CHARLESTON, S.C. – Ruby-throated hummingbirds are migrating to North America weeks earlier than in decades past, and research indicates that higher temperatures in their winter habitat may be the reason.

Researchers say the early arrival could mean less food at nesting time for the tiny birds that feed on insect pests, help pollinate flowers and are popular with birdwatchers.

“Hummingbirds are charismatic, and they do things that fascinate us,” said Ron Johnson, a scientist at Clemson University and one of the study’s authors. “They fly backward, and they hover, and they will come to feeders at homes so people can easily see them.”

Johnson and colleagues from Clemson; Taylor University in Upland; and the University of Nebraska last month published an article on the migration of the hummingbirds in The Auk, the Journal of the American Ornithologists Union.

The birds, which weigh little more than a nickel, fly hundreds of miles over the Gulf of Mexico from their wintering grounds in Central America to arrive in North America. The research compared data on their first arrival times from 1890 to 1969 with arrival times during the past 15 years or so.

The comparison found that the birds are arriving in North America 12 to 18 days earlier than in the past.

Jason Courtier of Taylor University said the historical data on hummingbirds is based on government surveys from about 3,000 naturalists around the country who recorded the first spring arrival time of bird species over the decades.

About 6 million such records exist and are being scanned into computer databases by the North American Bird Phenology Program. The research compared the historical documents with about 30,000 recent records on hummingbird arrivals.

James Van Remsen, the curator of birds at the Museum of Natural Science at Louisiana State University, said a wide body of literature shows other species of birds flying north earlier in North America and Europe.