Detectability of reptiles in standardised surveys: a test using grass snake Natrix helvetica models

The ability to detect snakes in the field may be influenced by phenotypic and morphological variables attributable to the target species. These variables include body size, colouration, and body posture. To test what effect these variables had on detectability by surveyors, plasticine model grass snakes were distributed along a predetermined transect in reptile habitat. Detections of different types of snake models along the transect were compared between two groups of inexperienced students and those of a single experienced observer. The experienced surveyor detected 72 % of all the snake models, compared to 53 % and 58 % by the inexperienced groups. All groups detected more larger snakes than smaller snakes, and more uncoiled snakes than coiled snakes. The presence of a yellow/black collar did not influence the detection of the snakes. The results demonstrate the observer bias that may be inherent in surveys of snakes due to variation in size and posture of the target animals. Accounting for such biases in the design of reptile surveys and providing appropriate training and experience for volunteers may improve the validity and interpretation of data collected within citizen science programmes.