple nest by the male wren as an ornament, an extended phenotype result of the same factor than the tail of the peacock, a signal of quality selected by sexual selection.

References
Evans, M. (1997). Nest building signals male condition rather than age in wrens Animal Behaviour, 53 (4), 749-755 DOI: 10.1006/anbe.1996.0311


Evans, M., & Burn, J. (1996). An experimental analysis of mate choice in the wren: a monomorphic, polygynous passerine Behavioral Ecology, 7 (1), 101-108 DOI: 10.1093/beheco/7.1.101