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 (<i>Calidris alpina arcticola</i>), 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. / Master of Science / Our understanding what influences bird populations is frequently related to identifying factors affecting population dynamics and breeding ecologies. Arctic-breeding migratory shorebirds face unique circannual challenges, as the Arctic is experiencing rapid and unprecedented climate change. Additionally, climate change and coastline development can significantly alter coastal environments used by shorebirds during non-breeding periods. Dunlin (<i>Calidris alpina arcticola</i>) are Arctic-breeding shorebirds that migrate annually to non-breeding areas in East Asia. Previous estimates of C. a. arcticola adult survival rates raised concerns about population stability, but it was unclear if these estimates reflected real survival patterns. Beyond survival rates, mate fidelity and implications of mate fidelity for this subspecies remains unknown. Understanding mate fidelity can clarify demographic constraints, as it can have significant implications for breeding dispersal and population structure. I used 19 years of data collected from a C. a. arcticola breeding site to obtain unbiased estimates of adult survival, factoring in fidelity to the breeding site, and, considered environmental and ecological variables from both breeding and non-breeding grounds thought to influence adult survival. Results indicated a slightly higher adult survival rate than previously estimated, but still low enough to remain depressive on population growth. Survival was positively linked to non-breeding habitat (intertidal areas). These results emphasize the need for conservation efforts aimed at intertidal areas along East Asian migratory routes and non-breeding locations. Using this same long-term dataset, I tested different divorce hypotheses to understand why C. a. arcticola divorce: which could be for better mates (better-option), to acquire better nesting habitat (habitat-mediated), because of settlement strategies upon arrival to breeding grounds (musical chairs), or due to differences in arrival timing to the breeding grounds between pair members and avoiding associated delayed reproductive costs while waiting for a prior year's mate (bet-hedging). I modeled divorce based on sex, individual breeding experience, arrival timing, annual reproductive output, and prior-year's seasonal environmental conditions at the breeding site. Divorce rates were higher in years when higher variation in mate quality were available breeding site, especially among males, who "upgraded" for higher-quality mates (better-option). Divorce behaviors also supported the bet-hedging hypothesis (divorce happens to avoid the cost of waiting for a previous mate to return) for males, and the habitat-mediated (divorce happens to acquire better breeding site habitat than the prior year) strategy in females. Divorced females had better hatching success then divorced males in the year of divorce, indicating they were the sex initiating divorce most frequently. This study provides insight into C. a. arcticola breeding ecology and demography, offering the first estimates of both true adult survival and mate fidelity rates for this subspecies. These findings contribute to our understanding of migratory shorebird dynamics and aim to support conservation efforts for migratory shorebird populations.

I conducted demographic and behavioral studies of Western Sandpipers (Calidris mauri) breeding on the Yukon-Kuskokwim River Delta, Alaska (1998-2005). In chapter one, I estimated apparent annual survival (product of true survival and site fidelity) while correcting for the probability of encounter for 237 males and 296 females. Overall return rates (individual returned to the site in a subsequent season) were lower for females (40%) than males (65%), as was apparent annual survival (&#177; SE, females = 0.65 &#177; 0.05, males = 0.78 &#177; 0.03), and encounter rate (females = 0.51 &#177; 0.07, males = 0.74 &#177; 0.04). In chapter two, I examined the effects of mate and site fidelity on nesting success (N = 430 nests). Annual divorce rate ranged between 37-83%, with 17-63% of pairs reuniting annually. Reuniting pairs initiated clutches earlier than newly formed pairs, and clutches that were initiated early in the season had higher nest success rates compared to late-season nests. When I controlled for clutch-initiation date, nests tended by individuals with prior breeding-site experience had higher daily survival rates compared to birds breeding at the site for the first time. The effect of site experience was greater for males than females. In chapter 3, I reported that Western Sandpipers exhibited aggregated breeding behavior on a 36 ha plot. Breeding aggregations occurred when dominant and/or older individuals excluded younger, subordinate individuals from preferred habitat. The pattern of habitat occupancy conformed to an ideal despotic distribution with aggregated nesting birds in less preferred habitat experiencing lower reproductive success. In chapter 4, I described and demonstrated the form and function of parent-chick communication in the Western Sandpiper. Through experimental playback of adult vocalizations to chicks in the field, I demonstrated: (1) chicks respond to the alarm call by vocalizing relatively less often and moving away from the signal source, (2) chicks respond to the gather call by vocalizing relatively more often and moving toward the signal source, (3) and chicks respond to the freeze call by vocalizing relatively less often and crouching motionless on the substrate for extended periods of time. I also describe two distinct chick vocalizations (chick-contact and chick-alarm calls). / Ph. D.

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