OP here, let me rephrase the topic. back in the late paleocene/early eocene, birds were rapidly diversifying and reached considerable size, while mammals stayed relatively small. whales were three meter long and tall as a dog, horses were big as a pincher, and everything else looked like an overgrown shrew. then suddenly, for no apparent reason, birds began to shrink and decline, they never reached truly massive size like the mammals, which caught up in an astoundingly short amount of time (basilosaurus came out not even 10 million years later). they lost competition with mammals on most fronts, as apex predators, as grazers, as megafauna, and were relegated to be either quick scavengers or small fauna predators. by the end of the pliocene, the only really big birds left were freaks of nature living in isolated environments. i wonder exactly what made mammals more successful than birds and what exactly happened to make them suddenly better.