Problems with Using Ptilochronology to Measure the Growth and Nutritional Status of Nestling Bluebirds

Abstract

Variation in habitat quality can influence birds' survival and reproduction profoundly, but many methods for assessing it are logistically difficult. Ptilochronology is a method of determining the nutritional state of a bird by measuring the width of the growth bars on its feathers and may be a useful proxy for assessing habitat quality. Since each growth bar is thought to represent 24 hr of feather growth, the average size of growth bars provides a record of the bird's nutritional state during feather growth. Ptilochronology has been validated in adult birds but may not be reliable when applied to nestlings. We compared the growth rate of remiges and the average width of the growth bars on them in Eastern Bluebird (Sialia sialis) chicks. On average, the width of growth bars was slightly less than 50% of that expected if each bar corresponded to the daily growth of the feather. The average width of the growth bars also failed to correlate with the concurrent growth rates of the wing and the remiges themselves, although growth rates of the latter were significantly related. Growth bars on nestlings' feathers were also narrower than those of adult bluebirds and their size varied considerably. These data suggest that nestlings' feathers undergo daily pulses of growth that are variable in duration and number, and consequently ptilochronology may not be a reliable method for determining the growth rate of the feathers and hence determining the nutritional status of nestling birds.