Those who promote the dino-to-bird idea, claim that "there are no other suitable candidates for avian ancestors."
They overlook the obvious candidates - PTEROSAURS.
"Other arguments, such as the putative differences between theropod and bird finger development, or lung morphology, or ankle bone morphology, all stumble on the lack of relevant data on extinct theropods, misinterpretations of anatomy, simplifying assumptions about developmental flexibility, and/or speculations about convergence, biomechanics, or selective pressures. The opponents of the theropod hypothesis refuse to propose an alternative hypothesis that is falsifiable. This is probably because there are no other suitable candidates for avian ancestors.".