The answer is mutation
of genes. The mutations were beneficial and gave the individuals of the population
advantage and hence increased their survival. The classic features of birds
evolved one by one and gave birds advantage over their dinosaurs counterparts, especially the feature of small size.
I believe the answer is: Different scientists use different data for investigation.
I say that because different scientists have different equipment, ideas, and have different methods. I hope I was right and i hope it helped and i wasn't to late to answer this question.
