Social and abiotic factors differentially affect plumage ornamentation of young and old males in an Australian songbird

Both abiotic environmental conditions and variation in social environment are known to impact the acquisition of sexual signals. However, the influences of abiotic environmental and social factors are rarely compared to each other. Here we test the relative importance of these factors in determining whether and when male red-backed fairywrens (Malurus melanocephalus) moult into a known sexual signal, ornamented breeding plumage. One-year-old male red-backed fairywrens vary in whether or not they acquire ornamentation, whereas males age two and older vary in their timing of ornament acquisition. It is unclear whether these processes are determined by the same or different factors and we examine both events using a combination of long-term breeding records and non-breeding social networks. We found that one-year-old males that paired prior to the start of the breeding season were more likely to acquire ornamented plumage, but rainfall did not influence whether one-year-old males acquired ornamented plumage. Thus, for young individuals, social cues appear to play a larger role than abiotic environmental factors in determining ornament acquisition. For older males, timing of ornamented plumage acquisition was constrained by rainfall, with drier non-breeding seasons leading to poorer physiological condition and later moult dates. Thus, sexual signal variation in older males appears to be a condition-dependent trait, driven by abiotic environmental and physiological factors rather than social cues. These findings reveal that factors influencing sexual signal expression can vary with age when age classes exhibit different forms of signal variation. Our results suggest that social environment may drive sexual signal variation in young individuals, whereas abiotic environmental variation may drive sexual signal variation in older individuals.