Use and Validation of a Molt Score Index Corrected for Primary-Feather Mass

Abstract

Abstract In European Starlings (Sturnus vulgaris), mass of new primary feathers increases linearly with time for most of the duration of molt (Dawson 2003). Here, we had two aims: (1) to confirm that mass of new primary feathers increases linearly in other species, and (2) to use that linear increase as the basis of a system to score the progress of molt. Increase in length of primary feathers during molt was recorded for captive Eurasian Magpies (Pica pica), Carrion Crows (Corvus corone), European Greenfinches (Carduelis chloris), and Eurasian Bullfinches (Pyrrhula pyrrhula). Wing shape differs among those species. Feather lengths during molt were converted into mass by using information on final feather length and mass and the distribution of mass from tip to base of feather. Rate of increase in total mass of new primary feathers was largely linear in all four species. A molt scoring method is described in which individual feather scores are weighted to account for the contribution of each particular primary feather's mass toward total primary-feather mass. When the method was tested on eight captive starlings, the increase in mass-corrected molt score was almost linear, unlike the increase shown by the standard scoring system, which exaggerated molt rate during the early part of molt and underestimated it later. In the four species studied here, mass-corrected molt score likewise closely tracked the actual increase in mass, unlike the standard molt score. Because it is based on feather mass, the method presented here is of greater physiological and energetic relevance than the standard method. Because the mass-corrected score increases more linearly with time, it has the additional advantage of enabling less complicated, and potentially more accurate, estimations of molt start dates and molt durations.