This approach assumes that countries in the same region have shared similar changes in socio-economic developments (rural development, urbanization) and are subject to similar environmental pressures (pollution, climate change). As a consequence, bird species are expected to have had similar changes in population numbers.
This may not be true, but we regard it as a better approximation of missing indices than alternatives. An alternative would be to extrapolate a species trend in a country beyond the years covered by the data, but this may lead to very unrealistic indices for missing years. Another alternative would be to assume that indices in years without data were equal to years with data, which is not very plausible.