The diverse hummingbird family (Trochilidae) has unique adaptations for nectarivory, among which is the ability to sustain hover feeding. As hummingbirds mostly feed while hovering, it is crucial to maintain this ability throughout the annual cycle and especially during flight feather moult, when feathers are missing from the wing structure. The presented research will focus on the wing of Calypte anna, a common hummingbird species in western North America, which consists of ten primary flight feathers whose deficit during moult may strongly affect wing aerodynamics.
The goal of this work is to quantify the aerodynamic characteristics and flow mechanisms during the wing’s annual cycle. Birds usually moult their feathers in a deterministic sequence that may have aerodynamic, physiological and behavioural ramifications. Unsteady aerodynamic loads and flow field measurements are correlated over a complete dynamically scaled Anna’s wing model and several wing geometries that follow the feather replacement sequence characterising a natural moult cycle. Using dynamic similarity, wing models were tested in water-glycerin solutions at biologically relevant Reynolds numbers and various angles of attack.
Results were taken over a model of a complete wing in order to depict the baseline aerodynamic characteristics and flow mechanisms involved. Unsteady vorticity concentration evolved from the wing’s leading-edge differs in both size and extent from attached vorticity structures typically found over insects, which themselves operate at Reynolds numbers which are two orders of magnitude smaller than those of hummingbirds. Estimation of the complete wing performance suggests it yields comparable lift coefficients and higher lift-to-drag ratios to those found over insects.
Measurements taken at several moult stages suggests that the wing’s integrity in the medial and leading-edge region affects the flow field considerably such that lift production drops disproportionately higher in relation to the loss of wing area, which explains why the leading feathers of the wing are essential to obtain high lift capacity during hovering. Furthermore, specific-power analysis suggests a distinct link to behavioural repertoire and mass reduction characterising moulting hummingbirds.
