||Kevin J. Gaston & Tim M. Blackburn, How many birds are there? Biodiversity & Conservation, April 1997, Volume 6, Issue 4, pp 615–625 p.621 top paragraph
||Marchant, J.H., Hudson, R., Carter, S.P. and Whittington, P. (1990) Population Trends in British Breeding Birds. Tring, Hertfordshire: British Trust for Ornithology.
||Abstract: "Here, [investigators] use a variety of methods to estimate the global number of individuals for a single taxon, birds."
||P.620 bottom paragraph: "Thus far [investigators] have ignored seabirds. It is much more problematic to derive density estimates for these species than it is for landbirds, because the area to use to calculate densities is unclear. However, having obtained estimates for the total number of individual landbirds, [they] can estimate the number of seabirds if [they] calculate the ratio between numbers of seabirds and landbirds. Marchant et al. (primary source) give estimates of the numbers of both land and seabirds breeding in Britain. [They] have already used the number of landbirds in Methods II and III (Table 2). The number of individual seabirds (auks, terns, gulls, skuas, sea ducks and grebes) is 6.67×10^6. Seabirds therefore comprise 5.5% of the British avifauna. If [they] assume that this relationship holds globally, then the number of individual birds in the world calculated using the various methods above, corrected to include seabirds, is summarized in Table 4."