Quote from late apex:
Aaron, your ball park estimate of 60 billion birds on Earth is of the right order of magnitude, not far off at all. And apparently even your density methodology is along the lines of what they actually use... impressive. According to this 1996 article by a couple of British biologists, in a peer-reviewed journal called "Biodiversity and Conservation":
"Here, we use a variety of methods to estimate the global number of individuals for a single taxon, birds. The different methods yield surprisingly consistent estimates of a global bird population of between 200 billion and 400 billion individuals (1 billion=10<sup>9</sup>)."
Nice, Apex!