Marking Nests Increases the Frequency of Nest Depredation in a Northern Population of Painted Turtles (Chrysemys Picta)

Abstract

Predators use visual and olfactory cues to locate turtle nests. Since 1999, we marked Painted Turtle (Chrysemys picta) nests at a long-term study site by inserting Popsicle™ sticks part way into the nest cavity. Because nest-marking provides a cue to potential predators, we tested whether nest-marking increases nest depredation rates. During the nesting season, 15 artificial nest pairs (N = 30 artificial nests in total) were created by digging and refilling holes (presumably emulating nest excavation by turtles) at a nesting site. Nests in each pair were 45 cm apart, but only one nest in each pair was marked with a Popsicle™ stick, and no eggs were placed in either hole. After one week, depredation was observed in nine of the 15 nest pairs, and all depredation events were directed towards marked nests. A Binomial Test revealed that this pattern was significantly nonrandom. It is possible that predators were responding to olfactory cues left by Popsicle™ sticks, and given that mammalian predators are common at our study site, we cannot rule out the possibility that such olfactory-oriented predators depredated artificial nests. However, we suspect that Common Ravens (Corvus corax) and American Crows (Corvus brachyrhynchos; nest predators that are visually oriented) were the primary predators in this study. Future experiments should use turtle eggs in both marked and unmarked nests to evaluate whether the markers represent a significant mortality factor for Painted Turtle eggs.