A New Method for Estimating Individual Speed of Molt

Abstract

We introduce a new method, Residual Raggedness Value (RRV), for estimating molt duration for individual birds captured only once during a molting period. The method is developed and tested using data from Willow Warblers (Phyiloscopus trochilus) in prebasic (post-nuptial) molt collected in Swedish Lapland during 1983-l 990. The wing raggedness value (RV) describes the amount of “missing” feather area in the wing of a molting bird. RV was positively correlated with stage of molt. The RRVs from this correlation were used to ob?ain an estimate of the size of the gap in the wing that was independent of stage of molt (i.e., RV controlled for stage of molt). Molt speed of recaptured birds was highly correlated with RRV at the first capture (rL = 0.57). Thus, an individual’s future molt speed could be predicted at the first capture with a high degree of accuracy. Compared to other methods for estimating molt duration, the RRV method produced estimates close to those obtained from recaptured birds. The widely used regression method for estimating molt duration (regressing date of capture on molt score) gave estimates that deviated substantially from both those obtained from the recapture and RRV methods. Our new method is a potentially powerful tool for increasing sample sizes of individual molt speeds in a studied population. This will facilitate understanding how individual birds may adjust the timing and duration of molt m relation to breeding and migration. The RRV method is probably applicable to most species that molt feathers sequentially. However, slow-molting species with few simultaneously growing feathers might be problematic.