It's been widely known in ecology for a while that some level of ecosystem disturbance is good for diversity. It's called the intermediate disturbance hypothesis. Basically, in a very stable environment, a few species can out compete the others leading to limited diversity. In environments that are frequently disturbed (e.g., frequent, intense forest fires), only the heartiest of species can survive, again leading to low diversity. Some level of disturbance tends to maximize diversity.

In the case here, many bird species have very specific habitat requirements, with some only occurring in forests, others only in meadows, and some that can go between. When you have fires that destroy parts of the forest, you open up habitat to become meadows as part of the successional pathway. Thus, you end up with forest birds remaining in the intact forest, and new species of meadow birds appearing.