Detectability of avian carcasses during recovery surveys

Volunteer performing carcass counts in the short grass

This summer, I am working with the Canadian Wildlife Health Cooperative (CWHC) on a research project looking at factors associated with the accuracy of estimating wildlife mortalities. Within a population, a large number of mortalities can occur in a short time period due to infectious diseases, environmental contaminants, or in the case of migrating birds, from anthropogenic sources during migration (e.g. collisions with windows and wind-mills). When these events occur, it is important to quickly obtain mortality estimates as carcass decomposition and/or removal by scavengers can bias results.

Various field investigation methods exist to search for carcasses, and these include line transects, circular quadrats, and complete counts; however, the accuracy of these methods has received little attention and the reported results are highly variable. To gain insight into this, my research focused on searcher detection in the field, defined as the probability for searchers to detect carcasses within a predetermined search area. This information will help investigators adjust mortality estimates from both the circular quadrat and complete count methods used in the field.

The decoy birds, which resemble Black-capped Chickadees, were purchased from Floral Supply Wholesaler and are 5 inches in length

The study was designed using the circular quadrat method to maximize the efficiency of searchers and provide replicates in different habitat types (i.e., shortgrass, long grass, bush, and trees). Specifically, I simulated an avian mortality event using artificial birds that resemble Black-capped Chickadees, within different habitat types, and had eleven volunteer searchers perform complete counts in ten circular quadrats (radius: 3.99 m, area: 50 m²). Searches were repeated within each habitat type to determine if detection varied between habitat types or searchers. The results from this study will inform field investigators on potential observer bias during carcass recovery surveys, which likely affects estimates of mortality events.

While working for them CWHC this summer, I am also participating in the Atlantic Veterinary College (AVC) Summer Research and Leadership Program. My supervisors for this program are Dr. Pierre-Yves Daoust (Director, CWHC Atlantic Region) and Dr. Raphael Vanderstichel (Department of Health Management, AVC).

I would like to thank Tim, Noel, and Karen Doiron for allowing us to use their land to perform these surveys, and the volunteers who participated in the searches; their support is greatly appreciated and without them this research would be impossible.

Please stay tuned for results, which will be posted on the Healthy Wildlife blog in the near future!

Decoy carcass placed within the tree habitat

Decoy carcass placed within the short grass

Submitted by Nicolle Davis, DVM Candidate, Atlantic Veterinary College, Class of 2017