Complex sentence: New nests are built each year generally, as the re-use of old ones might spread disease or parasites--nests become caked with faeces as the nestlings grow and the parents cannot keep up with its removal.
Split into simpler sentences: New nests are built each year generally, as the re-use of old ones might spread disease or parasites. Nests become caked with faeces as the nestlings grow and the parents cannot keep up with its removal.