Clutch Composition and Multiple Paternity in Ducks: Unpacking Monogamy, Extra-Pair Copulation, and Reproductive Dynamics

Ducks (family Anatidae) exhibit some of the most dynamic and complex reproductive strategies among birds. While many species, particularly in the Anas genus such as the mallard (Anas platyrhynchos), are often described as seasonally monogamous, this characterization fails to capture the full breadth of their mating systems. Field observations and molecular studies reveal that monogamy in ducks is social rather than genetic, with females frequently producing clutches fertilized by multiple males. This is driven by a combination of extra-pair copulations, intense male competition, and the physiological ability of females to store viable gametes from several mates.

Social Pairing and the Limits of Monogamy in Ducks

Most dabbling and diving ducks form seasonal pair bonds that last from winter through the nesting period. These bonds typically involve one male and one female, with the male often providing some degree of protection during the female’s fertile period and early incubation (McKinney, 1992). However, in nearly all duck species where genetic data are available, clutches frequently exhibit multiple paternity, meaning that embryos within a single nest can be sired by different males (Avise et al., 1990; Peters et al., 2009).

In mallards, genetic studies using microsatellite markers indicate that more than 30 percent of clutches contain offspring from at least two males, even when a stable social pair bond is present (Cheng et al., 1983). Similar results have been observed in other species, including wood ducks (Aix sponsa), northern pintails (Anas acuta), and ruddy ducks (Oxyura jamaicensis), suggesting that multiple paternity is a common reproductive strategy across diverse duck lineages (Sorenson et al., 1999; Peters et al., 2009).

Extra-Pair Copulation and Reproductive Competition

The primary behavioral mechanism behind multiple paternity in ducks is extra-pair copulation. In many duck species, including mallards, blue-winged teal, and gadwalls, these copulations are typically forced and occur without the female’s cooperation (McKinney, Derrickson, & Mineau, 1983). Males aggressively pursue unguarded females, particularly when they are nearing ovulation, during which the likelihood of successful fertilization is highest.

This behavior creates an evolutionary scenario characterized by sexual conflict. Males benefit by maximizing the number of eggs they fertilize, while females experience increased harassment, injury risk, and energetic cost. Despite these risks, forced extra-pair copulations may confer indirect genetic benefits to females by increasing offspring genetic diversity or introducing superior alleles into the brood (Birkhead & Møller, 1992).

The prevalence and intensity of forced copulations are correlated with operational sex ratios, habitat visibility, and breeding density. For instance, in dense prairie pothole regions with high male-to-female ratios, forced copulation rates in species like northern pintails and mallards can exceed 50 percent of all observed matings (Sayler, 1962; Sorenson et al., 1999).

Storage and Post-Copulatory Selection

Across duck species, females possess specialized structures in the oviduct called storage tubules. These allow viable male gametes to remain stored for extended periods, typically up to two weeks, although viability and fertilization potential decline after ten days (Birkhead & Fletcher, 1995). These adaptations enable females to produce a complete clutch using material from copulations that occurred well in advance of oviposition.

This biological mechanism introduces the potential for cryptic female choice, where females may selectively utilize genetic material from preferred males based on post-copulatory physiological mechanisms. This represents a second level of sexual selection that operates after copulation has occurred and may counterbalance the disadvantages of forced matings (Birkhead & Pizzari, 2002).

The ability to store and potentially select fertilizing material is widespread among ducks and has been confirmed in mallards, wood ducks, and northern shovelers. In redheads (Aythya americana), parasitic nesting can further complicate this dynamic, leading to mixed paternity within and across nests (Sorenson, 1997).

Broader Implications for Ecology, Genetics, and Conservation

The presence of multiple paternity in duck clutches has several important ecological and management implications. First, it contributes to increased genetic diversity within broods, which may enhance offspring fitness in unpredictable environments. This is especially relevant for species nesting in high-risk habitats where brood survival is low and reproductive success is constrained by predation or flooding (Arnold et al., 2007).

