Development of Badges of Status in Captive Male House Sparrows (Passer domesticus) in Relation to the Relative Ornamentation of Flock-Mates

The development of quality signals is expected to be largely influenced by environmental conditions among which the social environment could be an important factor, particularly in gregarious animals. Here, we investigated the role of the presumed social environment on the development of badge size in moulting male House Sparrows (Passer domesticus) over a 3-yr time span. House Sparrow badges are testosterone-dependent ornaments that are typically argued to be badges of status that is, signals of competitive ability and aggression. Immediately prior to moult, we created several groups of males, either with similarly sized badges or with badges of different sizes, and we measured each male's badge size before and after moult. The former groups were presumed to have a less clear social hierarchy than the latter groups because of group members advertising relatively similar competitive ability (and thus were expected to more frequently challenge one another in order to establish dominance). Conversely, the latter groups were presumed to have relatively higher social stability because birds would less frequently encounter rivals advertising similar agonistic threat. We predicted in groups of birds with similarly sized badges, both (1) larger changes (i.e. increases or decreases) in badge sizes and (2) higher increases in between-individual variation within groups. The first prediction was supported in one of the 3yrs, but there was no support for the second prediction in any year. We conclude that social environment has an effect on the development of badges of status, but that it is highly contingent on the specific nature and timing of group interactions.