Activity profiles and hook-tool use of New Caledonian crows recorded by bird-borne video cameras.

Jolyon Troscianko,Christian Rutz

doi:10.1098/rsbl.2015.0777

Abstract

New Caledonian crows are renowned for their unusually sophisticated tool behaviour. Despite decades of fieldwork, however, very little is known about how they make and use their foraging tools in the wild, which is largely owing to the difficulties in observing these shy forest birds. To obtain first estimates of activity budgets, as well as close-up observations of tool-assisted foraging, we equipped 19 wild crows with self-developed miniature video cameras, yielding more than 10 h of analysable video footage for 10 subjects. While only four crows used tools during recording sessions, they did so extensively: across all 10 birds, we conservatively estimate that tool-related behaviour occurred in 3% of total observation time, and accounted for 19% of all foraging behaviour. Our video-loggers provided first footage of crows manufacturing, and using, one of their most complex tool types—hooked stick tools—under completely natural foraging conditions. We recorded manufacture from live branches of paperbark (Melaleuca sp.) and another tree species (thought to be Acacia spirorbis), and deployment of tools in a range of contexts, including on the forest floor. Taken together, our video recordings reveal an ‘expanded’ foraging niche for hooked stick tools, and highlight more generally how crows routinely switch between tool- and bill-assisted foraging.

Highlights

New Caledonian (NC) crows Corvus moneduloides use a range of tool types to extract embedded prey [1,2] and are the only non-human animals known to manufacture hooked tools in the wild [3]
We present the first quantitative activity budgets for wild NC crows, and conservatively estimated that birds spent on average about 3% of their time handling and using tools (19% of foraging time), which is less than some of the values reported for woodpecker finches Cactospiza pallida [13]
Owing to the relatively short recording times per deployment, our video data cannot firmly establish whether individual crows differ in their reliance on tool-assisted foraging [8,13] (‘nil-returns’ may have been owing to chance)—this issue needs to be addressed with substantial observational datasets for a sample of subjects

Summary

Introduction

New Caledonian (NC) crows Corvus moneduloides use a range of tool types to extract embedded prey [1,2] and are the only non-human animals known to manufacture hooked tools in the wild [3]. Recent work investigated the energetics of ‘larva-fishing’ behaviour—where crows use (non-hooked) stick tools to extract giant wood-boring beetle larvae—by combining ‘camera-trap’ data from natural foraging sites [7] with stable-isotope-based estimates of diet composition [8]. We build on our earlier work with miniature bird-borne video cameras to document the natural behaviour of wild, free-ranging NC crows [9].

Objectives

Methods

Results

Conclusion