!'

Head. The acrosome and nucleus constitute the head. This was found to be helical in 18 passeridan species examined by Birkhead et al. (2006) with the exception of the rounded head of the Eurasian bullfinch. The number of head gyres ranged from 2.3 in Song thrush (Turdus philomelos) to 4.6 in Sedge warbler (Acrocephalus schoenobaenus); the corvids, Carrion crow (Corvus corone) and Rook (C. frugilegus) had 3.5 and 3.6 turns. All of the 18 species illustrated by Retzius have helical heads (6 Corvida, Figs. 8.37, 8.38; 12 Passerida, Figs. 8.41, 8.42). Acrosome. The very long, tapering acrosome can be three or four times the length of the albeit short nucleus (McFarlane 1963). Some acrosome:nucleus ratios are given in Table 8.6. These may be compared with ratios for paleognaths and non-passerines in Table 8.5. For references see species cited.