Now, as the summer solstice passes, I can hear birds singing outside my window before 5 a.m., just as it gets light. A few I even know — catbird, house wren, cardinal.

It is the males making the noise, announcing to the world each summer morning: “This is where I live. This is my territory.’’

And in about a dozen places in Connecticut, and in more than 4,000 others throughout all of North America, there are people listening to those songs and counting what they hear.

The North American Breeding Bird Survey is now taking place, as it has for nearly 50 years. While it resembles the far more famous Christmas Bird Count in some ways, it has important differences that make it one of the most important tools ornithologists have to trace what’s happening to songbirds on the continent.

“It’s the one I refer to most often when I look at trends in bird populations,’’ said Patrick Comins, director of bird conservation for Audubon Connecticut.

The survey began in 1966 as a response to the work of people like Rachel Carson, who in the 1960s began to notice the decline in the number of songbirds in the United States, and to realize springs were growing silent.

The difficult question was to somehow measure those declines over time.

Under the guidance of Chandler Robbins, a famed ornithologist at the Patuxent Wildlife Research Center in Maryland, a network of people began the task of trying to count nesting songbirds.

Researchers at the Patuxent center — run by the U.S. Geological Survey — and their counterparts at the Canadian Wildlife Service, carry on the work today.

Here is how the survey works. Each year, volunteers drive along a chosen 24.5 mile route — the same route every year.

At half-mile intervals, they stop and look and listen for exactly three minutes, noting what they see and hear within a quarter-mile radius. Then they move on, until they finish the 24.5 miles. It takes about five hours to complete a route.

Because of all the foliage in the trees, the volunteers must have good ears as well as good eyes — most of the bird IDs are made by knowing who is singing.

“About 80 percent of the birds we count are heard and not seen,’’ said Keith Pardieck, the survey’s coordinator at Patunxent. “As volunteers age and begin to lose their hearing a little, we have to ask them to step down.’’

They also have to be as rigorous as possible in keeping track of what they hear.

“If you hear a bird singing two seconds after the three-minute interval is up, you must not count it,’’ said Andrew Dasinger, the coordinator of the survey in the state.

The volunteers also can’t do anything to trick the birds into singing. They can’t playing recordings of songs to get the birds to respond. They can’t hoot like owls to get the bards riled up.

“There’s no pishing,’’ said Dasinger, of the pish-pish-pish noises birders use to get some action started in the underbrush.

Therefore, the volunteers have to really know what they’re doing. It’s a much more careful approach than the Christmas Bird Count, in which the number of volunteers, their expertise, and the places they look for birds, can vary a lot from year to year.

It also tries to control for weather. Volunteers make their counts on clear days with no rain and little wind. The Christmas counts can take place in snow or sun, in gales or still winter air.

The breeding bird survey has now gathered information on about 425 different species of birds. In 2008, it expanded into Mexico.

Because of the exactness and skill of its volunteers, ornithologists have used its data in about 450 scientific papers.

The survey has been used to study things like the effect of pesticides on songbirds, and how changes in habitat have caused some species to decline.

“A lot of new studies on the effects of climate change and how that may be changing the range of birds uses the breeding bird survey,’’ said Pardieck of the Patuxent center.

Comins said because of the very nature of the survey, it has species it cannot measure easily — shorebirds, rails and wading birds like egrets.

There are other surveys, including the Christmas bird count, that can help fill in those gaps.

“The more pieces in the puzzle, the bigger the puzzle,’’ Comins said. “Sometimes, we need a lot of information to learn about one small thing.’’

Contact Robert Miller at earthmattersrgm@gmail.com