Measurement techniques for curved shorebird bills : a comparison of low-tech and high-tech methods

The morphology of the bill is an important aspect of many avian ecological and biological studies. Most studies report the straight line length of the bill; however, for birds with curved bills, this may underestimate the overall length. Here we describe and compare low-tech physical and high-tech digital methods of measuring the curved length of the bill, using measurements from two shorebird species, Red-necked Avocet and Curlew Sandpiper. For the low-tech estimates we used (1) callipers and (2) string placed along the curve of the bill. For the high-tech estimates we used digital measurements of high quality photographs. Additionally, these methods are used to measure several other bill metrics, including bill height and middle bill profile. Whilst both methods can be used with relative ease, the combined use of both methods in intra- and inter-specific comparison should be avoided since they produce significantly different estimates. These differences suggest that further research should be undertaken to determine which method is more accurate. Our findings have implications for the current, standard physical method for measuring curved bills.