Background information

Population Status relative to circa 1970

Large Decrease
>= 50% decrease
Moderate Decrease
>= 25%, < 50% decrease
Little Change
< 25% decrease, <= 33% increase
Moderate Increase
> 33%, < 100% increase
Large Increase
> 100% increase
Data deficient
Insufficient data to determine population status

These categories reflect the current population status of the species relative to circa 1970 (late 1960s/early 1970s) or the closest available time period. This time period was selected as the reference point for assessing current population status of landbirds both because there are few data previous to this period for most landbirds, and because it is a reasonable conservation target for many species. However, desired conservation targets should be higher for species that were at very low population levels in 1970 (e.g. raptors and some species at risk). The Population Status is based on the magnitude and direction of trend and the annual indices from survey(s) that provide the best long-term information on the species, supplemented by other additional information as necessary.

The criteria for decreases and increases in the above table are symmetrical about zero in logarithmic scale and are equivalent in magnitude. For example, a 100% increase (Large Increase) is needed to reverse a 50% decline (Large Decrease). For more details see the Introduction and Methods section on this website.



The reliability with which the population status of a species can be determined varies depending on the quality and time period of data available for that species. The reliability of each data source used was assessed in relation to its statistical precision, coverage of the species’ breeding population or range, survey design, magnitude of the population change and other factors. The reliability of the Population Status is based on a cumulative assessment of all data sources used. For more details see Introduction and Methods section on this website.

Status category is likely to be correct, or at worst within one status category of the actual species status.
Significant uncertainty about the status category, but is likely to be within one status category of that assigned, and not off by more than 2 status categories.
Substantial uncertainty in status, such that actual status of the species may be 2 status categories different than assigned, and sometimes more.
Data deficient
No data, or else too much uncertainty in the data, or potential bias is too large in magnitude, to support any status category.