There are many synonyms for the term "bird pen". One such synonym is aviary, which is often used to refer to a large enclosure designed to keep birds. Another synonym is birdhouse, which is a small shelter designed for birds to nest in. A birdcage is another synonym for a bird pen, which is usually a small cage used to keep pet birds. Some other words that can be used to describe a bird pen include coop, roost, and perch. No matter what word you use to describe a bird pen, the most important thing is to make sure that it is designed to keep birds safe and comfortable.