Demography of a declining Dunlin (Calidris alpina arcticola): influences on adult survival and mate fidelity of an Arctic-breeding migratory shorebird

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2024-03-15
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Virginia Tech
Abstract

Understanding what restricts vital rates is crucial in conservation efforts. For migratory birds, vital rates can be impacted by conditions experienced throughout the year. Migratory shorebird populations are rapidly declining, including populations of Dunlin (Calidris alpina arcticola), an Arctic-breeding shorebird. Prior adult survival estimates (0.41– 0.60) appeared insufficient to maintain a stationary population, however, it was unclear if estimates were reflective of bias or a real survival signal. Additionally, C. a. arcticola mate fidelity has yet to be determined, and because demographic rates can be linked to breeding ecology, understanding factors affecting both adult survival and mate fidelity might illuminate specific constraints on demographic rates for this species. I used a Barker (1997) model to estimate true survival (unbiased relative to fidelity) rates of adult C. a. arcticola using 19 years (2003 – 2021) of mark-recapture data and environmental data, collected from a breeding area, Utqiaġvik, Alaska, U.S.A. Breeding site data were supplemented with resighting observations and habitat data from non-breeding sites in eastern Asia (Japan, China, Taiwan). I examined breeding site environmental (temperature, snow melt date, precipitation) and ecological (predator abundance and predator-prey cycles, food resources, shorebird nesting density) in conjunction non-breeding site habitat (area of intertidal extent) effects on survival estimates. True survival averaged 0.62 (95% C.I. 0.50 – 0.72), and marginally declined six percent throughout the study period. Survival was positively related to intertidal extent on non-breeding grounds and heavy precipitation events on the breeding grounds; with precipitation influence on survival likely being driven by outliers in the data. I propose intertidal habitat (which declined 22% across 19 years) is likely a core driver of low survival rates. These results enforce that low adult survival rates are suppressing C. a. arcticola populations, as while other demographic rates (reproductive output, breeding propensity) are comparable with North American Arctic-breeding C. alpina subspecies, both adult survival rates and estimated population growth remain relatively lower. I emphasize that conservation efforts should be focused at areas currently losing intertidal habitat within the East Asian-Australasian Flyway to mitigate future C. a. arcticola declines. Using C. a. arcticola breeding site data, I tested four hypotheses to understand divorce in C. a. arcticola: the better option hypothesis, in which divorce improves reproductive success by obtaining a higher quality mate; the habitat mediated hypothesis, when divorce might occur if an opportunity exists to nest at a higher quality site than the prior breeding season; musical chairs, in which divorce is related to site-specific settlement choices upon arrival to the breeding grounds; and bet-hedging, in which divorce is more likely when mates arrive to the breeding grounds asynchronously, and an individual will pair with a new mate to avoid the cost of waiting for a previous mate to return to it. I used a logistic regression model to investigate effects on C. a. arcticola divorce with environmental and ecological factors that might influence divorce. Of the females nesting in consecutive years, 20% of those females divorced; and in the cases of males nesting in consecutive years, 55% of those males divorced. Both sexes were more likely to divorce when there was greater availability of experienced mates on the breeding grounds, significantly in males (β = 0.81, 95% CI = -0.65 – 2.28), compared to females (β = 1.27, 95% CI = 0.28 − 2.25). The results indicate males divorce behavior supports the better-option hypothesis, in which males divorced to "upgrade" to a mate with more breeding experience than their prior mate. However, male divorce behaviors also supported the bet-hedging hypothesis, as evidenced by similar nest initiation dates between divorced and reuniting males, which indicated males may divorce to avoid reproductive costs associated with waiting for a later-returning mate. Female divorce behavior was linked to either the habitat-mediated hypothesis, in which individuals attempt to acquire better habitat than their prior breeding site, or the better-option hypothesis, both evidenced by divorced females improving their reproductive success from the prior year. Divorced females exhibited higher egg success rates compared to divorced males, indicating females are likely the sex breaking the pair. Together, the results present novel information concerning C. a. arcticola. The first chapter presents direct connections between intertidal habitat loss and lower adult survival, and enforce calls for restoration of Asian intertidal areas along flyways to aid the conservation of migratory shorebirds. The second chapter provides the first estimates of C. a. arcticola mate fidelity and insight towards better understanding migratory shorebird breeding ecology.

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survival, conservation, habitat, mate fidelity, climate change, shorebirds
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