An inexpensive, 3D‐printable breast muscle meter for field ornithologists

The size of the pectoral muscle is an important component of body condition in birds and has been linked to indices of fitness and migratory performance. Bauchinger et al. (2011. Journal of Ornithology 152: 507-514) developed, calibrated, and validated an aluminum "muscle meter" device that estimates the size of pectoral muscles noninvasively. To make this tool more widely available, we created a CAD model from 3D-scan data of the aluminum muscle meter that can be 3D-printed in durable plastic for similar to $30 USD. We tested this device on seven species of songbirds in Jamaica, The Bahamas, Cameroon, Equatorial Guinea, and Michigan. We demonstrate that the breast muscle meter measurements are (1) repeatable among users, (2) correlated with a four-category visual breast muscle scoring system, and (3) correlated with scaled mass index (an index of body condition). Muscle scores from our device outperformed the traditional four-category muscle scoring system in predicting scaled mass index. Finally, with our device, we quantified the increasing breast muscle size of American Redstarts (Setophaga ruticilla) from March through May as they prepared for spring migration. Given the precision of the 3D-scanning hardware used to generate our 3D image for printing, we produced a plastic muscle meter that is as precise and useful as the aluminum original, but more cost-effective and widely available.