Wait a Minute? Hiding Behavior of Burrowing Crabs and an Oversized Bill Explain Why Crab Plovers Prefer Armored Swimming Crabs

Abstract

In the Indo-West Pacific biogeographical region, a suite of shorebirds searches for crabs as food. While the majority of these shorebirds hunt on burrowing crabs, the endemic crab plover Dromas ardeola additionally feeds on swimming crabs with “fast and powerful claws”. Here, we examined the trade-off made by crab plovers between foraging for swimming crabs and burrowing crabs on the intertidal mudflats of Barr Al Hikman in the Sultanate of Oman during four winters (2012–2015). Foraging on burrowing crabs requires waiting for the occupant to emerge, whereas foraging on swimming crabs involves searching and extensive handling. We found that crab plovers strongly preferred swimming crabs. In fact, diet composition was exclusively a function of the densities of swimming crabs, that is, crab plovers stopped waiting for burrowing crabs above threshold densities of swimming crabs even if burrowing crabs were abundant. Using a two-prey functional response model, we could explain diet composition from an energy-maximization perspective, but only if waiting time was added as an identification phase independent of prey densities. This suggests that crab plovers exhibit selective attention and can only wait for a limited number of burrowing crabs at a time. We conclude that the preference for swimming crabs emerges from both the efficient handling of swimming crabs by the crab plover and the long hiding times of the burrowing crabs. Undoubtedly, it is the crab plovers' specialized bill which makes handling of swimming crabs profitable. We speculate that this bill uniquely evolved in the “escalated” environment of the Indo-West Pacific.