USDA National Wildlife Research Center Symposia


Date of this Version



Published in: Witmer, G. W., W. C. Pitt, and K. A. Fagerstone, editors. 2007. Managing vertebrate invasive species: proceedings of an international symposium. USDA/APHIS Wildlife Services, National Wildlife Research Center, Fort Collins, Colorado, USA. Also available online at


Relying on headlamp illumination for visual detection of cryptic nocturnal animals may present a challenge. To test how search light properties affect brown treesnake (Boiga irregularis) detection rate, we assigned eight biologists to search for dead snakes placed in roadside vegetation. Each person conducted 4 searches using lamps with varying properties: weak versus strong light, crossed by narrow versus wide beam. On each occasion, 100 snakes were placed randomly along the roadside transect. The mean number spotted per transect search was 13.5. Using an information theoretic approach, sequential order of transect runs was the only confounding variable included in the model with the highest support: 1.5 fewer snakes (95% CI = - 0.4 to -2.5) were found for every sequential transect search a person conducted. A narrow beam spotlight rendered almost seven fewer snakes per search than a broad beam floodlight (95% CI = -4.5 to -9.2). A weak light rendered 4.5 fewer snakes than a strong light (95% CI = -2.1 to -6.9). We suspect that the benefit of using a lamp with a floodlight beam is particularly pronounced when a complex, 3-dimensional forested habitat is surveyed and when the traveling speed is relatively high.