It's basic biology. All complex living things are united by their cellular structure... we are all eukaryotic organisms as opposed to bacteria who are prokaryotic.
The first big division between the multicellular organisms was between plants and Opisthokonts (animals and fungi) (yes that means you are more closely related to a mushroom than a potato)
They are the ancestors of the birds. Many of them are so bird like that it's hard to tell them apart. Wings, feathers, bird like behavior... the line between what is a dinosaur and what is a bird is very very fuzzy.
For example:
Archaeopteryx... widely known as "the first bird"
Microraptor... a gliding dinosaur closely related to both Velociraptor and Archy...
wa:do