Yes, true.
I just recently came a across a couple things with both Biologists, & Zoologists that are talking about birds being actually reptiles, but the subject is still controversial to some.
This is the next major milestone in evolutionary biology. The two most basic distinctions in Subphylum Vertebrata are cold/warm bloodedness and live birth. (Birds, monotremes, and sharks are the three that buck the convention of warm blood and live birth.) Recent discoveries (e.g., egg shell research - Yale) indicate that some dinosaurs may have been warm blooded, which makes sense, because large body mass, flight, and the metabolism of fibrous plant matter and/or meat all require massive amounts of energy. The laws of thermodynamics dictate that heat would be a product of the use of that energy, and at some point the storage of that energy for later use would be an advantage, especially to be able to traverse multiple climactic geographies (you know at some point if you're a brontosaurus or t-rex, you're going to eat all the food in your area and need to migrate). That was always a prevailing argument for the rise of mammals, but it's hard to imagine that the giant or flying dinos would have thrived if they relied solely on the environment to maintain their body temperature. Another supporting argument would be the ability to breathe air, a significant contributor to metabolism, which is a distinction between most (juvenile) amphibians and reptiles, and fish and aquatic mammals.
Had extinction events not occurred that greatly reduced the available food sources and favored the more efficient mammalian adaptations for thermoregulation, we might still have the giants roaming the earth today. (Not actually likely because...humans, but still an interesting thought. Or perhaps humans would not have evolved as quickly, or at all?

I bet our primate ancestors would have been tasty treats to meat eaters.)
Without birds, however they evolved, the human mind wouldn't have either. Besides microorganisms and peas, I can think of few other examples of cornerstones in biological science. Darwin's Finches and Archaeopteryx are as important to Biology as Einstein and Newton are to Physics.
To be able to see a living part of the history of life on this planet every day in my backyard, and knowing that the night sky and the oceans have been constants for millenia, it's astounding - the simultaneous feeling of being a giant and a speck of dust.

