Fig. 1

Experimental set-up for investigating hooked stick tool manufacture in New Caledonian crows, and key terminology. a Top view of set-up in an experimental chamber (approximately 3 × 3 m), as used in Experiment 1. In Experiment 2, only one ‘food log’ with a single extraction hole was presented. New Caledonian crow b examining seven plant stems of different properties in Experiment 2 (presented in randomised order on the ‘material log’), and c and d processing a basic tool (on ‘manufacture log’ in panel c). e Schematic drawing of a forked plant stem, and terminology used to score crows’ manufacture actions. The position of the basic tool is highlighted in white. f Crows can detach plant material from the stem at the: (i) root shaft; (ii) hook shaft; (iii) tool shaft (results in a non-hooked stick tool); or (iv) joint. Note that, by definition, detachment at the tool shaft is only possible through ‘cutting’, and detachment at the joint only through ‘pulling’. g Description of the two methods of detaching plant material from the stem. Colour-coding in panels e and g is the same as in Fig. 4. h Photo of a hooked stick tool showing three design features: the crafted hook; stripped bark at the hooked end; and moderate crow-induced curvature of the tool shaft