SAT Inferences Question

The Hawaiian archipelago is home to the kiwikiu (Pseudonestor xanthophrys), one of several bird species that can only be found on the islands. Due to Hawaii's uniquely varied geographic and ecological conditions, the kiwikiu became highly specialized as it adapted to survive in a specific island habitat. However, like many highly specialized species, the kiwikiu is particularly vulnerable to environmental stressors, such as disease, invasive species, and habitat destruction, that disrupt the delicately balanced ecosystems in which the birds live. Based on the text, why is the kiwikiu a highly specialized species?