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Sunday, September 28, 2014

Hummingbird Highway

Ruby-throated Hummingbirds, Archilochus colubris

Some of the most abundant jewels in Texas, black-chinned and ruby-throated hummingbirds may be small, but their fall migration is a feat of gigantic proportions! 

Measuring a mere three and one-half inches long with a three and three-quarter-inch wing span, the black-chinned hummingbird weighs only three to three and one-half grams, which is about equivalent to the weight of a dime plus a dollar bill. The male is dull metallic green above, gray below, black on the chin and upper throat, with an iridescent violet lower throat known as a gorget (pronounced gore-jet). The female lacks the characteristic coloring on the chin, upper throat, and lower throat.  Ruby-throated hummingbirds are also only about three and one-half inches long with the adult male having a black upper chin and ruby-red gorget, and they are a more eastern species with Austin being the westernmost border of their range. 

Like all hummingbirds, nectar serves as its main food source, fueling the tiny bird’s extreme metabolism.  These hummingbirds feed on several species of plants, most notably native penstemons, agaves, salvias, sages, and honeysuckles.  While artificial feeders supplement their diet, they also prey on insects and spiders, particularly during nesting season, which gives them the dietary fat and protein necessary to breed.

Black-chinned Hummingbird, Archilochus alexandri

The hummingbird’s unique skeletal structure allows them to fly forwards, backwards, sideways, and even on their backs!  This requires a wingbeat frequency of about 50 beats per second, and massive muscles that make up a third of their tiny body weight.  While the males perform an elaborate flight display during courtship, no pair bond is formed between the males and females.  Females build the tiny nest (out of spider webs, mosses, and various plant fibers), incubate the eggs, and raise the young, while the males are feeding and off chasing other females.  This is unusual among birds as a whole, since this class of animals exhibits the greatest amount of monogamy among vertebrates (animals with a backbone or spinal column). 

Ruby-throated Hummingbird (Female), Archilochus colubris
While most biologists believe that the shortening length of daylight hours triggers fall migration, black-chinned and ruby-throated hummingbirds begins their long journey south from Texas in September, to spend the winter mainly in western and southern Mexico.  The number of birds migrating south may be twice that of the northward trip in the spring, since it includes all immature birds that hatched during the summer, as well as surviving adults.
Amazingly, for a newly hatched hummingbird, there is no memory of past migrations, only an urge to put on a lot of weight, fly in a particular direction for a certain amount of time and hundreds of miles, and look for a good place to over-winter.  Once it learns such a route, a bird may retrace it every year as long as it lives!  There is evidence that fall and spring migration routes differ, with the hummingbirds following the Texas coast back into Mexico in the fall and crossing non-stop over the Gulf of Mexico on their way north in the spring.  Perhaps the hurricane season is a factor, and these birds have developed an innate sense to avoid the Gulf during its most precarious weather season. 

The timing of the fall hummingbird migration occurs when their natural food is most abundant.  However, you can enjoy this amazing spectacle up-close by keeping your feeders full of clear, fresh sugar water through at least the end of October, and enjoying the company of these enchanting little gems as they make a rest stop in your yard!