UC Santa Cruz

All organisms must allocate limited resources to reproduction and survival, producing life history trade-offs. One such trade-off is maternal care, which balances a female’s current reproduction against her future survival. Alternative reproductive tactics have evolved as a way of maximizing lifetime reproductive success by balancing the costs of parental care against the benefits that care provides to current offspring. Conspecific brood parasitism (CBP) is an alternative reproductive tactic where females lay eggs in the nest of a conspecific, providing no further care to those offspring. Some parasitic females may have their own nest in addition to laying parasitically. This results in three possible seasonal reproductive options: nesting and parasitizing, nesting without parasitizing, and parasitizing without nesting. While CBP is taxonomically widespread, it occurs at remarkably high rates in waterfowl. Detecting CBP is challenging, and historically researchers have relied on behavioral and morphological evidence to identify it. Recently, genetic methods of maternity assignment have allowed researchers to detect parasitism directly. However, genetic methods have not been examined under conditions common in waterfowl, where parasitic females may be highly related to their hosts and the candidate parent pool is often incomplete. Additionally, while females can flexibly transition between CBP reproductive options between years, the reproductive success of CBP reproductive options are often quantified by single-season estimates and are rarely considered in the context of the entire lifespan of the females that engage in this behavior.

My dissertation focuses on testing accuracy of both genetic and non-genetic methods of detecting CBP and quantifying long-term patterns of CBP in the wood duck (Aix sponsa) in California. In Chapter 1, I assessed the accuracy of detecting CBP using genetic assignments with simulated wood duck populations. Genetic methods of maternity assignment in the context of CBP in wood ducks rarely make assignment errors given a diverse set of microsatellite markers and a largely complete sample of candidate mothers, but the risk of false exclusion and misassignment does increase with related females in the candidate parent pool. In Chapter 2, I compared field-based genetic and non-genetic estimates of CBP. Non-genetic methods produced underestimates of CBP as compared to genetic estimates. However, in combination with genetic estimates of CBP, non-genetic methods uncovered CBP patterns in wood ducks that genetic methods alone would not have revealed. In Chapter 3, I used genetic assignments to quantify the frequency of CBP in wood ducks at the population level over five years, calculated single-season reproductive success of individual females’ reproductive options, and determined long-term patterns of CBP behavior of individual and their individual lifetime fitness estimates. The frequency of non-parasitic nesting females declined as density of breeding females and nest sites increased. Females that nested and parasitized had the highest reproductive success by single season measurements, while parasites that did not nest had the lowest reproductive success. Nesting parasitism contributed the most offspring to lifetime reproductive success but parasitism alone or nesting without parasitism could result in large fitness gains. Females nested more frequently as they aged, with or without parasitism, and continued to do so until they exited the population. Collectively, the results of my thesis suggest that genetic methods of maternity assignment are robust in CBP waterfowl, and female wood ducks flexibly transition into nesting reproductive options as they age, which ultimately results in higher reproductive success over the course of their lifetimes. My research is one of the first studies to assess lifetime reproductive success in a CBP system and highlights the importance of taking a long-term perspective in studies of alternative reproductive options to fully understand the costs and benefits of engaging in these behaviors.