Parental defence in shorebirds is mediated by embryonic calling, ambient temperature and predator latency

Avian embryos vocalise late in incubation and communicate with parents through the eggshell. Embryonic calling could conceivably evoke more intense parental defence, particularly when thermal conditions require rapid resumption of incubation, and/or render the eggs more vulnerable to detection by predators. Parental defence was experimentally induced by a standardised human (a predator analogue) approach in ground-nesting shorebirds (red-capped plovers Charadrius ruficapillus and masked lapwings Vanellus miles) for "vocalising” versus “not vocalising” clutches, accounting for air temperature, egg age and “predator latency” (investigator duration at the nest before departure). For clutches where embryonic vocalisations were present, Red-capped plover parents were more likely to employ high-intensity displays (e.g. injury-feigning), with warmer temperatures facilitating both an increase in the display intensity and recruitment of both parents (joint defence). For masked lapwings, warmer temperatures were more likely to cause closer parental approaches to the investigator. As predator latency increased, red-capped plover parents were more likely to approach closer. Predator latency was shorter at nests in which defending masked lapwing adults approached more closely and called more frequently, suggesting that lapwings influence investigator behaviour. The presence of embryonic vocalisations, among other factors, may mediate individual components of shorebird defensive strategies in a species-specific fashion.