Dual visible-thermal camera approach facilitates drone surveys of colonial marshbirds

Waterbirds are important indicators of wetland health, and understanding their status and trends is necessary for appropriate management and conservation. However, certain species are challenging to survey due to their sensitivity to disturbance and the difficulty of accessing their breeding habitats. This is especially true for colonially breeding marshbirds, for which a multi-species survey protocol that maximizes accuracy of counts and minimizes disturbance does not exist. Small drone aircraft have shown promise for conducting accurate and low-disturbance surveys of colonial waterbirds. However, complex marsh vegetation structures and the cryptic nature of some marshbird nests make them difficult to detect in aerial imagery. We used synchronous high-resolution visible imagery (0.8-2 cm/pixel) and thermal-infrared imagery (6-16 cm/pixel) captured from a drone to count nests of five species of marshbirds at eight colonies in Saskatchewan, Canada. We compared counts from the imagery to those obtained from traditional ground-based surveys, generally considered the most accurate survey method for these species. The two types of imagery proved highly complementary, as heat signatures helped detect and confirm nests not easily spotted in the visible imagery, while the detailed visible imagery allowed species identification. For four species (Western Grebe Aechmophorus occidentalis, Franklin's Gull Leucophaeus pipixcan, Forster's Tern Sterna forsteri, and Black-crowned Night-Heron Nycticorax nycticorax), drone-based counts (range 74-1524 nests per colony) were within 5% of ground-based counts (range 75-1582), while for smaller Black Tern Chlidonias niger colonies (range 3-7 nests), counts from the two methods were always within one nest of each other. Furthermore, an assessment of flight behavior before, during, and after drone surveys found no significant evidence of disturbance by the drone. Our drone-borne dual visible-thermal camera approach proved promising for surveying colonial marshbirds and could potentially be implemented in a range of situations where wildlife subjects are difficult to survey with visible or thermal imagery alone.

Automated summary

Waterbirds are important indicators of wetland health, but surveying certain species can be challenging due to their sensitivity to disturbance. This study used drones with dual visible-thermal cameras to accurately count nests of marshbirds in Saskatchewan, Canada. The results showed that drone-based counts were within 5% of ground-based counts, with no significant disturbance detected during drone surveys.