I should say: this would be for pure efficiency optimizations absent any other tradeoffs. What I think you should see in birds based on their more extreme body

I should say: this would be for pure efficiency optimizations absent any other tradeoffs. What I think you should see in birds based on their more extreme body mass constraints vs ground dwellers is different tradeoffs being worthwhile. Like, if there were an adaptation that required slightly more energy, but resulted in a brain that was much lighter; or doing various mental processes 20% less effectively for 50% less brain mass.