Revista Catalana d'Ornitologia Time-of-day bias in diurnal raptor abundance and richness estimated by road surveys

• Main Author: Pablo Vergara
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Circus cyaneus
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1088.9091; http://www.ornitologia.org/ca/queoferim/divulgacio/publicacions/rco/26_22_30.pdf

Road surveys are the most commonly used method for estimating the abundances of raptors, although biases may occur in relation to the time-of-day that surveys are conducted. However, very few studies have addressed the impact of time-of-day bias on the accuracy of survey results. In the present work, several raptor species were surveyed over the course of a year at different times of day. In six of the 11 studied species, the time of day (hours after sunrise) affected the number of detected individuals and the estimation of species richness. Even slight differences in the time-of-day (e.g. two hours) had a significant impact on results. Furthermore, when detected the time-of-day variation was specific for each species: Hen Harrier Circus cyaneus, Montagu's Harrier C. pygargus, Common Buzzard Buteo buteo and Common Kestrel Falco tinnunculus were detected more often around sunrise, whereas Griffon Vulture Gyps fulvus and Short-toed Eagle Circaetus gallicus were most commonly detected later in the morning (at least two hours after sunrise). No interactions between the time-of-day and season (breeding vs. non-breeding) were found for the studied raptor species. The present study highlights the need to control for time-of-day bias when applying road survey methodology to raptor populations.