Interesting, when you consider that a jaw with teeth is obviously a much more complex structure than a beak. It makes sense to me that a simpler basic structure would be more adaptable.
Smeghead’s comment with regard to lightness for flight reminds me that one of the larger prehistoric toothed birds, Hesperornis was aquatic and flightless–and moreover lacked external wings or forelimbs of any kind. Perhaps water birds were the last to lose their teeth.
No, many of the Enantiornithes of the Cretaceous were flying birds but retained teeth. They went extinct at the same time as hesperornithine birds, that is, at the end of the Cretaceous.
Obviously I’m 5 years late to this thread, but I thought I’d chime in anyway. Birds are covered in feathers as well as having flight feathers on their wings. Grooming these flight feathers with teeth would be far more difficult than using a beak, if not impossible. Irregardless of the type of beak one thing that can be said of all birds is that their beaks are generally smooth with is a huge advantage in grooming. Flightless birds such as emus, cassowaries, ostriches and penguins never reverted back to having teeth since there were no environmental pressures to lose their beaks.
Bats never developed beaks due to having membrane wings and can fly just fine with muzzles filled with teeth so that disproves the theory that birds developed beaks in order to save weight for flying.
Groomed wing feathers simply expend less energy which comes in very handy during periods that food is not readily available.
Of course not true, since flying birds with teeth were perfectly successful for something like 80 million years. And as has already been mentioned, toothlike projections on the beak in some modern flying birds don’t prevent them from grooming their feathers.
And of course this doesn’t “disprove” anything. Bats, pterosaurs, and many early birds and other flying dinosaurs were capable of flying even though they had teeth. And many animals have developed toothless beaks, even when weight isn’t a consideration, such as turtles.
But none of this disproves the possibility that saving weight might have been a factor in prompting the initial loss of teeth in the lineage leading to modern birds. For every flying organism, saving weight is a consideration, but there may be multiple routes to achieve the same result. Loss of teeth may be one way to save weight, even if it is not a necessary condition for flight.
Actually, feathers are quite expensive energetically, and may weigh more than a bird’s entire skeleton. They are also expensive because most birds need to shed and replace them once or twice a year to keep them in good condition.
A mouthful of teeth will most certainly inflict far more damage than just 2-3 toothlike projections on a birds beak.
I already pointed out that bats do not have flight feathers and therefore do not require beaks for grooming. That’s the connection I am making after all, --beaks and feathers. Take away the feathers from the get go and introduce a similar type of wing onto a bird such as a bat or pterosaurs wing, and the beak is not needed.
You mention feathers are quite expensive energetically ----thanks for proving my theory that birds need beaks to groom their feathers to sustain flight for longer periods of time when food is not so readily available as it was prior to the gradual cooling of the planet. Food was more plentiful and easily found back when birds had yet to develop their beaks. Not so today, nor in the past tens of millions of years.
Beaks may indeed be lighter than a muzzle with teeth, but unless those flight feathers are operating at their peak efficiency, the competition for the food will win out, as is most likely what happened when the birds who evolved beaks first then drove their toothed cousins into extinction.
The bottom line is that no meal is guaranteed, and todays climate as well as the climate of the past tens of millions of years is different than what the toothed ancestors of birds had to deal with. At some point something had to happen to inflict environmental pressures on birds to develop beaks, and it wasn’t just the weight saving design of the beak. I suppose we could just call it a two in one advantage. A great weight saving grooming device.
As I said 5 years ago, back in the Cretaceous Era there were plenty of toothed birds, and they seemed to be at no particular disadvantage to beaked birds. It happens that all modern birds are descended from a few types of beaked birds, and all the lineages of toothed birds and various other flying feathered dinosaur lineages. The beaked birds didn’t out-compete the toothed birds for the 80 million years of the Cretaceous Era, so beaks simply weren’t an overwhelming advantage.
Maybe beaks were a key feature that enabled some lineages of birds to survive the KT extinction, but it wasn’t enough to make beaked birds dominant in the Cretaceous itself. The notion that life was easier and food more plentiful back in the Cretaceous makes no sense. It wasn’t that beaked birds out-competed toothed birds in the harsh conditions of the Cenozoic. It was that a few type of beaked birds survived the KT extinction and founded all the modern populations of birds, but no toothed birds survived. One one side of the divide we have diverse flourishing populations of both types, after the event only one type.
Wow… somehow I missed most of the responses to my posts when this thread was live five years ago. 
Addressing why mammals don’t have beaks, my take: Mammals have very specialized teeth that usually mesh together with very fine tolerances. The teeth are not replaced because constantly losing and regrowing teeth would tend to eliminate the advantage of that close mesh. So while many animals (think crocodiles) have tear food into big pieces and swallow them whole, mammals have the advantage of being really good at biting off small pieces and chewing them thoroughly. This has all kinds of implications for the digestive system.
The first toothed birds would not have had mammal-like teeth, but reptile-like teeth. Obviously, that combination isn’t all bad, since they survived that way for so long… but they sure didn’t make it to the modern age.
I see what you did there.