FOOD AVAILABILITY, PARENTAL CARE AND MALE MATING SUCCESS IN RED-WINGED BLACKBIRDS (AGELAIUS-PHOENICEUS)

1. In species with biparental care, relative parental investment of th e sexes and male mating success may be influenced by the availability of food. We examined this hypothesis in red-winged blackbirds breeding at two marshes in Alberta, Canada that differed substantially in food availability. 2. At a marsh surrounded by woodland, females delivered greater biomass of food per hour to their nestlings than females at a marsh surrounded by agricultural land. As a result, nestling mortalit y due to starvation was lower, and unassisted females fledged more off spring at the woodland marsh than at the agricultural marsh. 3. Male p arental care was less important to offspring survival at the woodland than the agricultural marsh. As a result, very few males at the woodla nd marsh fed their young whereas all males at the agricultural marsh f ed at least one brood. Thus, male parental investment was related nega tively to food availability. 4. Male parental investment was also rela ted negatively to variance in male mating success. Males at the woodla nd marsh gained over twice as many mates and had significantly greater variance in mating success than males at the agricultural marsh. 5. O ur data suggest that abundant food reduces the importance of male pare ntal care to offspring survival. This allows males to reduce their par ental investment relative to the female's, and it reduces the cost to the female of mating with an already-mated male. As a result, more mal es than females are available as mates, competition for mates increase s among males and, consequently, the variance in male mating success i ncreases. These results are consistent with the hypothesis that relati ve parental investment of the sexes influences sexual selection and ma ting systems in birds.