[DouglasKC]

But today we have fossils of dozens of species of early dinosaur-like birds and bird-like dinosaurs. These include many obviously transitional forms.

How many?

So the conclusion -- not "assumption" -- that birds came from dinosaurs seems reasonable and warranted.

Nah, I think assumption is the right word here.

[JimSEA]

No assumptions. There are tens of thousands of bird and bird ancestors in numerous locations but best preserved in China. Birds are descended from a dinosaur Suborder Theropoda, Clade Coelurodauria. Hollow legged, feathered dinosaurs similar to the Velociraptors of movie fame. They weren't birds but had a common ancestor. The study of the bird / dinosaur relationship continues in earnest today with the study of many thousands of fossils.

130-Million-Year-Old Fossil from China is the Oldest Relative of Today's Birds

Feathered fossils from China reveal dawn of modern birds

[BroJoeK]

DouglasKC on transitional fossils: "How many?"

I would say, "all of them" since every fossil, without exception, is "transitional" between its ancestors and descendants, if any.

But this link lists seven transitional fossils, including archaeopteryx.

This illustration shows four pre-bird dinosaurs before we even reach the first bird.

And here is a more complete diagram:

DouglasKC: "Nah, I think assumption is the right word here."

The difference between "assumption" and "conclusion" is that conclusions can and sometimes do change with new evidence and better ideas.
Conclusions are reviewed & debated based on acknowledged facts, and that has certainly been the case with bird evolution.

So they are conclusions, albeit based on key assumptions underlying all of natural-science: 1) only natural explanations for natural processes and 2) processes we see today worked the same in Deep Time.