Limited evidence of biased offspring sex allocation in a cavity-nesting conspecific brood parasite

Sex allocation theory predicts that mothers should bias investment in offspring toward the sex that yields higher fitness returns; one such bias may be a skewed offspring-sex ratio. Sex allocation is well-studied in birds with cooperative breeding systems, with theory on local resource enhancement and production of helpers at the nest, but little theoretical or empirical work has focused on birds with brood parasitic breeding systems. Wood ducks (Aix sponsa) are conspecific brood parasites, and rates of parasitism appear to increase with density. Because female wood ducks show high natal philopatry and nest sites are often limiting, local resource competition (LRC) theory predicts that females should overproduce male offspring—the dispersing sex—when competition (density) is high. However, the unique features of conspecific brood parasitism generate alternative predictions from other sex allocation theories, which we develop and test here. We experimentally manipulated the nesting densi..., Field sites. We studied Wood Ducks in the Central Valley of California, USA, at four sites near the town of Davis, from 2013-2016: Russell Ranch (lat 38°32’03” N, long 121°52’05” W, USA), Putah Creek (lat 38°31’0” N, long 121°46’05” W, USA), Conaway Ranch (lat 38°38’24” N, long 121°42’0” W, USA), and Roosevelt Ranch (lat 38°49’15” N, long 121°48’39” W, USA). Sites were remnant oak woodland habitat, persisting in narrow riparian corridors (<20m wide on each bank, 2-5km long) along a stream (Russell Ranch, Putah Creek), slough (Conaway Ranch), or restored and managed wetland complex (Roosevelt Ranch) adjacent to agricultural fields.
We capitalized on an ongoing experimental manipulation of nesting female density to examine changes in offspring sex ratio. We installed a total of 72 nest boxes at Conaway Ranch (16 hectares) as a high-density treatment. We installed 16 nest boxes at Russell Ranch (8 hectares) and 6 nest boxes at Putah Creek (5 hectares) as low-density treatments in 1998-9..., , # Data from: Limited evidence of biased offspring sex allocation in a cavity-nesting conspecific brood parasite

[https://doi.org/10.5061/dryad.rbnzs7hkf](https://doi.org/10.5061/dryad.rbnzs7hkf)

This dataset includes two data frames: one population level (by year and site), and one individual level (by individual female by year and site).

## Description of the data and file structure

The population data frame includes numbers of male, female, and total ducklings (nest, parasite, and total) produced at 4 sites in each of multiple years of the study (2013-2016). It also includes site-year area (in hectares), number of nest boxes installed, number of adult female wood ducks using the site each year, calculated female density, calculated box density, calculated number of boxes available per female, and number of unused boxes. Last, it includes site-year averages of nearest neighbor distances between nesting females (in meters) and pairwise relatedness (*r*, calculated from multi-locus g...