Individuals and sampling

We trapped a total of 60 male and 60 female house sparrows using mist-nets in western Switzerland during the first two weeks of April 2014. From each bird, we measured body mass and tarsus length, and birds were assigned to 15 mixed outdoor aviaries at the Hasli Ethological Station (University of Bern, Switzerland) according to their body weight and an initial score of badge size. Thus, aviaries had on average birds of the same body weight and males with various badge sizes. After four weeks, all the females were transferred into a separate aviary, and we took a sperm sample from each male. We then collected a second sperm sample the day after, and a third sperm sample after 48 h from the last sample. This procedure ensured that any differences in sperm characteristics would be intrinsic differences in quality rather than differences due to depletion [49] or fresh sperm effects [61–63] (but see [64]), and only data collected after all males were manually depleted was used. Males were divided in three sampling bouts consisting of 5 aviaries, and each bout was processed 5 days apart.

To test the causality of the observed patterns, females were reintroduced to the aviaries and males were shuffled across aviaries according to their initial social rank. We maximized the number of positions that males could have gained or lost in the hierarchy (Additional file 1). Males were given three weeks to settle down the new hierarchical positions. The exact duration of spermatogenesis is unknown in house sparrows. However, spermatogenesis has been estimated to last between 11 and 15 days in non-passerine birds such as domestic fowls, Japanese quails and Barbary drakes [47], and a study by Bat & Maiti [48] on yellow-throated sparrows Petronia xanthocollis suggests that it may be shorter in passerine birds. We thus assumed that three weeks would cover at least one spermatogenesis cycle. At the end of these three weeks, we collected sperm samples following the same procedure as before.