Data from: Reduced fitness of females in a facultatively polygynous species: A 32-yr study of Savannah sparrows

The evolution of mating systems reflects a balance of the
often-conflicting interests of males and females. Polygyny, a mating
system in which males have multiple mates, presents a fitness benefit to
males, but the consequences for females are less clear. Females with
polygynous social mates may suffer reduced fitness, especially secondary
females who typically receive less male support. We use 32 years of
detailed reproductive data on a population of Savannah sparrows
(Passerculus sandwichensis) breeding on Kent Island, NB, Canada, to
investigate the effects of females’ social mating status on six indices of
female fitness: survival, clutch size, fledging success, number of
fledglings produced per nest and annually, and recruitment of offspring.
Secondary females produced fewer fledglings per nest and annually than did
monogamous or primary females, and their young were less likely to recruit
into the breeding population. Yearling secondary females also had lower
survival rates than older secondary females. Combined with higher rates of
partial brood loss among secondary females, our results suggest that
secondary females are unable to provide enough care to consistently fledge
all nestlings in their broods, likely due to reduced male provisioning.
Given that the sex ratio of breeders in the population is female-biased,
we suggest that polygyny persists despite its fitness costs because some
females must mate polygynously to “make the best of a bad situation.” Our
study demonstrates the value of detailed, long-term population monitoring
data for understanding mating systems and using multiple indices of
fitness to analyze the costs of polygyny.