What a good question? As most dinosaurs would tell us (if they weren’t extinct), there is evidence that being big can be bad for surviving. Being small has long been thought to help things survive extinction by having larger populations, greater genetic variation, shorter time to maturity, and fewer resources to suvive.
In a study done by paleontologists on a collection of bird ancestor bones approximately 65.5 million years old, it was found that there was a general increase in size for the birds in three of the four families. The big point of interest is the Ornithuromorpha, the last of these four families, got smaller over time and is the only family of birds who have members that managed to survive the Cretaceous–Tertiary extinction event.
So as Americans get bigger and stronger, I believe birds find their best chance at survival is if they stay small.
I couldn’t find any recent study on the size of birds, but based on observations of birds from the 1800’s to the present there is no obvious change in birds’ size unless helped along by farming practices.
Keep the questions coming and I’ll answer them as best I can. You can also chime in if I get something wrong.
Source: The DinoBase Public Forum http://dinobase.gly.bris.ac.uk/forum/viewtopic.php?id=538