DVD Players are slowly being replaced by Blu Ray Systems since they are becoming the new medium of entertainment.

Who is "they" in this prior sentence?
Blu Ray Systems
New York is very crowded compared to South Dakota, but they are also much smaller in square miles.

Who is "they" in this prior sentence?
New York
The bird perched on the limb and it bent.

Who is "it" in this prior sentence?
the limb