The complex anatomy of the infraorbital sinuses and diverticula lends itself to the establishment and persistence of upper respiratory tract infections. When combined with factors such as hypovitaminosis A (causing squamous metaplasia of the epithelial lining of the sinuses and decreased normal function and resistance to infection), irritation of the sinus linings (e.g. ammonia toxicosis in poorly ventilated, unhygienic cages and aviaries, cigarette smoke, or other aerosol pollutants), extremes of humidity (too dry, too moist) or choanal atresia (see below), it is little wonder that such infections are common in birds. Anatomic abnormalities such as choanal atresia, collapsed frontal hones, and traumatic injuries can also predispose to infection, as can inhaled foreign objects (e.g. millet seed, hand-rearing formula) and neoplasia (carcinoma, adenocarcinoma, fibrosarcoma, lymphosarcoma, and malignant melanoma).