Predatory risk favours intermittent incubation of a colonial burrowing shorebird, the Crab Plover Dromas ardeola

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Intermittent incubation in birds is currently interpreted as the result of trade-offs between the needs of the embryo and those of the parents. However, the low nest attendance in some species is still puzzling. We studied the Crab Plover (Dromas ardeola), a tropical burrow nesting shorebird with very low incubation constancy (less than 55%) due to frequent recesses from incubation (on average 5 recesses/h), mostly spent in the colony area without any apparent function. We aimed to test whether such unusual incubation rhythm can be partly explained by the need to scan for approaching predators. Data collected at Dahret Island (Eritrea) between 2006 and 2014 support the antipredatory function of the incubation rhythm: (1) many recesses on the colony surface were so short (50% less than 37 s in the hotter periods) that a thermoregulatory function can be dismissed; (2) adults on the colony surface responded to approaching predators with alarm calls that drove most incubating Crab Plovers to run out of the burrows and to escape in flocks; (3) sound intensity sharply decreased inside burrows, which explains the delayed escape observed for 9% of the incubating birds; (4) the delayed exit when predators were still in the colony area rules out their permanence inside burrows as an alternative antipredatory strategy; (5) after the visit of a predator, adults restarted incubation with very short in-bouts (2 min on average) apparently in order to scan frequently for approaching predators. We discuss how burrow nesting, alarm calls, and the antipredatory reactions of adults (escape into or out of the burrows, flock flight, mobbing) and chicks (escape into burrows) might have favoured colonial breeding in Crab Plovers.