The functions of belly‐soaking in Kentish Plovers Charadrius alexandrinus

We tested whether belly‐soaking (i.e. wetting of ventral plumage) in incubating Kentish Plovers Charadrius alexandrinus in a hot environment serves to cool overheated eggs, or whether belly‐soaking would mainly serve for adult thermoregulation, and egg wetting would be a mere consequence of the ventral feathers of incubating adults being wet. When ambient temperature was high (> 30 °C), body temperature of incubating Kentish Plovers increased with ambient temperatures. When adults departed from nests to belly‐soak, egg temperatures were not critical for embryos. However, eggs in exposed nests overheated to levels that may be lethal for embryos when adults left the nests to belly‐soak, and adults also employed belly‐soaking to cool down overheated eggs when they returned to their nests. Indeed, the cooling rate of eggs was much faster when they were wetted. We conclude that the primary function of belly‐soaking in the Kentish Plover is to dissipate body heat in heat‐stressed incubating adults, rather than to cool overheated eggs. Eggs overheated as a consequence of short‐term interruptions in nest attendance by heat‐stressed adults to belly‐soak, and Plovers employed secondarily belly‐soaking to cool down overheated eggs. These results indicate that adult thermoregulation and egg cooling are not mutually exclusive functions of belly‐soaking.