If birds evolved from dinosaurs that had high metabolisms and mammal-like complex teeth and then lost them and kept them lost even when they stopped flying (and we know birds have the ability to start growing teeth again, so it is actually significant that they didn't); then that would be, as you say, a significant problem with the theory that mammals' teeth are important to their metabolisms.
However, that is not the case. The dinosaurs birds evolved from, theropods, did not have complex, interlocking teeth, they had the standard rows of spikes common to vertebrates (as opposed to ornithopods, which had complex, differentiated teeth like mammals).
So the question is not, "how could birds have lost their teeth if teeth are crucial to a mammal's high metabolism". It is "how did theropods, avian and non, solve the problem of high metabolism that mammals require complex teeth to solve". This is a much less problematic question, since it suggests theropods evolved a different solution (there are always other solutions), rather than that birds had a good solution and lost it.
Re: Aw man, LJ ate my original comment:
:( I hate when that happens.
If birds evolved from dinosaurs that had high metabolisms and mammal-like complex teeth and then lost them and kept them lost even when they stopped flying (and we know birds have the ability to start growing teeth again, so it is actually significant that they didn't); then that would be, as you say, a significant problem with the theory that mammals' teeth are important to their metabolisms.
However, that is not the case. The dinosaurs birds evolved from, theropods, did not have complex, interlocking teeth, they had the standard rows of spikes common to vertebrates (as opposed to ornithopods, which had complex, differentiated teeth like mammals).
So the question is not, "how could birds have lost their teeth if teeth are crucial to a mammal's high metabolism". It is "how did theropods, avian and non, solve the problem of high metabolism that mammals require complex teeth to solve". This is a much less problematic question, since it suggests theropods evolved a different solution (there are always other solutions), rather than that birds had a good solution and lost it.