SEASONAL PATTERNS OF SPERM STORAGE IN THE YELLOW-HEADED BLACKBIRD XANTHOCEPHALUS-XANTHOCEPHALUS

The seasonal pattern of sperm storage was examined in a wild populatio n of Yellow-headed Blackbirds Xanthocephalus xanthocephalus by collect ing females at eight different stages of their breeding cycle. Sperm s torage tubules (SSTs) were present in similar numbers throughout all s tages of the breeding season, from arrival on the breeding grounds to departure for winter. In contrast, the size of SSTs varied enormously over the season, reaching maximum size just prior to clutch initiation , when SSTs were more than three times the length of those found in po st-breeding birds. The amount of sperm stored also varied over the sea son, in parallel with the growth and regression of SSTs. Sperm were fi rst observed in SSTs approximately 2-5 days before laying and increase d rapidly in abundance until clutch initiation. The amount of sperm st ored then decreased gradually during the egg-laving period so that by clutch completion approximately 70% of stored sperm had disappeared fr om the SSTs. The rapid seasonal development and subsequent regression of SSTs in the Yellow-headed Blackbird contrast sharply with the patte rn found in domestic poultry, where the imposition of continuous layin g regimes has led to the maintenance of SSTs at maximum size for exten ded periods of time.