# Background information

## Definitions

**Geographic Area** indicates the area from which data are obtained. Note that survey coverage varies and most surveys do not completely cover the entire geographic area indicated. To view annual indices of population change (if available), click on the graph icon next to the geographic area listed.

**Time Period** indicates the time period from which data are obtained. In some cases, two time periods (long-term and short-term) are reported for a survey. Many migratory bird surveys referred to on this website are conducted annually. However, others are based on periodic collection of data for a set number of years.

**Annual Trend** is the mean annual percent change in the bird population. The magnitude and direction of the Annual Trend reflect the vulnerability of the species. Species showing large declines are most vulnerable, whereas species with increasing trends are least vulnerable. The methods for deriving annual trends differ among surveys and are explained in the footnotes for this table.

**Limits** – The limits are the upper and lower limits that define the **Credible or Confidence Interval**, which is a measure of uncertainty around the estimates of the annual trend and indices. The 95% credible interval (i.e., the interval between the upper and lower credible limits) defines a range of trend values that have a 95% probability of including the true population trend value. The confidence interval is often interpreted in a similar way to the credible interval, but its formal statistical definition is more complicated.