PATTERNS OF COURTSHIP BEHAVIOR AND EJACULATE CHARACTERISTICS IN MALE RED-WINGED BLACKBIRDS

Sperm competition in birds is likely to have important effects on the behavior and physiology of reproduction in both sexes. For males, such competition should select for large sperm reserves and behavioral adj ustment of copulation when reserves are low. We investigated both thes e possibilities in free-living red-winged blackbirds (Agelaius phoenic eus), a highly polygynous species with apparently strong sperm competi tion. We found that the recent copulatory behavior of males did not af fect the propensity to copulate with a model female. Ejaculates collec ted from individual males at l-h intervals showed no evidence of sperm depletion, yet repeated ejaculates collected less than 10 min apart d id. Male ejaculate size was significantly larger if it was the first o ne of the day (i.e., after an overnight rest). The average ejaculate s ize was 12.5 (+/-12.5 SD) million sperm. Males captured during the bre eding season had an average of 111.7 (+/-52.8) million sperm stored in their seminal glomera. Because males average a peak copulation rate o f six per female per day, in one day a male might utilize all the sper m in his seminal glomera if more than two females on his territory are fertilizable. We hypothesize that polygyny and sperm competition in t his species have combined to select for rapid replenishment of the sem inal glomera throughout the day, in contrast to other species that hav e been studied. Testis size and sperm reserves of male red-winged blac kbirds are intermediate between monogamous species and species with in tense sperm competition. Several possible explanations for this are di scussed.