A limit to birds affinity for high vantage points
Birds find their niches based on many reasons. Their choice is based on primarily resource availability, predation risk, and competition. Keep in mind there are variations among species, most birds like to forage for food at a hight safe enough to avoid ground predation, at the same time to be able to see and find food without much competition. Therefore very high altitudes do not seem beneficial for food resources and its a cost in energy, thus reasonable altitudes are preferred. But again many variations depending on species and location.