Many animals make structures, including nests and tools. These artefacts are used for a range of functions, including raising offspring and foraging, and their performance impact an animal’s fitness. The evolution of construction behaviour, thus, depends on the factors that determine the performance of animal artefacts. To unravel these factors, one can ask three related questions: if the structure of an artefact is variable; if the structural variation is linked to differences in the manufacture behaviour; and if the structural variation affects the efficiency of the artefact. We investigated these questions using the New Caledonian crow. These crows make and use hooked stick tools to extract invertebrates from vegetation. Although tools made by a given population share key features, they vary appreciably in overall shape. Using wild-caught, temporarily captive crows, we investigated causes and consequences of variation in hook-tool morphology. We found that manufacture method, as well as bird age and raw-material properties, influenced tool morphology, and that hook geometry in turn affected crows’ foraging efficiency. Specifically, hook depth varied with both detachment technique and plant rigidity, and deeper hooks enabled faster prey extraction in the provided tasks. Older crows manufactured tools of distinctive shape, with pronounced tool curvature and hooks of intermediate depth. Comparable effects of extrinsic and intrinsic factors on the morphology, and ultimately on efficiency of artefacts, may be widespread in construction behaviour across the animal kingdom.