Delayed maturation of multiple signals in a migratory songbird

Many animals use multiple signals in sexual communication, but our understanding of the interactions between multiple signals, particularly in inexperienced breeders, is limited. In birds, delayed plumage maturation (DPM) is well documented; young birds appear duller than adults, despite reaching sexual maturity. Age-related changes in song structure are also common in songbirds, though the extent to which songs of yearling males differ from those of adults (delayed song maturation, DSM) and its prevalence in species with DPM is unknown. We tested for DSM in the mate-attraction (repeat) song of a species with dramatic DPM, the American redstart (Setophaga ruticilla). Repeat song structure of territorial yearling and adult males differed significantly, based on discriminant analysis of nine non-collinear song features. Combined with previous evidence of delayed maturation in territorial (serial) song, we provide the first evidence for DSM in different song types used in different behavioral contexts during the breeding season of a migratory songbird. Within adults, variation in repeat song was associated with pairing, earlier onset of breeding, and number of offspring sired, suggesting a potential benefit for expressing more adult-like song. We found no relationship between the expression of adult-like repeat song and plumage in either age class. These results indicate that delayed maturation of repeat song in yearling redstarts is not due to a generalized delay in signal development, and suggest further work is needed to determine whether mate attraction (for both yearling and adult males) provides greater pressure to sound more “adult-like” than to look more “adult-like”.