Seven cases have been reported in Espirito Santo state, with another discovered in Rio de Janeiro state.
The emergency declaration makes it easier for the government to now bring in measures to stop the highly infectious H5N1 virus from spreading.
Brazil is the world’s largest exporter of chicken meat, with annual sales of nearly $10bn (£8bn).
Authorities say the cases were found far away from Brazil’s main areas of production in the south of the country.
However, outbreaks in commercial flocks elsewhere have sometimes been observed to follow the discovery of avian flu cases in wild birds.
The finding of a case on a farm often triggers a cull of a large number of birds – and can sometimes prompt trade restrictions from other countries.
The health emergency has been declared across the whole country for the next 180 days as a precaution.
The world has been going through its worst ever outbreak of bird flu since October 2021, causing the deaths of more wild birds than ever before. Some mammals are also catching the disease.
Scientists remain unsure why this outbreak is proving so much worse than others, but the World Organisation for Animal Health (WOAH) has reported “devastating impacts on animal health and welfare”.