Reduction of wing area affects estimated stress in the primary flight muscles of chickens

Abstract

In flying birds, the pectoralis (PECT) and supracoracoideus (SUPRA) generate most of the power required for flight, while the wing feathers create the aerodynamic forces. However, in domestic laying hens, little is known about the architectural properties of these muscles and the forces the wings produce. As housing space increases for commercial laying hens, understanding these properties is important for assuring safe locomotion. We tested the effects of wing area loss on mass, physiological cross-sectional area (PCSA), and estimated muscle stress (EMS) of the PECT and SUPRA in white-feathered laying hens. Treatments included Unclipped (N = 18), Half-Clipped with primaries removed (N = 18) and Fully-Clipped with the primaries and secondaries removed (N = 18). The mass and PCSA of the PECT and SUPRA did not vary significantly with treatment. Thus, laying hen muscle anatomy may be relatively resistant to changes in external wing morphology. We observed significant differences in EMS among treatments, as Unclipped birds exhibited the greatest EMS. This suggests that intact wings provide the greatest stimulus of external force for the primary flight muscles.

Keywords: estimated muscle stress; feather loss; keel bone; laying hen; physiological cross-sectional area.

Figure 1.

Diagrammatic representation of the wing-feather clipping treatments. Unclipped, where no flight feathers were clipped; Half-Clipped, where the 10 primary flight feathers on both wings were clipped; Fully-Clipped where all primary and secondary flight feathers on both wings were clipped along the coverts.

Figure 2.

The deep side of the pectoralis (PECT) and supracoracoideus (SUPRA). (a) Dashed blue lines are examples of muscle fascicles measured in cm. (b) Solid black lines indicate pennation angles measured in degrees between two intersecting lines (orientation of the fascicle and central tendon). (c) The central tendon is indicated by the solid yellow line.

Figure 3.

LSM ± standard error (s.e.) of the estimated muscle stress (EMS, kPa) of (a) the pectoralis (PECT) and (b) the supracoracoideus (SUPRA) in white-feathered laying hens across three different wing-feather clipping treatments (Unclipped, Half-Clipped and Fully-Clipped). Different letters denote significant differences (p < 0.05).