Implications of flower orientation for hover-feeding
hummingbirds
Nir Sapir and Robert Dudley
Hummingbirds comprise a group of about
330 species that are common in many
New World ecosystems. The main food
source of hummingbirds is nectar and
they are important pollinators of various
plant species. They typically hover-feed
from flowers that may come in different
shapes, sizes and orientations. Many of
these flowers are oriented downwards
rather than horizontal, thereby requiring
that hummingbirds feed while hovering
with the bill oriented vertically upward.
Yet the implications of flower orientation
on feeding hummingbirds have not yet
been described. Hummingbirds very
commonly visit vertically-oriented flowers,
so we asked: is feeding from such flowers
energetically advantageous? Our specific
research questions were how do
hummingbirds change their body posture
and the way they flap their wings while
feeding from flowers that have different
orientations, and do they conserve energy
or pay a metabolic cost while feeding
from vertically oriented flowers?
We measured Anna’s Hummingbirds in
the laboratory at The University of
California at Berkeley. The birds were
introduced to artificial flowers that were
oriented horizontally, tilted 45º
downward, and pointing vertically
downward and their metabolism, body
posture and wing motions were
quantified. We found that when feeding
from vertically oriented flowers,
hummingbirds held their body in an
upright angle and flexed their heads
Anna's Hummingbird, picture by Eyal Shochat
towards their backs. In addition, the
stroke amplitude - that is the angle
between the birds’ maximal and minimal
positions of the wings - increased, and
the same was found for the stroke plane
angle in relation to the longitudinal body
axis, that is the angle between the plane
through which the birds flap their wings,
and the angle of the body. Metabolic
rates also increased by around 10 %
when feeding from vertically compared to
horizontally oriented flowers. Other wing
flapping variables, such as wingbeat
frequency, did not vary between flower
orientations, and neither did feeding
duration. Our results show that
hummingbirds are well adapted to feeding
from flowers that face downwards, and
that in order to do so they modify their
body posture and change the way they
flap their wings. In addition, the birds
pay a substantial energetic cost while
feeding from vertically oriented flowers,
refuting our hypothesis that feeding from
vertically oriented flowers may be
energetically beneficial.