He was. He just didn’t get the mechanism behind it right.
A crude way of explaining Lamarckian evolution would be to look at giraffes. Lamarckism suggests that because an animal that spends much of its life stretching its neck to reach food, it ends up with a slightly longer neck. This trait is then passed down to children, who might spend much of their lives stretching their necks, making them slightly longer. And so on.
He correctly identified that speciation occurs over many many generations, as a result of tiny incremental changes.
What Darwin did was to recognise the actual mechanism behind speciation - Natural Selection. Darwin was aware of and built on Lamarck’s work.
Weirdly, within the last thirty years, we’ve realised that the truth is not so clear cut. Epigenetic changes do occur as a result of the environment and are hereditary. While genes are still the main drivers of evolution, these epigenetic changes affect gene expression.
Taxonomically speaking, birds are dinosaurs.
There isn’t a place to put a line between them - all the things that make birds “birds” also apply to dinosaurs.
A super fun fact is that of the two main types of dinosaur, Saurischia (“lizard-hipped”) and Ornithischia (“bird-hipped”), birds actually evolved from the lizard-hipped group.