We conducted the fieldwork between March and June in 1994 and 1995 at Hastings Natural History Reservation and on the adjacent Oak Ridge Ranch, Monterey County, California, U.S.A.">Breeding density is predicted to influence the use of alternative reproductive strategies and may account for some of the variation in extrapair paternity rates found within and between avian species. By exploiting the extreme range of breeding densities occurring in a single, semicolonial population of Bullock's oriole, Icterus galbula bullockii, this study showed that, as predicted, both the number of nests containing extrapair nestlings and the mean proportion of extrapair nestlings in a brood increased with breeding density (high versus low breeding density: broods with extrapair nestlings: 62 versus 27%; mean: 48 versus 16%). As found in some other studies, extrapair paternity (both exclusion and assignment) significantly increased the variance in reproductive success among males. No difference was found in the mean reproductive success of males at different breeding densities. However, as predicted, the variance in reproductive success among males increased with breeding density. This study indicates that the distribution of individuals within a single population can lead to significant differences in the opportunity for sexual selection, and that density may be an important factor affecting variance in male reproductive success among populations.

We conducted the fieldwork between March and June in 1994 and 1995 at Hastings Natural History Reservation and on the adjacent Oak Ridge Ranch, Monterey County, California, U.S.A.">Single-locus minisatellite DNA profiling was used to assign paternity in a population of Bullock's orioles, Icterus galbula bullockii, and to determine the contribution of age to a male's success in obtaining extrapair paternity. There was a very low rate of intraspecific brood parasitism (2/202 = 1.0% of chicks). Older adult males lost less within-pair paternity and gained more extrapair fertilizations than did yearling subadult males. This resulted in adult males benefiting from an annual reproductive success more than double that of subadult males. Behavioural observations, used to determine the role of female choice in extrapair copulations (EPCs), indicated that females actively participate in EPCs and that they prefer to obtain them from older males. While it was possible that females obtained EPCs as an insurance against the possible infertility of their social mate, the results of this study fit best with the hypothesis that females were attempting to obtain better-quality genes for their offspring by obtaining EPCs with older, better-quality males.

"Extrapair paternity and variance in reproductive success related to breeding density in Bullock's orioles." Abstract:

Breeding density is predicted to influence the use of alternative reproductive strategies and may account for some of the variation in extrapair paternity rates found within and between avian species. By exploiting the extreme range of breeding densities occurring in a single, semicolonial population of Bullock's oriole, Icterus galbula bullockii, this study showed that, as predicted, both the number of nests containing extrapair nestlings and the mean proportion of extrapair nestlings in a brood increased with breeding density (high versus low breeding density: broods with extrapair nestlings: 62 versus 27%; mean: 48 versus 16%). As found in some other studies, extrapair paternity (both exclusion and assignment) significantly increased the variance in reproductive success among males. No difference was found in the mean reproductive success of males at different breeding densities. However, as predicted, the variance in reproductive success among males increased with breeding density. This study indicates that the distribution of individuals within a single population can lead to significant differences in the opportunity for sexual selection, and that density may be an important factor affecting variance in male reproductive success among populations.

We conducted the fieldwork between March and June in 1994 and 1995 at Hastings Natural History Reservation and on the adjacent Oak Ridge Ranch, Monterey County, California, U.S.A.">Single-locus minisatellite DNA profiling was used to assign paternity in a population of Bullock's orioles, Icterus galbula bullockii, and to determine the contribution of age to a male's success in obtaining extrapair paternity. There was a very low rate of intraspecific brood parasitism (2/202 = 1.0% of chicks). Older adult males lost less within-pair paternity and gained more extrapair fertilizations than did yearling subadult males. This resulted in adult males benefiting from an annual reproductive success more than double that of subadult males. Behavioural observations, used to determine the role of female choice in extrapair copulations (EPCs), indicated that females actively participate in EPCs and that they prefer to obtain them from older males. While it was possible that females obtained EPCs as an insurance against the possible infertility of their social mate, the results of this study fit best with the hypothesis that females were attempting to obtain better-quality genes for their offspring by obtaining EPCs with older, better-quality males.

