Data and Code for "Timing of seasonal bill colour change as a potential social signal in White-winged snowfinches"

Abstract The colouration of bare parts in birds can be temporally dynamic, changing within a season or even over the course of a few days. While bare-part colouration is generally assumed to be involved in visual signalling, the communicative function of individual variation in the timing of dynamic colour changes remains poorly understood. We observed substantial interindividual differences in the start date and duration of the change in bill colour from yellow to black during the non-breeding season in White-winged snowfinches. We asked whether this temporal variation is involved in signalling status or condition during competitive interactions. First, we correlated the timing of bill colour change with sex, age, body size, body mass, and aggressiveness to evaluate which individual traits may be signalled through bill colouration. We found that males and older individuals reached black bill colouration earlier than females and younger birds. An earlier change to black bill colouration tended to be associated with larger body size, relatively lower body mass, and higher aggressiveness. These patterns suggest that the timing of bill colour change may reflect individual aspects of social status or condition. Second, we conducted a field experiment to test whether bill colour functions as a signal. Snowfinches could choose whether to forage near a model bird with either a yellow or a black bill. Responses to model birds were highly variable, yet snowfinches showed a tendency to avoid the black-billed model, suggesting that black bill colouration may signal dominance over food resources. Overall, our results indicate that the timing of seasonal bill colour change reflects individual characteristics linked to competitiveness and may influence social decisions during winter foraging.