flying female hummerA 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 frequently resorbed.

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 as well.

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.