"Extrapair paternity and variance in reproductive success related to breeding density in Bullock's orioles." Abstract:

Breeding density is predicted to influence the use of alternative reproductive strategies and may account for some of the variation in extrapair paternity rates found within and between avian species. By exploiting the extreme range of breeding densities occurring in a single, semicolonial population of Bullock's oriole, Icterus galbula bullockii, this study showed that, as predicted, both the number of nests containing extrapair nestlings and the mean proportion of extrapair nestlings in a brood increased with breeding density (high versus low breeding density: broods with extrapair nestlings: 62 versus 27%; mean: 48 versus 16%). As found in some other studies, extrapair paternity (both exclusion and assignment) significantly increased the variance in reproductive success among males. No difference was found in the mean reproductive success of males at different breeding densities. However, as predicted, the variance in reproductive success among males increased with breeding density. This study indicates that the distribution of individuals within a single population can lead to significant differences in the opportunity for sexual selection, and that density may be an important factor affecting variance in male reproductive success among populations.

We conducted the fieldwork between March and June in 1994 and 1995 at Hastings Natural History Reservation and on the adjacent Oak Ridge Ranch, Monterey County, California, U.S.A.">Breeding density is predicted to influence the use of alternative reproductive strategies and may account for some of the variation in extrapair paternity rates found within and between avian species. By exploiting the extreme range of breeding densities occurring in a single, semicolonial population of Bullock's oriole, Icterus galbula bullockii, this study showed that, as predicted, both the number of nests containing extrapair nestlings and the mean proportion of extrapair nestlings in a brood increased with breeding density (high versus low breeding density: broods with extrapair nestlings: 62 versus 27%; mean: 48 versus 16%). As found in some other studies, extrapair paternity (both exclusion and assignment) significantly increased the variance in reproductive success among males. No difference was found in the mean reproductive success of males at different breeding densities. However, as predicted, the variance in reproductive success among males increased with breeding density. This study indicates that the distribution of individuals within a single population can lead to significant differences in the opportunity for sexual selection, and that density may be an important factor affecting variance in male reproductive success among populations.

We conducted the fieldwork between March and June in 1994 and 1995 at Hastings Natural History Reservation and on the adjacent Oak Ridge Ranch, Monterey County, California, U.S.A.">Single-locus minisatellite DNA profiling was used to assign paternity in a population of Bullock's orioles, Icterus galbula bullockii, and to determine the contribution of age to a male's success in obtaining extrapair paternity. There was a very low rate of intraspecific brood parasitism (2/202 = 1.0% of chicks). Older adult males lost less within-pair paternity and gained more extrapair fertilizations than did yearling subadult males. This resulted in adult males benefiting from an annual reproductive success more than double that of subadult males. Behavioural observations, used to determine the role of female choice in extrapair copulations (EPCs), indicated that females actively participate in EPCs and that they prefer to obtain them from older males. While it was possible that females obtained EPCs as an insurance against the possible infertility of their social mate, the results of this study fit best with the hypothesis that females were attempting to obtain better-quality genes for their offspring by obtaining EPCs with older, better-quality males.

"Extrapair paternity and variance in reproductive success related to breeding density in Bullock's orioles." Abstract:

Breeding density is predicted to influence the use of alternative reproductive strategies and may account for some of the variation in extrapair paternity rates found within and between avian species. By exploiting the extreme range of breeding densities occurring in a single, semicolonial population of Bullock's oriole, Icterus galbula bullockii, this study showed that, as predicted, both the number of nests containing extrapair nestlings and the mean proportion of extrapair nestlings in a brood increased with breeding density (high versus low breeding density: broods with extrapair nestlings: 62 versus 27%; mean: 48 versus 16%). As found in some other studies, extrapair paternity (both exclusion and assignment) significantly increased the variance in reproductive success among males. No difference was found in the mean reproductive success of males at different breeding densities. However, as predicted, the variance in reproductive success among males increased with breeding density. This study indicates that the distribution of individuals within a single population can lead to significant differences in the opportunity for sexual selection, and that density may be an important factor affecting variance in male reproductive success among populations.

We conducted the fieldwork between March and June in 1994 and 1995 at Hastings Natural History Reservation and on the adjacent Oak Ridge Ranch, Monterey County, California, U.S.A.