Complex sentence: Areas of excessive infestation of the parasite mistletoe badly damaged or killed the host trees and the reaction to this was to try and eradicate the bird seen as spreading the parasite, the mistletoebird.
Split into simpler sentences: Areas of excessive infestation of the parasite mistletoe badly damages or killsthe host trees. The reaction to this was to try and eradicate the bird seen as spreading the parasite, the mistletoebird.