Reproductive Success of Dunnocks, Prunella modularis, in a Variable Mating System. I. Factors Influencing Provisioning Rate, Nestling Weight and Fledging Success

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Abstract
Provisioning rate to the nestlings influenced nestling weight and nestling weight influenced survival to independence. Provisioning rates, nestling weights and reproductive success per breeding attempt varied across the different mating systems, being greatest where two males assisted a female to rear young (cooperative polyandry), less where only one male and a female reared young (monogamy) and least where one male divided his effort between two females (polygyny) or two males divided their efforts between two females (polygynandry). In polyandry (two males with one female) reproductive success was high if both males mated with the female because they then both helped to feed the young. Success was low, however, if the alpha male prevented the beta male from mating because then only the alpha male helped the female to feed the young and the beta male sometimes interfered with the breeding attempt. In polygynandry, failure rate was high because aggression between the females sometimes caused one of them to desert her clutch. When one or two males had two females with young at the same time, they tended to help feed at both nests if the females had similar numbers of young. If, however, one of the females had many more young the male(s) helped her full time and left the other female to provision her brood alone. Reproductive success varied with season but there was still variation through the season across the different mating systems. Reproductive success did not vary with habitat within the study area. The results of natural removal experiments and matched comparisons of reproductive success of the same territory but in different mating combinations support the view that the main determiant of reproductive success is the mating system because this influences the number of adults who feed the young.