Spotlight: Crab Plovers are Childcare Innovators
When I think of a shorebird, I do not think of a creature that breeds in a burrow. And I certainly don’t picture a striking white and black bird, with long grey legs, partially webbed feet, and a heavy black bill, standing guard over multiple burrows to shoo wayward chicks back in when they stick their heads out for a peek. Enter the Crab Plover Dromas ardeola, evidently the childcare innovator of the shorebird world.
Crab Plover. Photo: Peter Wächtershäuser, Wikimedia Commons
Mohammed Almalki and colleagues, scientists based in Saudi Arabia and the UK, do think of Crab Plovers when they think of shorebirds. They report on this delightful bird in the Wader Study Group Bulletin (volume 121 issue 3). The Crab Plover is endemic to the Indian Ocean, its heavy black bill is specialized for crab eating (hence the name), and it is the only shorebird species to nest in an underground burrow.
Why go to the trouble of digging a two-meter long burrow for nesting? This bird breeds in extremely hot environments – on desert islands and coasts around the Arabian Peninsula – and during the hottest time of the year (April to August). Indeed, in June 2012, Almalki and colleagues measured outside temperatures at their study site ranging from 30°C at night to 50°C in the middle of the day, on average. Yet within the 11 burrows they measured, using tiny temperature dataloggers called iButtons, the inside temperature was relatively stable at 35.0 ± 0.18°C. So the burrow serves a purpose. Keeping the eggs within the optimal range for incubation (rather than temperature more suitable for cooking). The burrow incubates the eggs, allowing the adults to go forage.
The burrow also serves as protection against predation, keeping eggs safely hidden inside. But what happens when the chicks hatch? Chicks are not safe if they emerge from the burrow; yet guarding a burrow means time away from foraging and time spent in the hot sun. As part of their study on breeding ecology, Almalki and colleagues individually colour marked adults, labelled nest-burrows (with numbers on plastic spoons), and recorded behavior using cameras. In doing so, they made an interesting observation. They were able to document several occasions when most adults had left the colony, but one adult remained, to prevent the chicks from leaving their burrows while their own parents were away.
Beyond observing cooperative care in Crab Plovers, and finding stable temperatures inside active nest-burrows, the study revealed other key findings on the breeding ecology of these wonderful birds. By combining molecular sexing of blood samples with standard morphological measurements, the authors found that bill length is the most reliable morphological trait indicating the sex of Crab Plovers (males have longer bills, wings and tarsus lengths than females). They also found that both males and females feed their chicks (though females tend to feed more often). Finally, this study examined day-night cycles in chick feeding, finding higher feeding rates during the day than at night.
Professor Tamas Szekely of the University of Bath, a co-author in the study, sums it up well, “It was exciting to work with Crab Plovers. They are very unusual shorebirds because of their specialised habits, and a lot remains to be learned about their breeding behaviour.”