Phenotyping avian bill sizes; combining the collection of standardized still images with software to obtain observer-independent measures of avian bill shapes

Avian bill size is a morphological trait with evolutionary and ecological importance. Obtaining large-scale observer-independent measures of bill length and bill depth has proven to be challenging. We developed a device, the Bill Phenotyping Box, that allows taking standardized still images from wild small passerine birds in the field. We combine this with dedicated software that, based on a single human action, determines both bill length (tip to nostril) and depth (at 2/3 of the distance from the tip). We tested for consistency of observations by correlated measurements from two independently taken still images, which was high (r=0.85). We showed that the measurement depends on the angle of the bill position relative to the camera of the device but this error is relatively small compared to the between-observer variation in hand-based measures. We show that these hand-based measures are strongly observer-dependent and that calibrating observers to each other may take at least 50 measurements, which is not feasible in field studies with dozens of observers. We thus have developed a new method that allows large scale observer-independent bill morphological measurement on wild passerines. Both the specifications of the device and of the software is openly available