Acoustic cues from within the egg do not heighten depredation risk to shorebird clutches

Egg predators use an array of olfactory and visual cues to locate eggs. Precocial avian embryos within eggs can produce vocalizations for a period prior to hatching, which may be audible to predators. Here, we investigated, under field conditions, the embryonic vocalizations emitted from eggs of a shorebird species, the Red-capped Plover Charadrius ruficapillus. We characterize the acoustic properties of the vocalizations and the circumstances under which they are emitted, then test whether such vocalizations are used as an acoustic cue by predators to locate eggs. Embryonic vocalizations typically occurred between 0 and 5 days before hatching (henceforth the "vocalization period"). Within the vocalization period, the maximum acoustic frequency (kHz) of vocalizations increased with egg age (perhaps as a consequence of embryonic development) and the minimum acoustic frequency (kHz) increased with ground temperature (perhaps as mode of communication with parents regarding thermal needs). An artificial nest experiment compared the survival of nests with and without acoustic cues (prerecorded embryonic vocalizations played continuously from the nest). Corvids were the major egg predator (accounting for 76% of cases of artificial nest predation). However, the presence of vocalizations did not affect the time taken for predators to locate and depredate eggs. Our results suggest that embryonic vocalizations are important signals that may aid in communication with parents but that they do not increase predation rates. Further research involving a greater diversity of predators (e.g., acoustic predators) is required to examine whether vocalizations from the egg incur costs under other predator regimes.