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Peering through binoculars, flipping through field guides and pointing enthusiastically into the tree canopy, birders who took part in the 11th annual C&O Canal Midwinter Bird Survey may have turned a few heads among hikers and bikers along the towpath Saturday.

While it may have seemed out of the ordinary to some, the data the bird enthusiasts collected was a "snap shot" of the mid-winter bird population in the C&O Canal — when paired with other data, the information may help over time in determining long-term trends and broader environmental shifts that may affect those changes.

"Say we see numerous populations of woods ducks in a certain location, then for 10 years we don't see anything at that location," said Scott Bell, natural resource program manager for the C&O Canal National Historical Park, giving an example of the kind of data the counters aim to collect. "We might come to the conclusion that maybe the population has moved, maybe their food structure has changed."

Birders divide up most of the 184-mile towpath into slices of several miles to cover every year for the count, which is organized by the D.C. Audubon Society and the National Park Service. They carefully record the bird species they find and the data is aggregated and posted online. The C&O Canal count is one of many mid-winter bird counts across the region and the country — including Christmas bird counts — that aim to track winter bird populations.

"It helps to train your observation skills," said Janet Millenson, a Potomac birder who trekked the canal from Violet's Lock to Riley's Lock this year, spying 42 species of birds.

According to Peter Vankevich, a board member of the D.C. Audubon society who started the count, the C&O Canal is scientifically ideal for collecting specific data because the mile markers along the canal make it easy to record each bird's location. "You can take one mile or three miles and just look at that data," Vankevich said.

While it's difficult to link shifts in bird population directly with a warming planet, some long-time birders say that some bird species are shifting slowly north — a pattern that might be affected by warming temperatures. Birds that migrate up to the Washington area from the south in the spring — including the yellow throated warbler, a small gray and black bird with a bright yellow "throat" — are arriving earlier each year, according to Mike Bowen, a Bethesda resident who has birded in the area for 40 years.

Bowen said he spotted a warbler this winter in the Washington area. "This winter there's been a lot of warblers not normally seen," he said.

For birds like northern finches which fly to Maryland from the north, scarce food supply in places like Canada and New England could mean more northerly species on the count, Millenson said. She said she has spied several northern species already this year.

Drawing conclusive evidence that links winter bird populations to global warming or other environmental factors is difficult, however, and may only be possible after years of data from various bird counts are collected and analyzed, Bell said. But with 11 years of counting the C&O Canal under their belt, the D.C. Audubon Society will have a significant amount of data to add to the mix. "They've been doing this for so long I think it's going to be a very good piece of the puzzle once someone starts putting this together," Bell said.