Dear Student,
Evolution, as a whole is defined as a gradual and continuos process of change and adaptation of species towards the changing environment , which has been passed to generations as heritage gene modification. It is leading to structural and functional changes for surviving in the competition of life.
The detailed study of fossils of dinosaurs , and parallel study of modern organisms has found a link with dinosaurs and modern day birds. During the period of dinosaur extinction. Some fossils have shown resemblance to modern day reptiles also. This proves that birds and reptiles are modern day evolved dinosaurs. Thus, they are ancestrally related.
Regards,