By Terry W. Johnson
For the past several weeks, hummingbird fanciers have enjoyed watching squadrons of ruby-throated hummingbirds displaying their aeronautical skills in backyards across the state. These tiny dynamos have been flying between feeders and flowers, gorging themselves on sugar water and nectar. When they haven't been feeding, they seem to have been trying to keep others from enjoying the sweet bounty.
While we find their activities entertaining, to hummingbirds it is a matter of life and death.
Hummingbirds don't simply show up at our feeders at this time of year by chance. Far from it, they converge on our backyards because they are preparing for their perilous fall migration or are stopping by to refuel as they head far to the south. Some ruby-throats that hail from the Midwest, Northeast and Canada actually begin their migration in early July. However, as far as we know, Georgia hummers commence their migration later in the summer.
The first birds to embark on this long journey are adult males. In the Peach State, most of them depart by the end of August. Females and those birds hatched this year follow later. By October, the vast majority of our hummingbirds have left for their winter homes.
The fall migration is triggered by declining day length. As the days grow shorter, chemical changes are triggered in the hummingbirds' bodies. These changes manifest themselves in hummingbirds going on a feeding binge. In preparation for the journey, each bird must store the food needed to make this epic trip. This requires each bird to increases its body weight by two-thirds or more. This is the equivalent of a 170-pound man putting on an extra 85 pounds!
Ruby-throated hummingbirds can put on weight in a hurry. This is illustrated by the fact that during the summer of 1996 an immature hummingbird I banded and later recaptured increased its weight by 73 percent in just 29 days.
The fuel for the fall migration is stored as fat. As the birds accumulate fat, they become so rotund that they aren't able to maneuver as easily as they could prior to their dramatic weight gain. If you held one of these birds in your hand, it would feel like a bloated sponge.
Nobody knows how a hummingbird knows it is time to leave. However, when the time is right, the bird takes off and heads toward its wintering grounds in southern Mexico and Central America. Each bird travels alone. This means that a hummingbird hatched in the spring instinctively knows where it is going. However, whatever path it chooses for its first migration appears to be the one it will use its entire life.
Unlike many migratory birds, ruby-throated hummingbirds seem to migrate principally during the day. The only exception seems to be when they are flying over the Gulf of Mexico. The length of this trip requires the birds to fly in the dark.
Aside from this leg of the trip, a migrating ruby-throat's day consists of refueling early in the morning, flying throughout most of the daylight hours and then stopping in the late afternoon to feed until dark.
How long a bird remains at a stopover site such as your backyard depends on the weather and how much stored fat it is carrying. Consequently, birds might stay a week or more to replenish their fat reserves.
Heavy rain and strong winds can delay migration. Wind direction also plays a key role in determining when and how far the birds migrate. Southbound migration seems heaviest when the birds can take advantage of a good tailwind.
The birds appear to migrate at a speed of approximately 25 mph. While we don't know for sure at what altitude the birds travel, hot air balloonists have seen ruby-throats apparently migrating southward at altitudes up to 500 feet above the ground. This dramatically changes when the birds fly over the open Gulf of Mexico. Here, far from shore, southbound hummers have been spotted by fishermen and workers on oil and gas platforms winging their way along just above the waves.
Compared to some birds, the ruby-throated hummingbirds migration is a leisurely affair. Ruby-throats apparently migrate only about 23 miles a day.
Some ruby-throated hummingbirds reach their wintering grounds by flying overland down the Texas and Mexico coasts. However, most ruby-throats raised in Georgia and elsewhere in the East fly across the Gulf of Mexico. The shortest distance across this broad expanse of water is approximately 500 miles. It is thought such a flight takes a ruby-throat 18-20 hours.
To make the flight a hummingbird has to beat its wings 2.7 million times. A successful crossing means each bird needs to carry at least 3/40th of an ounce of fuel, or fat. The larger females can carry enough fuel to make a 24.3-hour flight and can travel roughly 645 miles. Males can carry enough fuel to stay aloft for 26 hours. This enables them to traverse some 604 miles.
If the birds encounter a tropical storm while over open water, they have little chance for survival. Even a strong headwind can be devastating. A hummingbird flying into a 20 mph wind is literally pushed backward.
Each year untold numbers of hummingbirds are lost in migration. However, since the fall migration stretches over several weeks, the timing insures that millions do make it.
As more and more natural stopover sites are destroyed each year, I am convinced that backyards festooned with feeders and flowerbeds ablaze with nectar plants will play an increasingly greater role in insuring that hummingbirds find enough food along their migration route.
If you want to do your share to help the birds that bring you so much pleasure, make your backyard a haven for hummingbirds. Together we can make a difference.
Terry Johnson is a former Nongame program manager with the Georgia Wildlife Resources Division, a noted backyard wildlife writer and expert, and executive director of TERN, the friends group for Wildlife Resources' Nongame Conservation Section. His ""Out my backdoor"" column appears monthly in the Georgia Wild e-newsletter.