Two Modes of Primary Replacement During Prebasic Molt of Rufous Fantails, Rhipidura rufifrons

Abstract

We documented unusual variation in the sequence of primary replacement during the prebasic molt of Rufous Fantails (Rhipidura rufifrons) captured in 2009 and 2010 on Saipan, Northern Marianas Islands. Most captures (62%) followed a typical replacement sequence, starting at the proximal primary (P 1) and proceeding distally to the outermost primary (P 10), but some individuals (38%) commenced molt from medial feathers, either P 2, P 3, or P 4 with replacement proceeding bidirectionally from the point of origin in multiple molt series. The distribution of centers for medial origin was P 2 (3%), P 3 (77%), P 3 and/or P 4 (12%), and P 4 (8%), indicating this pattern was distinct from the typical (P 1) pattern and the commencement point was not fixed to a specific primary. Females and individuals undergoing the second prebasic molt were more likely to have a center among primaries 2–4 than males and older birds. Two females that showed a center at P 1 during molt in 2009 showed a center at P 3 or P 4 during molt in 2010 represented exceptions to this finding but also indicated the starting point could vary inter-annually in an individual. Multiple molt series among primaries are poorly documented in passerines and we provide the first evidence for this molting strategy in the Rhipiduridae. Our results suggest a molt center among primaries 2–4 in younger and female Rufous Fantails may have evolved to enable more-rapid replacement of primaries while maintaining better foraging ability among selectively disadvantaged individuals.