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 ol..., , , # Reduced fitness of females in a facultatively polygynous species: a 32-yr study of Savannah sparrows

[https://doi.org/10.5061/dryad.msbcc2g7d](https://doi.org/10.5061/dryad.msbcc2g7d)

## Description of the data and file structure

We used 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. We mark all individuals breeding within the study area and find and monitor all nest attempts to produce reproductive data including hatching date; number of eggs, nestlings, and fledglings; and fledging or failure date of the nest. All nestlings and unbanded adults are banded and measured. Assignment of territories and mating status (monogamous or polygynous) are based on field observations of...,