Timing and duration of moult in the Bullfinch Pyrrhula pyrrhula: an appraisal of different analytical procedures

Abstract

The method most commonly used to assess the timing and duration of moult in birds is based on the moult scores of individuals caught in moult, while non-moulting individuals caught at the same time are ignored. Underhill and Zucchini (1988) produced a statistical model of avian moult which incorporated data from non-moulting birds and which could be used to produce potentially more reliable estimates of the key parameters. They distinguished three types of moult data Our aim was to evaluate the Underhill-Zucchini model, using records of Bullfinch Pyrrhula pyrrhula moult. We found that only data incorporating observations of non-moulting as well as of moulting birds (but without individual moult scores) gave unbiased estimates of moult rate (and duration) as judged from rates obtained from recaptures of the same individuals. Using such data, the estimated mean starting date of moult in adult Bullfinches near Oxford varied between 10 and 18 August in four years, the standard deviation in starting dates between 11 and 14 days, and the mean moult duration between 67 and 77 days. Only mean starting date differed significantly between years.