Cloacal Protuberance and Cloacal Sperm in Passerine Birds: Comparative Study of Quantitative Relations

Size of cloacal protuberance, a putative criterion of male sex and breeding condition in many passerines, and number of sperm in cloacal lavages were determined in males of 21 passerine species (n = 275 individuals), at Foley, Missouri, 1 to 14 May 1984. The objective was to test the quantitative relations of these two measures and to compare their vernal temporal changes. Relative size of the cloacal protuberance (cp) was scored, providing a cloacal protuberance index (CPI: 0 to + + + +). Numbers of sperm in lavages without massage were estimated from counts using phase contrast microscopy of dried smears, giving a cloacal sperm index (CSI: 0 to 5.0 = 0 to 1.2× 107 sperm/lavage). Correspondence between CPI and CSI was generally slight to lacking. Indeed, some males lacking a cp had abundant cloacal sperm, either as migrants and/or during the beginning of their breeding and nesting, while some others of the same species with a well developed large cp lacked cloacal sperm. The CSI was equivalent in first-year and older male Indigo Buntings (Passerina cyanea), although secondary sex, or dimorphic, characteristics (CPI and plumage) were greater in the latter. It is concluded: (1) that in examined passerines, presence of a large cp can be used as a criterion of male sex, but that its absence or small size is not a valid criterion of either sex; (2) cloacal release of normal sperm in large numbers occurs in males both with and without a cp, and therefore the cp is not a reliable criterion of breeding capability.