I've got a general question about the evolution of birds. Am I to believe that all birds evolved from a single family of feathered dinosaurs, or did they evolve from separate families of feathered dinosaurs? It's kind of odd that birds with different bodies like chickens, pigeons, owls and penguins could have a single solitary ancestor while ostriches could evolve from feathered versions of an Ornithomimus or Struthiomimus. I would certainly be interested in your opinion on this.
Good question, by the way.
It appears that along the way of evolution of dinosaurs, that some became feathered, and we aren't really sure why. Some believe that the feathers were used for display during courtship....either against another male or female, or to attract another male or female. For the most part, it is the male bird that has the wildest, and brightest colors today. Others believe that it had some other reason, such as insulation.
Did they all come from the same line? Since the DNA of almost all dinosaurs is no longer obtainable for the most part, no one can say whether there were several whose lines led to birds or only one or two. Today, birds are birds.
(We know, for example that there were several types of hominids existing at the same time, and some likely interbred. So, which hominid is the direct ancestor to today's humans? No one knows. If they interbred, the DNA is no longer pure, so to speak. Did they all have a common ancestor? Sure. Very likely all life on this planet had a common ancestor. Google stromatolites. They still exist.)
And they are distinguished by being feathered. To you and me, they all look different. To the anatomist, they look exactly the same.... they all have a head, beak, neck, four limbs a body and a tail.... and feathers. The size of any of these is immaterial. Animals of all sorts adapt to their environment, and keep up with it if it does not change so fast as to stress them into extinction.
The differences in sizes and shapes are the results of adjusting to the environment. From, say, the first feathered dinosaur, those feathers were an advantage. And interestingly, all bird have teeth in the embryonic stage before hatching....ALL!!! Just like most dinosaurs!!!
And as the planet changed, and with the extinction of all large dinosaurs and about all life millions of years ago with the asteroid that struck in the Gulf of Mexico, only the smallest of animals of any kind survived... and of course among them were small mammals, and small dinosaurs. The larger ones were gone.
I can certainly understand your confusion of the differences in birds, but that all is simply the outer "coat". Inside, they are all alike. Ostriches developed huge "drumstick" legs, because with their ever growing size, flight wasn't going to be an option. So, 4000 generations down the road you have an ostrich today in Africa, able to run, kick, defend itself, and it has no reason to fly.... the wings are used for sexual display now.
Small mutations that work tend then to become dominate within that population.... so don't be sidetracked by outer appearances. In fact, all animals with an internal skeleton are exactly alike.....all have a head, body, four limbs and a tail. Some have a neck, but not all.(Fish don't have necks.) And some of the limbs are not even visible. Whales for example have a pelvic girdle, but not legs on the outside. You can see that pelvic girdle only in the bare skeleton, and yet, they have hair, breathe air, bear live young and nurse them, just as do all mammals.
All that is needed for the diversity of birds to appear is death, and time. Another one I find interesting are the Galapagos Island cormorants. They cannot fly. Why not, they have wings, right?
Nature tends to get rid of "expensive" items. And huge chest muscles to power those wings are expensive to maintain. Thus, they are used for swimming, not flying. And since there were no predators when cormorants either drifted there on wood or were blown there in a storm, wings were pretty useless, and the muscles to run them very expensive. Thus, over the eons, Galapagos Island cormorants lost the ability to fly, but developed extensive muscles in their legs. They did not have to fly after their pray, they swam under water using those powerful legs to catch fish. And this has worked for them for millions and millions of years. Only when humans discovered those island, and brought with them cats, and rats, has that bird had problems surviving on some of the islands.
So do not let the size or shape of body parts of the bird fool you.
Read the book, "The Ancestor's Tale"...beautifully written, and 600 pages long, yet you will not be bored. Rather than a book that starts at the beginning of life, when RNA first found a way to split apart, and then team up to another molecule of DNA, which was the beginning of reproduction, this books starts from a modern world, and marches back to the beginning. One of the very best books out there on mutations, and why and how animals change.