in Britain 16 %: globaly ~4 %
||Kevin J. Gaston & Tim M. Blackburn, How many birds are there? Biodiversity & Conservation, April 1997, Volume 6, Issue 4, pp 615–625 p.622 2nd paragraph
||Sibley, C.G. and Monroe Jr., B.L. (1990) Distribution and Taxonomy of Birds of the World. New Haven: Yale University Press
||Abstract: "Here, [investigators] use a variety of methods to estimate the global number of individuals for a single taxon, birds."
||P.622 2nd paragraph: "[Investigators] think, however, that neither problem is likely to have had a significant effect on the overall magnitude of estimates in Table 4. The estimate for seabirds is far from ideal. Britain is an island, and the percentage of species in its breeding avifauna which comprises seabirds (16%) is quite high: the figure for the global avifauna is around 4% (primary source). The percentage of individual birds which are seabirds in the British avifauna is thus also likely to be high, suggesting that if anything [their] estimate of the global number of seabird individuals is likely to be an over estimate. This ignores the possible effect of Antarctica in particular, which contributes significantly to global seabird numbers. However, the percentage of all bird individuals that are seabirds is still likely to remain small."