As I expected, ruby-throated hummingbirds, after seeming to disappear in May, have started returning to my yard in Decatur — and once again are busy sipping nectar from the feeders. They're back because their nesting season has all but wound down. Now, they're turning their attention to the long journey ahead — their annual trek to winter grounds in Mexico and Central America.

Many ruby-throats will be leaving within the next few weeks. For thousands of them, the arduous trip will require flying nonstop some 500 miles over the Gulf of Mexico — a mind-boggling feat for such tiny birds that weigh no more than a nickel apiece. Other ruby-throats will follow a coastal route south. No one knows for sure what proportion of the birds follow that route and why they choose it instead of the sea route.

Robert Lubeck / Nationalgeographic.com (ENLARGE)

Ruby-throated hummingbirds fly South to Central America and Mexico for the winter.

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Regardless of which route the little birds follow, flying from the upper reaches of North America to the tropical wilds of Mexico and Central America requires considerable energy. To get it, ruby-throats gorge on nectar and insects to double their body mass in as few as 10 days in late summer, prior to departing south. Fattening up, then, will be the main goal of ruby-throats during the next several weeks. That's why it's important now to have your hummingbird feeders up and filled with fresh nectar (one part sugar to four parts water).

Among the ruby-throats that will be showing up in our yards during the next several weeks will be those that nested in northern climes — as far north as Canada — during the summer, and are now passing through. They add greatly to the numbers of hummingbirds that are already here and breed in Georgia. In general, the peak of Georgia's fall hummingbird migration is in August, although we have some migrating ruby-throats until late October. The adult males go through first, followed by the adult females and finally the young birds.

An interesting fact: Several studies indicate that the ruby-throat's southbound migration in late summer and early fall — at least that portion over land — is nearly synchronous with peak flowering of jewelweed, suggesting that this common wildflower is an important nectar source during this time and may influence the timing of migration.

By the way, we get a few questions from readers each year asking if putting out feeders will delay ruby-throated hummingbirds departing on their fall migration. There is no scientific evidence to support this — the urge in these little birds to migrate apparently overcomes any desire to stay behind and sip nectar from backyard feeders.

More migrations

Ruby-throated hummingbirds aren't the only birds in Georgia that start heading south in July. Marietta birder Bob Zaremba, who keeps close tabs on migrations at Kennesaw Mountain National Battlefield Park in Cobb County, notes that some warbler species begin migrating through the area as early as the first week of July. They're also headed to winter homes in Latin America. Here's what Zaremba says on his Web site at www.georgia-birding.com/KMT/fall07.htm

"The fall [migration] season begins in July with the arrival of the first migrants around the 4th of the month. The first arrivals are usually black-and-white warbler, Northern parula and black-throated green warbler. The migration slowly builds over the next few weeks as cerulean and worm-eating warblers begin to pass through. By the end of the first week of August, prairie and hooded warblers are making an appearance. The migration picks up in late August and into September, peaking towards the middle of the month."

For upcoming fall migration bird walks at Kennesaw and other birding sites around metro Atlanta (most starting in August), visit the Atlanta Audubon Society Web site at www.atlantaaudubon.org.

Elsewhere in Georgia, swallow-tailed kites, Mississippi kites, purple martins and tree swallows also are beginning already to gather in flocks to migrate south for the winter.

Still nesting

While many Georgia birds are preparing for fall migration in July, our year-round resident American goldfinch normally waits until about this time of year to breed. The reason for this is not clear, but there appears to be a close relationship between the flowering of thistles and the start of nest building. Thistles are favorite food plants and a source of nesting material for goldfinches. Georgia's goldfinches may still be laying eggs as late as mid-August. As a result, most goldfinch pairs have time to produce only one brood in a season.

In the sky

The moon will be full on Thursday night, rising out of the east about sunset, says astronomer David Dundee of the Northwest Georgia Museum of Science. July is sometimes known as the Buck Moon because this is normally the month when the new antlers of buck deer push out of their foreheads in coatings of velvety fur. Mercury is low in the east just before sunrise. Venus is low in the west just after sunset. Mars and Saturn set in the southwest less than an hour after sunset. The giant planet Jupiter rises out of the east at sunset and will appear near the moon Thursday night.