Ptilochronology: A Critical Evaluation of Assumptions and Utility

Ptilochronology has been proposed as a method that will open up many new lines of investigation in nutritional ecology, in foraging behavior, and of life-history variables that are affected by (or affect) a bird's nutritional status. The method depends on measurements of a series of daily growth bars to estimate a feather's growth rate, and thereby to quantitate a bird's nutritional status while the feather was growing. The reliability of the method as proposed depends on the fulfillment of several previously unstated assumptions. We identified seven key assumptions and, in experiments with captive White-crowned Sparrows (Zonotrichia leucophrys gambelii), examined the likelihood that six of the assumptions could be fulfilled. The seventh assumption, no less crucial than the others, could not be tested empirically. None of the six tested assumptions were consistently fulfilled. Only lethal kinds or levels of nutritional privation consistently slowed feather growth. Moderate or even severe sublethal privation did not produce consistent effects on feather growth. In short, ptilochronology as originally conceived is fraught with uncertainty. It may yield reliable results in very limited, carefully controlled conditions, but this remains to be proved.