200 - 400 billion (10^9) birds
||Kevin J. Gaston & Tim M. Blackburn, How many birds are there? Biodiversity & Conservation, April 1997, Volume 6, Issue 4, pp 615–625 abstract & p.621 2nd paragraph
||Abstract: "Here, [investigators] use a variety of methods to estimate the global number of individuals for a single taxon, birds." P.617 bottom paragraph: "The first method is based on estimates of the overall density of birds at local study sites, and scaling these up to the area of the globe." P.618 5th paragraph: "The second method involves extrapolation of estimates for the number of birds in a geographic region. Such estimates were available for four countries and two continents." P.619 2nd paragraph: "The third method uses the same data as the second, but a different form of extrapolation." P.619 3rd paragraph: "The fourth method uses allometric scaling of species densities to produce an expected range of densities for species based on their body sizes."
||Abstract: "The different methods yield surprisingly consistent estimates of a global bird population of between 200 billion and 400 billion individuals (1 billion = 10^9)." P.621 2nd paragraph: "The four different methods [investigators] applied yield a reasonably consistent estimation of a global total of 200 to 400 billion individual birds (2-4 × 10^11): Wood (1982) gives an estimate of 100 billion, but provides no indication of how this was determined. Given the variety of methods, this consistency is gratifying. However, despite the general consistency, there is still some considerable variation in the estimates (in particular in those from Method II), and since the methods are all crude, [they] would do well first to assess how reliable [they] believe these estimates to be."