If evolving makes you better, then why did avian dinosaurs evolve into birds instead of becoming even bigger birds?

This is a fundamental misunderstanding of evolution, and is based on the human tendency to assume that things will always get bigger, better, taller, etc. When evolution is illustrated in textbooks, it is often illustrated as a tree, or a ladder, or some other progression from simpler to more complex.

But evolution doesn’t have a goal, and it doesn’t move in any specific direction. Everything that is, is pure mathematics. Living things that reproduce and survive best are those that increase in number. Always. If being dumber, slower, or simpler is advantageous to survival, and sometimes it is, then the species will head in that direction, instead. Everything has compensations. A larger brain requires much more energy. So does being hot-blooded. It’s not always worth it when resources are scarce. Being bigger is certainly not advantageous when times are tight. The bigger an animal is, the more fuel it needs to stay alive. There are also serious limits on movement when it gets too big. Blue whales can largely ignore this by being in the water, which supports most of their weight and allows them to move using comparatively little energy. But they need to eat about four tons of krill per day.

Avian dinosaurs evolved because flight is enormously advantageous. Their wing design allowed them to be very competitive with pterosaurs, which had trouble maneuvering within dense forest canopies. (Pterosaur wings are long and narrow, a good design for flight, but they get in the way in dense undergrowth. Birds can have very short, wide wings.) However, to be able to fly, you have to be relatively small, and definitely as light as possible. Otherwise, you would simply have to use too much power, more than you could reasonably find.

When the asteroid struck, creating a blanket of winter across the globe, vast expanses of vegetation died. There was no fuel for the large animals. They died. Only small animals could make a living by scavenging what was left. And the small, maneuverable birds that could fly to find new food resources. Why they did, but the pterosaurs didn’t, is up for debate. I personally think it has something to do with the fact that pterosaurs didn’t care about their young. Perhaps the way they built their nests did not allow their eggs to survive through the winter. But that’s just a guess. Birds incubate their eggs using body heat (well, most of them do), so that may be why they fared better. (Edit… I have since learned that the pterosaurs of the time were almost all large animals. To compete with birds, they became large, as the pterosaur form allows for a much larger flying animal than the bird form. So it stood to reason that they would go extinct with all the other large animals.)

Once the dust settled, why didn’t the birds grow back? The truth is that flying is REALLY advantageous. It is extremely useful. Leaving him would require a special set of circumstances. It would have to be easier to survive when it’s big than when it can fly, and that’s rare.

Which is not to say that it hasn’t happened.

But other mass extinction events have shown that the largest animals are often the first to go, and humans wiped out some of these giants as well (like the moa). Still, there are large flightless birds in Africa, South America, and Australia; They easily compete with mammals.

The largest animals today may be mammals, but there are still more species of dinosaurs alive today than there are species of mammals. So which of the two groups is REALLY the more successful? Are we going by the size, or by the numbers? What makes you think that the birds are not already better, just as they are? The size is not everything.