Low incubation investment in the burrow-nesting Crab Plover Dromas ardeola permits extended foraging on a tidal food resource

摘要

We used GPS data-loggers, video-recordings and dummy eggs to assess whether foraging needs may force the low incubation attentiveness (55%) of the Crab Plover Dromas ardeola, a crab-eating wader of the Indian Ocean that nests colonially in burrows. The tidal cycle was the major determinant of the time budget and some foraging trips were more distant from the colony than previously known (up to 26km away and lasting up to 45h). The longest trips were mostly made by off-duty parents, but on-duty parents also frequently left the nest unattended while foraging for 1-7h. However, the time spent at the colony area (47%) and the time spent roosting on the foraging grounds (16%) would have allowed almost continuous incubation, as in other species with shared incubation. Therefore, the low incubation attentiveness is not explained by the need for long foraging trips but is largely dependent on a high intermittent rhythm of incubation with many short recesses (5.8 +/- 2.6recesses/h) that were not spent foraging but just outside the burrow or thermoregulating at the seashore. As a result, the eggs were warmed on average only 1.7 degrees C above burrow temperature, slightly more during high tide periods and when burrow temperature was lower between 20:00 and 10:00h, only partly counteracting the temperature fluctuations of the incubation chamber. These results suggest that low incubation attentiveness is due to the favourable thermal conditions provided by safe nesting burrows and by the hot tropical breeding season, a combination that allows simultaneous foraging by parents and the exploitation of distant foraging grounds. Why Crab Plovers engage in many short recesses from incubation still remains to be clarified but the need to thermoregulate at the seashore and to watch for predators may play a role.