Summary: | Using rufous hummingbirds (Selasphorus rufus Gmelin) as test subjects, this paper
addresses issues surrounding the energetic cost of acceleration. Hummingbirds have been
the subjects of extensive aerodynamic and energetic studies, but current aerodynamic
theory ignores acceleration as an energy expenditure during flight. Using four
experiments, this thesis explores acceleration as an important part of the daily energy
expenditures of rufous hummingbirds (Selasphorus rufus).
In experiment 1,1 estimate velocity and acceleration continuously in
hummingbirds. This experiment indicates that for short flights (< 3.5 m), hummingbirds
accelerate for 100% of the flying time. For longer flights, a section of constant maximum
velocity is inserted between two "acceleration envelopes" (one positive; one negative).
These envelopes are independent of the length of the flight. Since most of hummingbirds'
daily flights are less than 2 metres long (patch to patch and between flowers in patches),
this suggests that most of the foraging time of hummingbirds is spent accelerating. This
experiment shows that hummingbirds actively accelerate and may passively decelerate.
Experiment 2 is a preliminary theoretical exploration of the possible energetic costs
of acceleration. Using current aerodynamic theory, a thought experiment was performed
to assess this cost. Current aerodynamic theory predicts a high cost for accelerating
flights. However, due to its steady-state assumptions and small range of applicable
velocities around Vmp, aerodynamic theory probably underestimates the cost of
acceleration.
In experiment 3,1 estimate the power required for acceleration by measuring the
acceleration of hummingbirds in relation to the inclination of flight. This experiment
indicates that hummingbird flight is energy limited and acceleration may cost as much as 3
times that of steady-state flight. It also shows that the power required for acceleration is
close to the maximum sustainable power output estimated in other stud In experiment 4,1 estimate the power required for acceleration by measuring total
power input and mass changes and by estimating energy expenditures from a complete
time budget. Since the costs of all other component activities have been measured
previously, the only term unknown is the cost of acceleration. This experiment supports
the conclusion that acceleration is an expensive activity approximating the cost of
maximum sustainable power output.
|