Developing a photography-based harvest survey to estimate age and subspecies composition of midcontinent sandhill cranes

Midcontinent sandhill cranes (Antigone canadensis) are managed as a single population, but hunting regulations are structured so harvest is targeted towards the more numerous lesser sandhill cranes (A. c. canadensis). However, research indicates that greater sandhill cranes (A. c. tabida) have been disproportionally exposed to harvest at a rate exceeding their proportion within the midcontinent population. In addition, harvest has increased 22% per year in the U.S. Central Flyway states. The midcontinent population appears to be growing in recent years, but variability in annual abundance estimates has increased substantially. With limited resources and harvest management uncertainty increasing, we developed methods for a citizen science, photography-based harvest survey to estimate age and subspecies composition of harvested midcontinent sandhill cranes. To develop survey methods, we collected physical parts from 284 sandhill cranes in North Dakota in 2019 and 2020. We manually measured the culmen and tarsus using calipers, and digitally measured these parts using photographs and computer software. All digitally derived measurements were 2.5% to 5.9% larger than manual measurements; therefore, we developed linear models that adjusted digital measurements, facilitating subspecies prediction using an existing morphometric-based technique. In 2021, we requested an equal number of hunters to participate using 2 data collection methods to test if hunters could reliably take photographs suitable for digital measurement. Collection method 1 involved photographing the head and leg simultaneously, and Collection method 2 involved photographing the head only. Hunters submitted a total of 239 photographs. Only 80 of these photographs were submitted using Collection method 1, and 72% were suitable for digital measurement. Conversely, hunters submitted twice as many photographs using Collection method 2, and 88% of these photographs were deemed suitable. Although obtaining the tarsus measurement slightly improved subspecies predictability, Collection method 2 increased participation and usable data. We believe our results could be used to develop an operational survey of age and subspecies composition of midcontinent sandhill cranes, wherein a sample of crane hunters throughout the midcontinent population range would be asked to electronically submit a photograph of the head of each bird they harvest. A time series of age and subspecies composition of this population would provide managers with valuable information and improve harvest management at minimal additional cost and burden, compared to a traditional parts collection survey administered by mail.