Digiscoping Vouchers for Diet Studies in Bill-load Holding Birds

Abstract

We conducted an observer-based diet study of Caspian Terns (Sterna caspia) incorporating digiscoping to collect digital photographic vouchers of fish. Observer-based diet studies pose several problems, ranging from differential experience and abilities of observers to the lack of vouchers for confirming identification. One solution to these problems is to recover fish either directly from birds (i.e., trapping, shooting) or ones dropped on the colony. This may not, however, be desirable or achieve adequate sample sizes. Some species are difficult to capture or methods may cause significant disturbance and/or nest abandonment. To overcome these problems, we used a relatively inexpensive new photographic technique known as digiscoping. By holding a high-resolution point-and-shoot digital camera up to the eyepiece of a spotting scope or pair of binoculars, we could take digital photographic vouchers of fish as terns brought them into the colony. During the study, we made 16,423 observations, identifying 92% of the fish to the family (3%), genus (15%), or species (74%) level. We supported these observations with over 2,500 digital photograph vouchers. Vouchers confirmed identification of all fish taxa and allowed for later identification, by us or qualified fisheries biologists, of several species unidentifiable in the field. The ability to have independent experts confirm observations and identify previously unidentifiable observations greatly expanded our understanding of the range of fish taken by Caspian Terns.