PMC

(A) A New Caledonian crow holding a stick tool angle-ways in its straight bill, which positions the tool tip as close as possible to the line of intersection between the mid-sagittal and mid-frontal planes of the bill. The tool is held securely by a strong bite from the tomia on the right side of the bill and a strong precision grip at the bill tip. (B) A watercolour based on a photograph of a woodpecker finch using a tool angle-ways in its decurved bill. A secure grip on a tool requires the woodpecker finch to position the tool more laterally in the bill, holding it with the tomia on both sides of the bill rather than the bill tip. This more secure holding method positions the tool tip further from the mid-sagittal plane of the bill, probably making fine manipulation of the tool tip more difficult. (C,D) Illustrations showing the distance of the tool tip from the mid-sagittal plane (dashed line) when a tool is held securely in either a decurved (C) or straight bill (D). The watercolour in B is reproduced with permission of Guido de Filippo.

A △D (%); B C_D. For the sailfish, ⋄, original bill (shown only in B); ○, short bill; □, long bill; △, smooth bill; ▽, rough bill. For the swordfish, ⋄, original bill (shown only in B); •, short bill. Here, we use the Reynolds number based on the body length, Re_BL, because of the change in the bill length. Error bars are also plotted in this figure.

The 3D reconstruction of a NCC’s skull is shown with a tool held angle-ways in the bill, mostly on the right side of the front of the bill and at the bill tip. The straight bill enables the tip of a straight tool to be positioned on the mid-frontal plane of the bill (pink plane) and as close as possible to the mid-sagittal plane of the bill (blue plane). The dashed black line indicates the assumed ideal position of the tool tip for its visually-directed manipulation. Probing with the tool tip creates a unique set of forces and moments that act on the bill in three dimensions. Since the moment arms of the tool tip force FT are functions of distances a and b, as b increases the moments acting on the bill become large quickly. The tool must be held securely by increased reaction forces acting on the bill at S1 (RU₁ and RL₁) and S2 (RU₂ and RL₂). As a consequence of a strong precision grip, the bill and the tool become a rigid, combined structure, which also exposes the bill to a twisting, vertical bending and lateral bending moment with respect to its anatomical axes. Moments Mx and Mz should be far greater than any twisting moment My. Thus potentially dangerous levels of stress that might occur on the upper mandible (e.g. above the nares and at the nasal-frontal hinge, as indicated by the red oval) would be due to moments Mx and Mz. The NCC’s deep, short stout bill would be advantageous to counteract the forces and moments acting on it.

Survival of males (left panel) and females (right panel) in relation to bill hue categories (tertiles). Note that data are shown for bill hue tertiles but bill hue was entered as continuous variable in the analyses. In both sexes individuals with low redness survive worst. In females a quadratic relationship of survival with bill hue was detected (see main text).