From spoonbill to Spoon‐billed Sandpiper: the perceptual dimensions to the niche

According to Begon et al. (2006), a niche is not a place but an idea. A niche is a summary of an animal’s tolerances and requirements. This determines where an animal would do best and where it likes to be; in other words, how an animal would select its habitat. Habitat selection is usually represented graphically in terms of prey selection, food intake rates and predator avoidance (Piersma 2011), but when this is done for particular animal species, their morphological, physiological, behavioural and sensory design features are mostly taken for granted (e.g. Wiens 1989, Morrison et al. 1990, GossCustard et al. 2006). How helpful even a limited understanding of, in this case, prey detection mechanisms could be for predictive models of habitat selection and foraging distributions became clear early on in our work on Red Knots Calidris canutus. The precise arrangement of pressure sensors in the bill tip and their wiring to and in the brain explained the considerable capacity of Red Knots to detect hard objects such as bivalves and snails in wet soft sediments (Piersma et al. 1998). However, these sensory insights then also helped us to understand the rather low givingup densities of available prey at some sites (Piersma et al. 1993), the near-absence of soft-bodied prey such as polychaete worms in their diets (Piersma 1991), and the foraging distributions of Red Knots over intertidal mudflats at a variety of spatial and temporal scales (Piersma et al. 1995, van Gils et al. 2006b, Quaintenne et al. 2011). Thus, if a little understanding of even a single ‘design’ feature helped so much to make sense of this species’ ecology, imagine what an understanding of multiple design features would do (van Gils et al. 2006a). This takes me back to a symposium that I once attended at Leiden University. I was there to tell the story of the specialized bill-tip organ of Red Knots and how this helped us understand their food and habitat choices. The audience consisted of functional morphologists in the tradition of Leiden’s van der Klaauw (1948) and see, for example, Gerritsen & Sevenster 1985, Zweers et al. 1995, van der Leeuw et al. 2003). Apart from the widespread whisperings during the conference dinner about functional morphology rapidly becoming a dying trade, the composition of the nationalities around the table struck me as odd. Whereas the behavioural, ecological and ornithological meetings that I was used to were usually dominated by scientists from English-speaking countries, none of those were there; the audience was German, Dutch, Austrian and French only. Here were the scientists who could tell one bone, one set of muscles or one sensory organ from another and know how to study their morphology and functionality, and there we were, apparently losing these remarkable, if somewhat inward-looking, specialists. Still, as witnessed by a steady trickle of new birdrelated publications (e.g. Guillemain et al. 2002, Cunningham et al. 2010a, 2010b, Demery et al. 2011), the trade that merges insights from morphology, sensory physiology and ecology is alive and well. In this issue of Ibis, Martin and Portugal (2011) describe the visual fields of four ecologically distinct but phylogenetically related wading birds from one family, two ibises and two spoonbills, and interpret their findings in the context of the foraging ecology of these threskiornithids. They were in for a surprise when finding that even such tactile foragers with long bills have fields of vision that enables them to look binocularly at freshly captured prey. Clearly, careful scrutiny of captured prey, during handling between the tips of their mandibles before ingestion, is important enough for these spoonbills and ibises to give up the possibility of complete celestial coverage by having highly placed eyes. The ingestion of Three-spined Sticklebacks Gasterosteus aculeatus in full self-defence must be something like eating barbed wire. It involves the disarmament of the spines by careful head-up positioning of the fish before swallowing, and I can see why spoonbills need to use binocular visual input to do this efficiently. That spoonbills and ibises are large bodied, and thus have less to fear from avian predators than have smaller tactile-feeders such as ducks, may explain why the large wading birds have given up complete celestial vision, whereas the ducks have not (Martin 1986b, 2007, Guillemain et al. 2002, Martin et al. 2007a). Such trade-offs are paramount in the designs of all animals, and it is tribute to Graham Martin that he has built up such an extraordinary portfolio of comparative studies at the interface between morphology, sensory physiology and ecology (e.g. Martin 1994, 2009, 2011, Martin & Katzir 1995, Martin & Coetzee 2004, Martin *Email: theunis.piersma@nioz.nl