The semen of some passerine and non-passerine birds was examined. That of passerine birds was thick in consistency and contained relatively few spermatozoa, while that of non-passerine species was more liquid in nature with many spermatozoa.
The two types of spermatozoa were found to differ considerably in morphology and activity, the passerine form showing a spiral configuration and a helical forward movement. The differences were also pronounced at the ultrastructural level.
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