Population trend estimates are the simplest measure of population change over a time period. They are expressed as a percent change per year, and in these Tables they are presented for many regions and several time periods.
Here is the header and one line of data for a "typical" trend output:
Area|--------1966-1994 trends--------| |--1966-1979---||--1980-1994---|
Trend P N ( 95% CI ) R.A. Trend P N Trend P N
ALB 4.9 15 -4.2 14.1 0.14 1.3 4 7.0 13
Trends are presented for 3 time periods: 1966 - 1994, 1966-1979, and 1980 - 1994. This should allow you to get an idea of overall change and change for 2 subintervals.
Areas: Trends are provided for (1) states and provinces, (2) physiographic strata (abbreviated as "S" and the strata number), (3) U. S. Fish and Wildlife Service Administrative regions (e.g. RE5), (4) BBS regions (EA, CE, and WE), (5) United States excluding Alaska, (6) Canada (within the survey area), and entire survey area (SUR).
Trend:Estimated trend, summarized as a % change/year.
P:Because the trends are estimates, we conduct a statistical test to determine whether the trend is significantly different from 0, and results of the test are presented as stars '*' indicating the significance of the trend. No stars indicates that we cannot reject the null hypothesis that the trend is different from 0.
N:Number of survey routes in the analysis.
( 95% CI )95% confidence interval for the trend estimate. estimated as a multiplicative (constant rate) change in counts over time, with covariables to adjust for differences in observer quality. Regional trends are estimated as a weighted average of the route trends.
R. A.:Relative abundance for the species, in birds/route. This number is an approximate measure of how many birds are seen on a route in the region.
Trend estimates are summaries of population change, and do not provide information on other patterns of population change (such as cycles) over time. Consequently, data from several time periods are presented to allow observation of temporal pattern, and regional maps of trend should be consulted to evaluate regional patterns in population change.