A female ruby throat?
September 22, 2020
When I sat on the deck at dawn, three hummingbirds argued over the
nearby feeder, ignoring me but fighting among themselves. That
was when I realized that all had white throats and there was not a ruby
throat in sight. It would seem that the migration has begun and
the males must be on their way south. The females and immature birds
hang around another couple of weeks to build up their stamina, but the
males move toward the Gulf, their summer work done.
If we had to choose one creature that we most treasure on the farm, it
would have to be the hummingbird. We eagerly await their arrival each
spring and anticipate their departure with regret come September.
Many of our greenhouse plants spend the summer on the deck and we
maintain three feeders (out of sight of each other) so there is much
nectar for them to eat and we spent many hours observing their antics.
Hummingbirds collect nectar as a source of energy. Plants produce the
sweet syrup as a reward for potential pollinators so it is a win-win
situation for both the bird and the flower; the bird receives food, and
the flower has its pollen spread and its ovules fertilized. The
secretion of nectar usually begins when the flower opens and increases
each time the flower is visited. After pollination, the nectar is
Flower nectar is produced in glands called nectaries. These can be
located on any part of a plant, but the most familiar types are those
located in flowers. Depending on the species, a flower's nectaries can
be situated on almost any part of the blossom, but most commonly are at
the base of the petals and sepals so that any visitors looking for
nectar must brush the flower's pollen onto its reproductive organs.
Common nectar-consuming pollinators include bees, butterflies, and
moths, hummingbirds and bats.
The composition of nectar varies from plant species to plant species,
but carbohydrates make up the largest fraction by weight. The
various types of nectars can be ordered into three equal groups
according to sugar content: predominately sucrose, predominately
glucose and fructose. Some nectars also contain amino acids, and
all twenty of the normal amino acids found in protein have been
identified in various nectars, as well as other substances such as
organic acids, alkaloids, flavonoids, vitamins and oils.
In addition to the floral nectaries which attract pollinating
creatures, many plants have extrafloral nectaries which provide a
nutrient source to animals which in turn provide protection from harm.
These are highly diverse in form, location and size, and have been
found in leaves, stems, fruits and virtually all aboveground plant
parts. They range from single-celled fine outgrowths, hairs and scales,
to complex cup-like structures. Extrafloral nectaries have been
reported in thousands of species of plants, almost all of which flower
Extrafloral nectaries were originally believed to rid the plant of
waste products but now it is understood that their nectar attracts
predatory insects that will eat both the nectar and any plant-eating
insects around, thus protecting the plant. Foraging
predatory insects show a preference for plants with extrafloral
nectaries, particularly some species of ants, wasps and lady bugs, and
these have been observed to directly defend the plants.
Interestingly, sugar concentrations from these extrafloral nectaries
varies greatly depending on their type and location--particularly the
amount of vascular tissue beneath them. If the transportation cells
laid end to end throughout the plant that carry sugars and other
molecules (called phloem) make up most of the vascular tissue, the
nectar may contain up to 50 percent sugar. On the other hand, if xylem
predominates (the system of tubes and transport cells that circulate
water and dissolved minerals), the sugar content may fall to as little
as 8 percent.
A hummingbird in flight has the highest metabolism of all creatures
except for some insects and so has the greatest need for stored
energy. Its heart rate can reach as high as 1,260 beats per
minute, and it must consume more than its own weight in nectar each
day. Nectar is a poor source of some vital nutrients, however, so
hummingbirds meet their needs for protein, vitamins, and minerals by
preying on insects and spiders.
To avoid the cold and the scarcity of food when flowers stop blooming
and insects stop flying, ruby-throats go south. Some adult males start
migrating south as early as mid-July, but the peak of southward
migration for this species is late August and early September. By
mid-September, essentially all of the ruby-throated we see at feeders
are migrating through from farther north.
For a hummer that just hatched, there's no memory of past migrations,
only an urge to put on much weight and fly in a particular direction
for a certain amount of time. Once it learns its route, however, the
bird may retrace it every year as long as it lives. The initial urge is
triggered by the shortening days as autumn approaches, and the birds
start south when they have accumulated sufficient fat. It has
nothing to do with temperature or the availability of food; in fact,
hummingbirds migrate at the time of greatest food abundance. Do
maintain your feeders until the syrup begins to freeze, however, as you
may save some late travelers.