Depends on your regular vets confidence with dealing with birds! I suggest ringing around and finding out yourself about each vet.
My first 'avian vet' was not avian certified, but had studied zoo med and so had lots of dealings with birds. He lacked the official qualification but he was more than knowledgeable about bird conditions and ailments etc for me to trust him with Merlins life.
So no, an 'avian vet' isn't always necessary. Just find a vet who understands a birds medical needs.
My vet was "Ian Mostyn BVM&S CertZooMed MRCVS" Which means he's a Member of the Royal College of Veterinary Surgeons and has a Bachelors Degree in Vet Medicine + Surgery and is Certified to practice 'Zoo Med' which is basically exotics and fancy animals that arn't cats/dogs/bunnys.
So look out for ZooMed type awards. MRCVS is a UK thing, so ignore that!
I should also mention that before I got Merlin into this vet's practice I had previously seen a self proclaimed Avian Vet, two of them actually and they were horrific with Merlin. I'm now seeing one of the ACTUAL few Avian Vets in the country and the only reason I'm seeing him over Ian Mostyn was that I moved and so I'm closer to him now. His surgery is avian care only and he is just fantastic, but had I not had the opportunity to have Merlin in his practice I would have stayed with Ian, the non-avian vet without a doubt.
Ian, despite not being an certified avian vet, was caring, professional, knew exactly how to handle Merlin. He knew specifically about flight feathers, how to do clips (we didn't get Merlin clipped) he knew about bird anatomy, bone structure, their respiratory system and infectious illness'. Merlin has a splayed leg too, which we discussed and resolved for the most part.
Seeing an Avian Vet isn't the bee all and end all, your vet just needs knowledge, experience and equipment, the certification is just a bonus.