Why does some trend information reflect the aggregate of the BBS and CBC, while some focuses on only one source of data?
Some species breed in the United States but winter south of the United States. For these species, we provide trends based only on the BBS because there are no CBC data for them. Some species winter in the 48 contiguous states, but breed farther north (farther north than there are roads). For these species, there are no BBS data, so the trends are based only on the CBC. Other species, such as permanent resident species or short-distance migrants are entirely or partially covered by both surveys. For these species, our best estimate of their total population change is a combination of the BBS and CBC trends. Looking at the counts individually and together helps us to better understand the differences in more local population trends as opposed to those observed across the lower 48. A local story that appears better than the national picture simply demonstrates that we can make conservation gains close to home but still need to act at the broader level to protect the birds, their habitats
