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

Abstract

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 a conspecific brood parasite, 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 theory, which we develop and test here. We experimentally manipulated nesting density of female wood ducks in 4 populations from 2013 to 2016, and analyzed the resulting sex allocation of >2000 ducklings. In contrast to predictions we did not find overproduction of male offspring by females in high-density populations, females in better condition, or parasitic females; modest support for LRC was found in overproduction of only female parasitic offspring with higher nest box availability. The lack of evidence for sex ratio biases, as expected for LRC and some aspects of brood parasitism, could reflect conflicting selection pressures from nest competition and brood parasitism, or that mechanisms of adaptive sex ratio bias are not possible.

Keywords: alternative reproductive tactic; density; female philopatry; local resource competition; local resource enhancement; nest box study; offspring sex ratio; waterfowl.

Figure 1.

The proportion of males produced as a function of female density for (a) parasitic ducklings, (b) nest ducklings, and (c) all ducklings. Each point represents ducklings produced in one population in 1 year (n = 12: 4 years from Russell Ranch, 3 years from Putah Creek, 3 years from Conaway Ranch, and 2 years from Roosevelt Ranch), in the Central Valley of California, USA.

Figure 2.

The proportion of males produced as a function of resource availability (i.e. nest boxes per female) for (a) parasitic ducklings, (b) nest ducklings, and (c) all ducklings. Each point represents ducklings produced by a single female in 1 year (n = 284) in the Central Valley of California, USA.

Figure 3.

Proportion total male ducklings produced by females using each of 3 alternative reproductive tactics: Nesting female (laid only in the nest she incubated), nesting parasite (laid in the nest she incubated and in nests incubated by other females), and parasite female (laid only in nests incubated by other females—did not incubate a nest herself). A Z-score of 0 indicates an even sex ratio, consistent with the binomial expectation for a given sample size; positive values represent increasingly male-biased offspring, and negative values represent increasingly female-biased offspring.

Figure 4.

Sex ratios of ducklings hatched in other nests (parasitic ducklings, left boxplots) vs. own nest (nest ducklings, right boxplots) for nesting parasite females at 4 sites across a range of female density. A Z-score of 0 indicates an even sex ratio, consistent with the binomial expectation for a given sample size; positive values represent increasingly male-biased offspring, and negative values represent increasingly female-biased offspring.