Movements and Activity of Juvenile Brown Treesnakes (Boiga irregularis)
Understanding the spatial ecology and foraging strategy of invasive animals is essential for success in control or eradication. We studied movements and activity in juvenile Brown Treesnakes on Guam, as this population segment has proven particularly difficult to control. Distance between daytime refugia (from telemetry of 18 juveniles, 423–800 mm snout–vent length) ranged from 0–118 m (n = 86), with a grand mean of 43 m. There were tendencies for shorter snake movements on nights directly following a full moon and on dry nights, but variation among snakes was of a larger magnitude and would greatly reduce chances to detect moon or rain effects unless corrected for. Snake activity was estimated from audio recordings of signals from “tipping” radio transmitters, analyzed for pulse period and amplitude. Activity was highest in the hours immediately after sunset, and gradually declined throughout the night before dropping abruptly in conjunction with sunrise. Snake activity was higher on rainy nights, and tended to be highest during waning moons and when the moon was below the horizon. We conclude that small Brown Treesnakes forage actively and appear to move far enough to regularly encounter the traps and bait used on Guam for control purposes, suggesting that alternative explanations are required for their low capture rates with these control tools.
Distribution of linear distances between successive daytime refugia for 18 Boiga irregularis from Guam with snout–vent lengths ranging from 423 to 800 mm, studied for varying durations; data obtained between 26 September 2011 and 31 January 2012.
Influence of moon phase on 86 linear distances between successive daytime refugia for 18 juvenile Brown Treesnakes in Guam was evaluated by phase-shifting the proportion of the moon disc illuminated. We assumed the distances snakes moved varied as a function of the lunar cycle, but that the longest versus the shortest movements needed not coincide with full and/or new moons; we therefore shifted the sinusoidal moon function (or really, the data on the proportion of the moon disc that was illuminated) in one-day steps to find the best fit to the movement data. Shown here is the support for models varying in lunar phase shift (top panel), the Moon Phase coefficient in the respective models (middle panel), and the associated fixed model intercepts (which are negatively correlated with the moon phase coefficients) (lower panel). Large dots indicate the −26 d phase shift rendering the best model fit. Besides the intercept and moon phase, these models also contained a random snake effect, specified as a random intercept.
Activity of seven Boiga irregularis (423–780 mm SVL), measured as the proportion of one-hour time periods telemetry signal variation was detected, from one hour prior to sunset until after sunrise the following day. Dots are means of raw data (and thus unbalanced across snakes); vertical bars indicate ±1 SD. Sample sizes for each 1 h time slot are listed above the graph. Because sunset is the reference time for all data, and because these data were collected on and off over a period of 3½ months (14 October 2011–1 February 2012), the time of sunrise varied by ±17 minutes around the arrow indicating sunrise.
Contributor Notes
Associate Editor: M. J. Lannoo.