Complex sentence: On oceanic islands (where mammals are often scarce), small birds--mainly passerine--may make up the bulk of its diet while elsewhere birds are only important food during a few weeks each summer when inexperienced fledglings abound.
Split into simpler sentences: On oceanic islands (where mammals are often scarce), small birds--mainly passerine--may make up the bulk of its diet. Elsewhere, birds are only an important food during a few weeks each summer when inexperienced fledglings abound.