To all the people saying Turians don't look like birds: that's been a running joke in Mass Effect since like ME1. Even Grunt, during the Citadel party, mutters "they don't look like birds...they look like cats!"
It's also worth noting that just because they are referred to as "avian" does not mean that they are "anatomically and physiologically identical to earth bird species". It just means they are "avian-like" from a human perspective, attempting to describe an alien species in familiar terms. Maybe their bones are partially hollow. Maybe they have a unidirectional air flow system in their lungs and the absence of a diaphragm. Maybe they have a brain structure analogous to the optic tectum of earth birds and used for the same purpose. But they don't have wings (not even apparent evidence that they ever HAD them to begin with), nor feathers, and their armored skin is only roughly analogous to scales. They are more similar to a heavily anthropomorphized theropod in appearance than a bird, and even that isn't really descriptive.
Similarly, the Salarians are "amhibious" but are clearly not homologous to earth amphibians except in the rudimentary sense that both are amphibious. They're just words that humans use to describe alien-ness.
I also doubt that an animal that could fly could even evolve on Palaven to begin with, given the conditions that they listed. If they could, they'd probably be extremely small animals. It is roughly similar to Earth in all ways, with the exception of the massively increased exposure to radiation. Thus, almost all animals on Palaven apparently have the thick, armored plates on their skin that we see with the Turians. Undoubtedly this would add some weight, possibly offsetting any benefit that decreased body size and mass would have on lift capability. But that's a side point, just something I always found interesting from a biology standpoint. Point is - just because something is described as "avian" does not mean they have to resemble birds to the point that it is easily recognizable.