Introduced species
People brought cats and stoats to islands.  When birds live on islands with no mammal predators, they sometimes do not build nests in trees.  Instead, they nest on the ground.  This makes it easy for the cats and stoats to eat the chicks and eggs.  Some birds, for example the kiwi and kakapo do not fly.  The animals that like to eat the kakapo were mostly birds of prey that flew high overhead.  They would find the kakapo by seeing it.  The kakapo would stand very still so the bird of prey would not find it.  But this does not work against a mammal predator that finds its prey by smelling it.  The kakapo would stand very still, and the introduced mammal would catch it.