Complex sentence: At the beginning of the 19th century the Little Tern was a common bird of European shores, rivers and wetlands, but in the 20th century populations of coastal areas decreased cause of habitat loss, pollution and human disturbance.
Split into simpler sentences: At the beginning of the 19th century the Little Tern was a common bird of European shores, rivers and wetlands. In the 20th century populations of coastal areas decreased cause of habitat-loss, pollution and human disturbance.