Estimating organ size in small migrating shorebirds with ultrasonography: An intercalibration exercise

Organs, even of fully grown adult birds, mammals, and reptiles, may show su bstantial size changes in relation to specific performances. These changes are difficult to study, because measurements usually can only be obtained f ollowing the death, of the animal. We explored the use of ultrasonographic imaging, a relatively simple noninvasive technique, to measure size of pect oral muscles and stomach in two small shorebird species (red knots Calidris canutus and golden plovers Pluvialis apricaria). Accuracy of ultrasound me asurements in estimating organ mass in red knots was reasonably high. Depen ding on the equipment used, the error of individual measurements was 20%-25 % for the pectoral muscles and 26%-44% for the stomach. In plovers the tech nique was less accurate, probably because of the low variability of the org ans involved. Ultrasound scanning is particularly suited to measure rapidly changing organ sizes over short time intervals. We demonstrate this with a n example in which changes in individuals in size of pectoral muscle and st omach were monitored in captive red knots following a change in diet. Ultra sound measures will enable studies on the links between body composition an d future behavior and physiology.