Trend Analysis

• The aim of our work is to describe the status of bird populations in each of our study areas.
• We accomplish this using a two-fold approach: 1) by describing the trajectory of the population path of each species through time, and 2) by evaluating whether the trend of the population path represents a significant increase or decrease since the study began (Link and Sauer 1997).
• We use a set of criteria to ensure that our analyses provided reliable population information. Stands are included in the analysis only if they have been sampled in at least six years. Data are included for a species if it is observed on a minimum of five stands per study area and in at least three years on each stand.
• For species that were observed on a minimum of five stands in each of the three national forests, we pooled all stands and carried out an additional regional (three national forest combined) analysis. Although this regional analysis does not include lands in non-federal ownerships, it should give an indication of population trends at a larger scale than the individual national forest.
• We use a non-parametric route regression procedure similar to that described by James et al. (1996) to characterize population trajectories. This method allows us to use relative abundance from the sampling unit to describe populations across entire study regions, without assuming a specific pattern of population change (e.g., a linear population trajectory). In addition, because we use a non-parametric technique, we are not limited to data that meet assumptions of normality.
• For each sampling unit, a non-linear estimate of trajectory is calculated for each species by using locally-weighted regression (LOESS) to model species abundance as a smooth function of year.
  
  Note: Go to species trends link to obtain individual species trend graphs

   

  James, F. C., C. E. McCulloch, and D. A. Wiedenfeld. 1996. New approaches to the analysis of population trends in land birds. Ecology 77(1):13-27.

Link, W. A., and J. R. Sauer. 1997. New approaches to the analysis of population trends in land birds: comment. Ecology 78:2632-2634.