Population size estimates are based on the scheme suggested by James Ferguson-Less and David Christie in their monumental "Raptors of the world" (2001) with input from raptor experts Tom Cade, Nigel Collar, Stephen Debus, and Bernd Meyburg, and the user is referred to their discussion of the strengths and limitations of this approach. Their scale is as follows (with designations ending in 0.5 indicating that the population size estimate falls in the mid-range of the increment).
0 = Extinct
1 = 1-10 individuals
2 = 11-1003 = 101-1,000
4 = 1,001-10,000
5 = 10,001-100,000
6 = 100,001-1,000,000
7 = over 1 million birds
As pointed out by the former authors, the method works best for the rarest species, but it is increasingly less precise for the more common species (where, fortunately, the details are less important!). Also, the estimates are most reliable for the best studied species in the best studied regions, and the projections for many of the scarcer tropical species could be off by one or two orders of magnitude.
At the outset of the GRIN project, nearly all of the estimates are those provided by Ferguson-Lees and Christie, but it is hoped that GRIN participants will pointed out any suggested refinements for species they have studied.
The inclusion of this feature is intended to compliment the Conservation Status portion of the database and is meant to assist the prioritization of research and conservation efforts.