Birdcages could be replaced in a sentence with a number of synonyms such as Aviaries, cages, coops, enclosures, huts, or pens. Each of these words could be used to describe an enclosed area in which birds are kept captive. Aviaries are typically larger than birdcages and may be used more for display or breeding purposes. Cages, on the other hand, are smaller and often used for single or a few bird pets. Coops and pens suggest the housing of fowl in a more farming or agriculture-oriented environment. Regardless of which synonym is used, the fact remains that these spaces are used to hold and protect birds.