This is the 5824th most frequent Portuguese word.
aviária
"Related to birds or birdlife."
In this case, 'aviária' refers to the 'avian' type associated with birds as part of the noun phrase describing the flu.
A gripe aviária é causada por um vírus que afeta aves.
Avian flu is caused by a virus that affects birds.
Here, 'aviária' is used adjectivally to describe the reserve as related to birds.
Visitamos uma reserva aviária que abriga diversas espécies de aves.
We visited a bird reserve that shelters various bird species.
For this usage, 'aviárias' pertains to the traits or aspects relating to birds, described by the adjective modifying 'characteristics'.