Patterns of Growth and Asymmetry in Nestling Tree Swallows

Abstract

Growth rates of three bilateral characters of nestling Tree Swallows Tachycineta bicolor were monitored to (a) determine the suitability of these characters in assessing fluctuating asymmetry (FA) and (b) test the hypothesis that fluctuating asymmetry of any or all of these characters is correlated with other growth parameters that may be indicative of genetic and/or environmental stress. Examination of the distribution of differences between right and left sides for tarsus, bill, and primary feather measurements reveals either directional asymmetry and/or a significant departure from a normal distribution suggesting that such characters may not be suitable for assessing fluctuating asymmetry. Further, there was little evidence that FA was correlated with any of the three growth parameters calculated from a Richards growth curve or with an overall index of growth performance for individual nestlings. Age-related trends were evident for both tarsus and primary feather asymmetry, with maximum values of asymmetry occurring during the mid or latter part of the nestling period, respectively, and declining before nestlings fledged. Such a pattern may be a normal component of growth not previously reported and related to peak growing periods for those characters.