Implications of floral orientation for flight kinematics and metabolic expenditure of hover-feeding hummingbirds

Nectar-bearing flowers are characterized by many different shapes, sizes and orientations, which may affect the way hummingbirds feed from them. Many hummingbird-pollinated flowers are oriented downwards, thereby requiring that trochilids feed while hovering with the bill oriented vertically upward. We measured body orientations, wingbeat kinematics and hovering metabolic rates for Anna's Hummingbirds (Calypte anna) feeding from artificial flowers that were oriented horizontally, tilted 45° downwards and pointing vertically downwards. When feeding from vertically oriented flowers, hummingbirds employed an upright body position combined with dorsal head flexion. Additional kinematic adjustments included an increased stroke plane angle relative to the longitudinal body axis and an increased stroke amplitude deriving from increases in the minimum positional angle of the wingbeat. By contrast, wingbeat frequency, the stroke plane angle relative to horizontal, the ratio of the minimum to maximum positional angles of the wingbeat and the upstroke/downstroke ratio did not vary during feeding from different flower orientations. Metabolic rates increased by an average (±SD) of 10·8 (±8·8)% for feeding from vertically compared to horizontally oriented flowers. Feeding from pendent flowers comes with a substantial metabolic cost that may influence floral selection by hummingbirds and thus the evolution of associated pollination syndromes.