In 1966, the first Breeding Bird Surveys were conducted under the auspices of the U.S. Fish and Wildlife Service and the Canadian Wildlife Service. With the conclusion of surveys last month, the program has contributed 50 years of data on the dynamics of the breeding birds of North America.

The idea for the survey was the brainchild of Chandler Robbins. You may recognize Robbins as the primary author of the 1966 “Birds of North America.”

The basic protocol of a Breeding Bird Survey involves conducting a bird census along 24.5 miles of secondary roads. Beginning about a half-hour before sunrise during the peak of the breeding season (May into early July, depending on latitude), an observer starts at the fixed first stop of the route. For three minutes, he or she counts all the birds heard or seen. The observer then drives half a mile to the next stop, counts for three minutes, continues to the next stop, counts and so on until 50 stops have been sampled. It generally requires about 41/2 hours to complete a route. Most birds recorded are heard and not seen.

The observer uploads the bird abundance at each station to the BBS website along with data on temperature, wind and cloud cover.

The following year, the observer repeats the route, stopping at precisely the same 50 stations. It’s easy to locate stops precisely with GPS devices now.

The routes are assigned by the USFWS or CWS. The roads chosen are generally on less traveled roads. As you can imagine, traffic noise makes it hard to hear singing birds.

Of course, a single survey is of limited use in detecting population changes. The power of the BBS lies in the fact that thousands of surveys are run each year. Initially, 600 routes were established, all east of the Mississippi River. The program expanded to western North America, reaching 2,000 routes by 1968. Now there are nearly 3,700 routes in the United States and southern Canada.

Analyzing BBS data presents a challenge because of many confounding factors like differences among observers in identification skills, visual, and auditory acuity and weather. Biologists do not claim the data can be used to determine actual abundances of each species but rather offer a way to compare abundances from year to year (relative abundances). The development of maps showing population changes is one of the strengths of the BBS (https://www.pwrc.usgs.gov/bbs/).

The BBS data are available to anyone, and hundreds of ornithologists and biologists have it as the basis of scientific papers. One landmark paper based on BBS data was authored by Robbins and colleagues in the Proceedings of the National Academy of Sciences in 1989.

In this paper, the authors studied the changes in relative abundance of eastern breeding birds from 1966 until 1987. They showed that a striking decline in the abundance of North American breeding birds in the New World tropics occurred from 1978 to 1987 after a period of stable populations from 1966 to 1977. Most permanent residents and short-distance migratory species did not show such declines. The data therefore strongly suggest that changes on the wintering grounds such as deforestation were driving these declines of long-distance migrants.

The BBS continues to be one of the most powerful tools we have to gauge changes in bird populations. But the BBS is not Chan Robbins’ sole accomplishment. He worked on impacts of DDT on birds, influencing Rachel Carson to write “Silent Spring” in 1964. He retired in 2005 after 60 years with the USFWS.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at

whwilson@colby.edu