New Caledonian crows drop candle-nuts onto rock from communally-used forks on branches

Abstract

Many birds and non-human primates purposely drop or throw down objects from a height to achieve a goal. Tool-using New Caledonian Crows, Corvus moneduloides, are one of the few birds that drop nuts, and do so when standing on a substrate. In this initial study of their nut-dropping, we watched free-living Crows at Sarraméa drop Aleurites moluccana `candle-nuts' from branches onto rock at two locations 200 m apart. Over three consecutive days, Crows dropped nuts 208 and 85 times, respectively, from two branches at one of the locations, and 74 times over three days from one branch at the other location. Most nuts that broke or were damaged and prised open were dropped 2-3 times. Aside from 18 unseen cases, birds released all 349 nuts from forks in the branches, not directly from their bills. Our findings confirm early reports that suggested that Crows select branches non-randomly for nut-dropping, drop candle-nuts onto anvils from predetermined locations on branches and communally use nut-dropping branches. In addition, we report here new behaviour associated with animals dropping objects from a height: (i) the placing of objects on a substrate to release them, and (ii) the repeated dropping of objects from the same release position in three-dimensional space by the use of forks in branches. Nut-dropping from forks is well suited for experimental work to investigate the evolution, maintenance and transmission of a novel, object-related foraging behaviour.