Embryonic vocalizations mediate parental care in Masked Lapwings (Vanellus miles) but not Red-capped Plovers (Charadrius ruficapillus)

Embryonic vocalizations in birds may enable communication between embryos and incubating parents but responses of parents to embryonic vocalizations have not been measured experimentally. In two ground-nesting shorebirds, we compare parental care of eggs without embryonic vocalizations, those with natural embryonic vocalizations, and those in which we experimentally introduced embryonic vocalizations, while accounting for prevailing temperatures. We continuously recorded nests (one block of up to 24 h per nest) of two ground-nesting shorebirds, the Red-capped Plover Charadrius ruficapillus and Masked Lapwing Vanellus miles and quantified parental behaviours. We predict an increase in egg thermoregulation behaviours (time spent incubating or shading) and 'fussing' (a combination of rising and re-settling, shuffling, orientation change and egg turning) when vocalizations were naturally present or when they were experimentally introduced in comparison to when no vocalizations were present (control). In Lapwings, the rate of fussing increased at the nest only when natural vocalizations were present; however, preening increased when natural or introduced vocalizations were present. Additionally, the frequency of bill to egg touches (inspections) increased only in the presence of introduced vocalizations. For Plovers, treatment had no influence on parental behavioural; however, the frequency of fussing, bill to egg touches and shell tossing increased with air temperature. For both species, there was no influence of the presence of the embryo's vocalizations (natural or introduced) on the parent's egg thermoregulation behaviours. Our results highlight that acoustic communication across the eggshell may occur and can influence parental care in shorebirds, but this appears to be species-specific.