Second, multiple paternity generates strong reproductive skew among males, resulting in unequal gene flow across generations. A small number of successful males may father a disproportionate number of offspring, which can reduce effective population size and accelerate sexual selection for traits such as plumage brightness, courtship behavior, and body condition (Eadie et al., 1998).

Finally, this reproductive strategy complicates efforts to monitor hybridization and genetic introgression in wild populations. In areas where wild and domestic or game-farm ducks interbreed, such as the Great Lakes region and parts of the Mississippi Flyway, extra-pair copulations can facilitate the rapid spread of domestic alleles, even when female fidelity is high (Lavretsky, McInerney, & Peters, 2020). For conservation programs utilizing artificial nesting structures like hen houses or nest boxes, this underscores the need for genetic monitoring of entire broods, not just maternal lineages.

References

Arnold, T. W., Clark, R. G., & Koons, D. N. (2007). Factors that affect clutch size variation in North American dabbling ducks. The Condor, 109(3), 556–565. https://doi.org/10.1093/condor/109.3.556

Avise, J. C., Nelson, W. S., & Sibley, C. G. (1990). Multiple paternity in a natural population of a lek-breeding bird. Science, 250(4987), 1233–1235. https://doi.org/10.1126/science.250.4987.1233

Birkhead, T. R., & Fletcher, F. (1995). Storage and the selection of male genetic material in the reproductive tract of female birds. Biological Reviews, 70(4), 321–346. https://doi.org/10.1111/j.1469-185X.1995.tb01184.x

Birkhead, T. R., & Møller, A. P. (1992). Sperm competition in birds: Evolutionary causes and consequences. Academic Press.

Birkhead, T. R., & Pizzari, T. (2002). Postcopulatory sexual selection. Nature Reviews Genetics, 3(4), 262–273. https://doi.org/10.1038/nrg774

Cheng, K. M., Burns, J. T., & McKinney, F. (1983). Forced copulation in captive mallards III. Competition and fertilization success. Auk, 100(2), 302–310. https://doi.org/10.2307/4086826

Eadie, J. M., Sherman, P. W., & Semel, B. (1998). Conspecific brood parasitism, population dynamics, and the evolution of cooperative breeding in birds. In P. A. Gowaty (Ed.), Female competition and choice (pp. 306–339). Academic Press.

Lavretsky, P., McInerney, N. R., & Peters, J. L. (2020). Genetic introgression supports the gradual replacement of wild North American mallards by domestic variants. Scientific Reports, 10, 12682. https://doi.org/10.1038/s41598-020-69541-3

McKinney, F. (1992). Courtship, pair formation, and signal systems. In B. D. J. Batt (Ed.), Ecology and management of breeding waterfowl (pp. 214–250). University of Minnesota Press.

McKinney, F., Derrickson, S. R., & Mineau, P. (1983). Forced copulation in waterfowl. Behaviour, 86(3–4), 250–294. https://doi.org/10.1163/156853983X00210

Peters, J. L., Zhuravlev, Y. N., Fefelov, I., Logie, A., & Omland, K. E. (2009). Evidence for the independent evolution of mating preferences in a pair of hybridizing mallard species. Evolution, 63(6), 1587–1600. https://doi.org/10.1111/j.1558-5646.2009.00631.x

Sayler, R. D. (1962). A study of the social relationships in a population of mallards. The Wilson Bulletin, 74(3), 284–305.

Sorenson, L. G. (1997). Effects of brood parasitism by redheads (Aythya americana) on host reproductive success. Ecology, 78(7), 2221–2230. https://doi.org/10.1890/0012-9658(1997)078[2221:EOBPBR]2.0.CO;2

Sorenson, L. G., Eadie, J. M., & Sherman, P. W. (1999). Ectoparasitism and conspecific brood parasitism in wood ducks: Does parasitism cause host females to desert their broods? Behavioral Ecology, 10(6), 705–710. https://doi.org/10.1093/beheco/10.6.705