Complex sentence: His study of harriers resulted in the discovery that the Montagu's Harrier was breeding in southern England, and he was also involved in the first British records of Cirl Bunting, Cattle Egret, Little Gull and Gull-billed Tern.
Split into simpler sentences: His study of harriers resulted in the discovery that the Montagu's Harrier was breeding in southern England. He was also involved in the first British records of Cirl Bunting, Cattle Egret, Little Gull and Gull-billed Tern.