Flight performance trade-offs and functional capacities of ruby-throated hu
mmingbirds (Archilochus colubris L.) were studied using an integrative appr
oach. Performance limits were measured by noninvasively challenging birds w
ith two strenuous forms of flight: hovering in low-density gas mixtures (a
lift assay for the capacity to generate vertical force) and fast forward ni
ght in a wind tunnel(a thrust assay for the capacity to generate horizontal
force). Functional capacities during hovering were measured by simultaneou
sly collecting metabolic data using respirometry and information on wingbea
t kinematics for aerodynamic analysis. Intraspecific differences in flight
capacity, presumably reflecting diverse selective forces because of sexual
dimorphism, migration, and plumage renewal, were then compared. Birds with
Shorter wings (adult males) or with increased body weight displayed a reduc
ed hovering capacity, although their maximum flight speed was unaffected by
such morphological changes. Birds undergoing molt of their flight feathers
exhibited a diminished performance during both hovering and forward flight
. Hovering capacities in relation to variation in wing morphology and body
mass were congruent with aerodynamic predictions, whereas performance capac
ities in fast forward flight differed from theoretical models. Kinematicall
y, hovering hummingbirds operate within a narrow range of wing-beat frequen
cies, and modulation of aerodynamic forces and mechanical power is achieved
primarily through variation in wing-stroke amplitude. Although differing i
n hovering performance, both sexes. of nonmolt birds demonstrate similar me
chanical and metabolic capacities, whereas molting inflicts high energetic
costs. Relatively invariant physiological capacities may thus ultimately co
nstrain the extent of intraspecific trade-offs between morphology and perfo
rmance, providing mechanistic insights into the multilevel functional desig
n of the hummingbird flight system.