Shorebird embryos exhibit anti-predator responses

Ground-nesting shorebirds rely on egg crypsis as one of their main anti-predator defence strategies. Avian embryos vocalise late in incubation and communicate with parents through the eggshell but calls may compromise crypsis. Conceivably, embryos may cease calling when exposed to signals that suggest a predator is nearby (acoustic crypsis) but this has not been studied. Signals of nearby predators may include bird calls (predator, parental alarm), parental heart rate (higher when predators are nearby) or changes in light and temperature (incubators leave nests when predators are nearby). Experiments using eggs of Red-capped Plovers Charadrius ruficapillus and Masked Lapwings Vanellus miles examined whether the number of embryonic vocalisations varied with exposure to the above factors. For both species, exposure to predator calls caused fewer embryonic calls (although, this effect was evident for only one variant of predator calls for Red-capped Plover). For Red-capped Plovers, there was a tendency (p = 0.068) for exposure to light/changed temperature and noise to be associated with fewer calls. Parental heart rate (tested only in lapwings) and exposure to light/temperature and noise change had no influence on the number of embryonic vocalisations for Masked Lapwings. We show that embryos are able to respond to predator calls and possibly light/temperature/noise levels as cues, in a manner consistent with anti-predator behaviour before hatching.