Will free-ranging predators stop depredating untreated eggs in pulegone-scented gull nests after exposure to pulegone-injected eggs?

Abstract

Pulegone is a chemical derived from plants of the mint family ( Mentha spp.) that irritates the trigeminal nerve of mammals when inhaled and causes gastric distress when consumed. We examined whether free-ranging mammalian predators would stop depredating untreated eggs in gulls’ nests that smelled of pulegone. Prior to the nesting period, we distributed pulegone-injected eggs around a gull colony so that local mammalian predators could learn that opening such eggs was painful. We then selected 165 ring-billed gull ( Larus delawarensis) and 165 California gull ( Larus californicus) nests in the colony and randomly assigned them to one of the three treatments: (1) nests treated by placing two drops of pulegone (each 1 ml) on the ground on opposite sides of the nest, (2) nests treated by spraying 2 ml of pulegone around the nest's periphery, or (3) nests left untreated as a control. For both gull species, there were no differences among treatments in proportion of nests that were depredated. In a second experiment, which was also preceded by an exposure period when pulegone-injected eggs were distributed, 400 ring-billed and 150 California gull nests located in three colonies were selected to receive one of the two treatments. Half were treated by placing 2 ml of pulegone in a cup and burying it beneath the nest and half were left untreated as a control. Although 30–40% of the control and treated nests were depredated, differences between treated and control nests were not detected. Application of pulegone to nests did not decrease viability of those eggs that escaped depredation. Nevertheless, these results failed to support the hypothesis that egg survival of colonial birds could be increased by training free-ranging predators not to open untreated eggs in pulegone-scented nests.