Low incubation investment in the burrow‐nesting Crab PloverDromas ardeolapermits extended foraging on a tidal food resource

Abstract

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 26 km away and lasting up to 45 h). The longest trips were mostly made by off-duty parents, but on-duty parents also frequently left the nest unattended while foraging for 1-7 h. 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 AE 2.6 recesses/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°C above burrow temperature, slightly more during high tide periods and when burrow temperature was lower between 20:00 and 10:00 h, 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.

A Modified Leg-noose Trap for Crab-plovers (Dromas ardeola) at Burrow Nests

“…Using leg-noose traps, we caught 59 adult Crab-plovers between 2005-2012, during the incubation phase (late May to early Jul), when Crab-plovers stayed longer within the colony area and frequently entered their burrows in accordance with their intermittent brooding pattern (De Marchi et al 2008, 2015. We started employing the shock absorbers in 2009 in order to avoid captured plovers escaping because of breaking nooses, a problem that occurred with 16.66% of the birds, and in order to avoid a consistent proportion (50%) of the 30 captured birds being wounded on their legs by the nooses, albeit superficially.…”

“…The Crab-plover Dromas ardeola (Charadriiformes, Dromadidae), one of the few truly colonial shorebirds (Chiozzi et al 2011) and the only one that nests in burrows (Rands 1996), breeds on islands free of terrestrial predators, lays a single egg, raises a nidicolous chick, and regularly commutes from breeding sites to distant feeding areas (Rands 1996;Hockey and Aspinall 1997;De Marchi et al 2006, 2015. The burrows serve as shelter against avian predators, and also as solar incubators (De Marchi et al 2008, 2015 that allow parent Crabplovers to leave their nests unattended for periods up to several hours (De Marchi et al 2008, 2015.…”

“…The burrows serve as shelter against avian predators, and also as solar incubators (De Marchi et al 2008, 2015 that allow parent Crabplovers to leave their nests unattended for periods up to several hours (De Marchi et al 2008, 2015. Crab-plovers are very shy about any change around the burrow entrance (GDM, pers.…”

“…During a multi-year study on the breeding biology of Crab-plovers, we had to choose a trapping device that allowed the capture of both members of a pair for morphometric sexing (De Marchi et al 2012), and for applying GPS dataloggers which needed to be recovered for data downloading with a successive recapture (De Marchi et al 2012, 2015.…”

“…We tried Havahart H double door cage traps, which proved ineffective because the Crab-plovers avoided entering their nests even when the traps were left overnight. This cautious behavior might stem from the species' low incubation needs because of partial solar incubation (De Marchi et al 2008, 2015.…”