Bird is a creature type.
They are exactly what you would expect from them, birds. But through the years they have made some developments.
The first bird in Magic was Birds of Paradise, although it began life with the somewhat questionable creature type mana birds. After that a lot of sets introduced creature types like falcon, roc and Giant Albatross. Eventually, Wizards decided they did not need all these different kinds of birds and changed them all retroactively into birds.
Until Onslaught there never were many birds at once in any set. Onslaught Block however introduced the Aven. A race of giant humanoid birds. Since then the creature type has had a number of new members and even a "lord". The type has persisted through Future Sight and Shards of Alara to become one of the more prominent creature types in Magic.