Feather growth bands and photoperiod

Abstract

Growth bands are alternate dark/light bands perpendicular to the feather rachis. Previous studies indicate that pairs of dark/light bands are grown every 24 h, with light bands being produced at night, and dark ones during the day. Thus, the dark:light width ratio could reflect the photoperiod under which a feather was grown. We tested this hypothesis by inducing feathers to grow under contrasting photoperiods, using red-legged partridges Alectoris rufa as a model species. We first validated the assumption that a pair of dark/light band is produced every day. Secondly, we show that dark/light width ratios remain close to 1:1, irrespective of the photoperiod under which feathers were grown. Dark:light width ratios of feathers grown in summer (15 light-hours: 9 darkness-hours) and winter solstices (9l:15d) did not show any consistent pattern of variation within individuals. Thus, the dark/light banding patterns are not simply the product of light regimes and are not indicative of photoperiod. This finding, together with reports of ‘‘aberrant’’ growth band patterns (e.g. two growth bands produced over 24 h instead of one) challenges our current knowledge of growth bands. We propose that the normal circadian periodicity of growth bands is primarily driven by circadian rhythms: band formation starts at a point of critically low physiological activity (e.g. during night resting), and thus every 24 h irrespective of photoperiod. Our experiment emphasises that our knowledge of growth bands is weaker than previously appreciated, and that the study of dark/light band patterns on feathers could shed new light on interesting phenomena such as unusual avian biological rhythms and the functioning of internal clocks. Detecting ‘‘aberrant’’ banding patterns could therefore allow identifying bird species with unusual activity patterns or physiological rhythms.