The Ecological Significance of Tool Use in New Caledonian Crows

Clever Crows Understanding the adaptive significance of animal tool use requires reliable information on the foraging behavior in the wild. New Caledonian crows consume a range of foods and use sticks as tools to extract wood-boring beetle larvae from their burrows. These larvae, with their unusual diet, have a distinct isotopic signature that can be traced after consumption by the crows in the crows' feathers and blood. By comparing the stable isotope profiles of crows' tissues with those of their food sources, Rutz et al. (p. 1523) estimated the proportion of larvae in crow diets, providing a proxy for tool-use dependence in individual crows. Just a few larvae can satisfy a crow's daily energy requirements, highlighting the substantial rewards available to competent tool users and their offspring. Thus, tool use provides New Caledonian crows with access to a very nutritional food source that is not easily exploited by beak alone. Stable isotope analysis reveals the nutritional benefits of tool use in wild New Caledonian crows. Tool use is so rare in the animal kingdom that its evolutionary origins cannot be traced with comparative analyses. Valuable insights can be gained from investigating the ecological context and adaptive significance of tool use under contemporary conditions, but obtaining robust observational data is challenging. We assayed individual-level tool-use dependence in wild New Caledonian crows by analyzing stable isotope profiles of the birds’ feathers, blood, and putative food sources. Bayesian diet-mixing models revealed that a substantial amount of the crows’ protein and lipid intake comes from prey obtained with stick tools—wood-boring beetle larvae. Our calculations provide estimates of larva-intake rates and show that just a few larvae can satisfy a crow’s daily energy requirements, highlighting the substantial rewards available to competent tool users.

[1]  Andrew L Jackson,et al.  Erroneous behaviour of MixSIR, a recently published Bayesian isotope mixing model: a discussion of Moore & Semmens (2008). , 2009, Ecology letters.

[2]  Jackie Chappell,et al.  Morphology and sexual dimorphism of the New Caledonian crow Corvus moneduloides, with notes on its behaviour and ecology , 2004 .

[3]  J. Goodall,et al.  Tool-Using and Aimed Throwing in a Community of Free-Living Chimpanzees , 1964, Nature.

[4]  J. Mann,et al.  Why Do Dolphins Carry Sponges? , 2008, PloS one.

[5]  Toshisada Nishida,et al.  Natural history of a tool-using behavior by wild chimpanzees in feeding upon wood-boring ants , 1982 .

[6]  Animal Physiology at Cambridge , 1958, Nature.

[7]  B. H. Becker,et al.  DIET–FEATHER STABLE ISOTOPE (δ15N AND δ13C) FRACTIONATION IN COMMON MURRES AND OTHER SEABIRDS , 2007 .

[8]  K. Nagy.  FIELD METABOLIC RATE AND FOOD REQUIREMENT SCALING IN MAMMALS AND BIRDS , 1987 .

[9]  Gavin R. Hunt,et al.  Tool Use by the New Caledonian Crow Corvus moneduloides to Obtain Cerambycidae from Dead Wood , 2000 .

[10]  G Yamakoshi,et al.  Dietary responses to fruit scarcity of wild chimpanzees at Bossou, Guinea: possible implications for ecological importance of tool use. , 1998, American journal of physical anthropology.

[11]  G. Ruxton,et al.  Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis. , 2006, The Journal of animal ecology.

[12]  A. Kacelnik,et al.  Behavioural ecology: Tool manufacture by naive juvenile crows , 2005, Nature.

[13]  K. Hobson,et al.  Assessing Avian Diets Using Stable Isotopes I: Turnover of 13C in Tissues , 1992 .

[14]  Sabine Tebbich,et al.  The ecology of tool-use in the woodpecker finch (Cactospiza pallida) , 2002 .

[15]  M. Coode,et al.  Flora Vitiensis Nova, a New Flora of Fiji , 1984 .

[16]  G. Ruxton,et al.  Using daily ration models and stable isotope analysis to predict biomass depletion by herbivores , 2006 .

[17]  J. Ashby.  References and Notes , 1999 .

[18]  B. Heinrich,et al.  Age and Mouth Color in Common Ravens , 1992 .

[19]  Robert G. Clark,et al.  Assessing Avian Diets Using Stable Isotopes II: Factors Influencing Diet-Tissue Fractionation , 1992 .

[20]  J. Myers,et al.  Assimilation efficiency in birds: a function of taxon or food type? , 1989 .

[21]  D. Levey,et al.  Effects of elemental composition on the incorporation of dietary nitrogen and carbon isotopic signatures in an omnivorous songbird , 2003, Oecologia.

[22]  Stuart Bearhop,et al.  Factors That Influence Assimilation Rates and Fractionation of Nitrogen and Carbon Stable Isotopes in Avian Blood and Feathers , 2002, Physiological and Biochemical Zoology.

[23]  Richard Inger,et al.  Applications of stable isotope analyses to avian ecology , 2008 .

[24]  Christian Rutz,et al.  Tool use by wild New Caledonian crows Corvus moneduloides at natural foraging sites , 2010, Proceedings of the Royal Society B: Biological Sciences.

[25]  K. Hobson,et al.  Isotopic fractionation and turnover in captive Garden Warblers (Sylvia borin): implications for delineating dietary and migratory associations in wild passerines , 2003 .

[26]  G. Hunt,et al.  New Caledonian Crows drop candle-nuts onto rock from communally used forks on branches , 2002 .

[27]  G. Hunt,et al.  Tool manufacture by New Caledonian crows : chipping away at human uniqueness , 2006 .

[28]  Richard Inger,et al.  Source Partitioning Using Stable Isotopes: Coping with Too Much Variation , 2010, PloS one.

[29]  A. Kacelnik,et al.  Video Cameras on Wild Birds , 2007, Science.

[30]  K. Hobson,et al.  Trophic relationships among Antarctic fulmarine petrels : insights into dietary overlap and chick provisioning strategies inferred from stable-isotope (δ15N and δ13C) analyses , 2000 .