Tool Use by Bristle-Thighed Curlews Feeding on Albatross Eggs

Abstract

In strictest sense, tool use is the use of an external object as a functional extension of mouth or beak, hand or claw, in attainment of an immediate goal (van Lawick-Goodall 1970). Tool use by birds has been reported for a number of species but is considered uncommon (Morse 1968, van Lawick-Goodall 1970). Examples include Woodpecker Finches (Cactospiza pallida) and Green Jays (Cyanocorax yncas) probing for insects with twigs (Lack 1947, Gayou 1982), Blackbreasted Kites (Hamirostra melanosternon) and Egyptian Vultures (Neophron percnopterus) dropping stones onto ratite eggs (Chisholm 1954, van Lawick-Goodall and van Lawick-Goodall 1966), and a Fan-tailed Raven (Corvus rhipidurus) striking a ping-pong ball with a rock (presumably mistaking ball for an egg; Andersson 1989). Here, we report five cases of tool use by Bristlethighed Curlews (Numenius tahitiensis) feeding on albatross eggs and suggest that tool use is an extension of behavior commonly employed by foraging Bristle-thighed Curlews. To our knowledge, tool use has not been reported previously for any shorebird. Bristle-thighed Curlews breed in western Alaska and winter on islands in central and south Pacific (AOU 1983). We observed curlews on Tern Island (2345'N, 16610'W) and Laysan Island (25?46'N, 171?44'W) in Northwestern Hawaiian Islands, which extend for 2,000 km WNW of main Hawaiian Islands and comprise northern limit for wintering Bristle-thighed Curlews. The natural history of Tern Island and Laysan Island is described by Amerson (1971) and Ely and Clapp (1973), respectively. In Northwestern Hawaiian Islands, Bristlethighed Curlews forage in terrestrial habitats, feeding primarily on spiders, land crabs, and a wide variety of insects (J. S. Marks and R. L. Redmond, unpubl. data). Food items too large to be swallowed whole often are slammed against ground until they are broken (J. S. Marks, pers. observ.). In a typical slamming event, curlew holds food item in tip of its bill, raises it overhead, and then vigorously slams it against ground. This sequence is repeated until food item is broken into pieces. Usually, curlew slams food item against a flat rock. On Laysan and Tern islands, most commonly slammed items are ghost crabs (Ocypode laevis and O. ceratophthalma). Flightless chicks on breeding grounds slam nonfood items such as lichen, moss, and plastic flagging (R. E. Gill, pers. comm.), and hatching-year curlews newly arrived on Laysan often slam seabird feathers, old crab shells, and seaweed. Adult curlews confine slamming behavior to food items and in Alaska have never been observed to slam food during brood-rearing period (R. E. Gill and B. J. McCaffery, pers. comm.). Therefore, slamming behavior is probably innate. Between 2 and 10 January 1991, Hall observed four cases (involving two banded adults and at least one unbanded bird of unknown age) of tool use by curlews feeding on abandoned eggs of Black-footed (Diomedea nigripes) or Laysan (D. immutabilis) albatrosses on Tern Island. In each case, curlew picked up a small piece of coral (<2 cm diameter) and threw it at egg repeatedly, breaking a hole in shell (Fig. 1). The curlew then inserted its bill into hole, enlarged opening, and fed upon Marks saw an unbanded adult curlew use same behavior to open a Black-footed Albatross egg on Laysan Island on 13 February 1991. Except for substitution of a stone for a food item, use of stones to open eggs is identical to slamming behavior and very likely is derived from it. Tool use by Bristle-thighed Curlews is similar to that by Egyptian Vultures in that stones are thrown forcefully at eggs rather than dropped onto them (van LawickGoodall and van Lawick-Goodall 1966). Bristle-thighed Curlews are well-known predators of seabird eggs (Bailey 1956, Ely and Clapp 1973). On Laysan Island and Tern Island, curlews break smaller eggs (e.g., shearwater, petrel, and tern eggs) by pecking them with their bills or by slamming or dropping them on ground. Bailey (1956) reported that Bristlethighed Curlews eating albatross eggs would tap eggs with their curved beaks and drain contents. In hundreds of hours of observation, we never saw a curlew open an intact albatross egg with its bill. We have seen curlews pecking at albatross eggs that have been opened by Ruddy Turnstones (Arenaria interpres), however, and it is possible that turnstones initially opened eggs that Bailey observed. Bristle-thighed Curlews often feed by probing, and Received 20 April 1992. Accepted 10 July 1992.