Walking on a heavily wooded trail by a north Texas pond, Craig Miller spots a large white bird lifting up from the water's edge.
"Good, we got a great egret," said Miller, adding it to the list of close to 60 species sighted by early afternoon on the cold, crisp day after Christmas at the Heard Wildlife Sanctuary in McKinney, a fast-growing city just north of Dallas.
Miller and a his group of three other volunteers are part of an army of tens of thousands of citizen scientists taking part nationwide in Audubon's annual Christmas bird count, which started on December 14 and ends on January 5.
This season marks the count's 110th year, making it the longest-running wildlife census on record and one that has contributed hugely to data about changing bird ranges linked to global warming and habitat change.
Greg Butcher, director of bird conservation for the National Audubon Society, said "there are hundreds and hundreds of scientific publications based on Christmas bird count data."
Among them is an Audubon study released earlier this year that looked at 305 widespread species that are commonly seen on the bird counts. It found that 177 of them, or 58 percent, had increased their range significantly northward over a 40-year period from 1966.
"This is clearly linked to climate change," Butcher said in an interview, referring to rising temperatures which most scientists attribute to the burning of fossil fuels such as coal.
Butcher said birds also shifted their territory in response to things like the stocking of fish in large numbers where they were not found before and habitat changes. He also noted that 70 of the 305 birds in the study had moved south.
Many of these changes have been observed first-hand at the McKinney count, which has been conducted annually since 1976. It takes place in a 15-mile radius divided into nine zones with the Heard sanctuary at its center.
This year's McKinney count coordinator, Rodney Thomas, a 54-year-old pharmacist, went through some of the changes as he and his fellow volunteers took a midday break from counting for a hot cafeteria lunch.
Leafing through the data from previous bird counts, he said the white-winged dove, which was long associated with south Texas, first appeared on the McKinney count in 1990.
"Then by 2001, 122 were reported," he said.
Changes linked to rapidly vanishing habitat have also been noted in the area. McKinney, according to U.S. census data, was America's fastest growing city from 2000 to 2008, as its population more than doubled to over 121,000.
Thomas said he first did the McKinney count in 1990 and "Zone 5" at that time "was mostly open fields. Now it's pretty much developed with housing and strip malls."
Volunteer counter Gailon Brehm, a semi-retired engineer, said because of habitat loss in the area "we have lost many grassland birds." Birds no longer found here he said included the northern bobwhite, also known as the bobwhite quail.
Bird watching is a popular past time. A report in July by the U.S. Fish and Wildlife Service found that 48 million Americans watched birds and they contributed $36 billion to the U.S. economy in 2006.
Andrew Wang, 20, and his father Henry, who is an immigrant from China, were on their first Christmas bird count on Saturday in McKinney and were planning to take up the hobby.
"We are going to start compiling a list of birds we see," said Henry as he followed Miller, binoculars at the ready.
The final tally on Saturday from the McKinney count was 104 species from all nine zones. Elsewhere, the count will go on until January 5, giving Butcher and other scientists a fresh batch of data to study.