Browsing by Author "Catlin, Daniel Herbert"
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- Demography of a declining Dunlin (Calidris alpina arcticola): influences on adult survival and mate fidelity of an Arctic-breeding migratory shorebirdHermanns, Lindsay F. (Virginia Tech, 2024-03-15)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.
- Factors affecting piping plover (Charadrius melodus) nest site selection following landscape and predator community changesDorsey, Sharon Selena-Lee (Virginia Tech, 2024-01-18)The dynamics of coastal landscapes following major storm events and human interventions significantly impact nesting habitat use by species like the piping plover (Charadrius melodus) along Atlantic coastlines. Our study focused on Fire Island, New York, assessing changes in vegetation succession, plover nesting habitat selection, and suitable nesting habitat availability from 2010 until eight years after Hurricane Sandy. We analyzed classified imagery to quantify vegetative cover changes across the landscape and at nest sites. Results showed an increase of 11.5% change in vegetative cover across the study area, and nest sites experienced an increase in vegetation cover from 0.1% to 6.2% between 2015 and 2020. Selection for four habitat variables (i.e., distance to ocean, least-cost distance to bay, elevation, backshore width) was consistent throughout the study, but post-hurricane nests were situated farther from development and closer to bay areas (Euclidean distance) compared to pre-hurricane. Moreover, suitable nesting habitat peaked immediately post-stabilization but declined in subsequent years. Understanding these landscape shifts provides insights into species prioritization of habitat characteristics during nesting. Piping plovers face predation threats during breeding seasons while balancing consideration of the aforementioned habitat considerations, leading to specific nest placement strategies. Our study examined the relationship between nest site selection and the area visible from nests (viewshed) and evaluated its addition in nest site selection models including habitat variables. Piping plovers exhibited a preference for nest sites with increased predator visibility compared to random selection, indicating a strategic selection process. Initially influential (β = 0.43; CI = 0.28 – 0.58), the role of this predator visibility ('viewshed') diminished (β = -0.13; CI = -0.23– -0.04) as the landscape underwent ecological succession. Topographic variation caused greater visual obstruction at nest sites than vegetation. This study on piping plovers offers insights into the interaction between landscape changes, habitat selection, and predator visibility. The evolving importance of viewshed in nest site selection underscores the dynamic nature of nesting strategies in response to changing environments, and incorporation of this variable can improve the predictive ability of other models as it did for this study. These findings have broader implications for ground-nesting bird species and highlight the importance of considering landscape changes and predator visibility in land management strategies to safeguard vulnerable avian populations.
- North American Tree Bat (Genera: Lasiurus, Lasionycteris) Migration on the Mid-Atlantic Coast—Implications and Discussion for Current and Future Offshore Wind DevelopmentTrue, Michael C. (Virginia Tech, 2022-01-18)In eastern North America, "tree bats" (Genera: Lasiurus and Lasionycteris) are highly susceptible to collisions with wind energy turbines and are known to fly offshore during migration. This raises concern about ongoing expansion of offshore wind-energy development off the Atlantic Coast. Season, atmospheric conditions, and site-level characteristics such as local habitat features (e.g., forest coverage) have been shown to influence wind turbine collision rates by bats onshore, and similar features may be related to risk offshore. In response to rapidly developing offshore wind energy development, I assessed the factors affecting coastal and offshore presence of tree bats. I continuously gathered tree bat nightly occurrence data using stationary acoustic recorders on five structures (four lighthouses on barrier islands and one light tower offshore) off the coast of Virginia, USA, across all seasons, 2012–2019. I used generalized additive models to describe nightly tree bat occurrence in relation to multiple factors. I found that sites either indicated maternity or migratory patterns in their seasonal occurrence pattern that were associated with local roosting resources (i.e., presence of forest). Across all sites, nightly occurrence was negatively related to wind speed and positively related to temperature and visibility. Using predictive performance metrics, I concluded that the model was highly predictive for the Virginia coast. My findings were consistent with other studies—tree bat occurrence probability and presumed mortality risk to offshore wind-energy collisions is highest on nights with low wind speed, high temperature and visibility during spring and fall. The high predictive model performance I observed provides a basis for which managers, using a similar monitoring and modeling regime, could develop an effective curtailment-based mitigation strategy. Although information at fixed points is helpful for managing specific sites, large questions remain on certain aspects of tree bat migration, in part because direct evidence (i.e., tracking of individuals) has been difficult to obtain so far. For instance, patterns in fall behavior such as the timing of migration events, the existence of migratory pathways, consistencies in the direction of travel, the drivers of over-water flight, and the activity states of residents (or bats in stopover) remain unstudied in the mid-Atlantic. The recently established Motus Wildlife Tracking System, an array of ground-based receiver stations, provides a new technique to track individual bats via the ability to detect course-scale movement paths of attached very high frequency radio-tags. To reveal patterns in migration, and to understand drivers of over-water flight, I captured and radio-tagged 115 eastern red bats (Lasiurus borealis) and subsequently tracked their movements. For the bats with evidence of large movements, most traveled in a southwesterly direction whereby paths were often oriented interior toward the continental landmass rather than being oriented along the coastline. This observation challenges earlier held beliefs that bats closely follow linear landscape features, such as the coast, when migrating. I documented bats traveling across wide sections of the Chesapeake and Delaware bays confirming the species' ability to travel across large water bodies. This behavior typically occurred in the early hours of the night and during favorable flying conditions such as low wind speeds, warm temperatures, and/or during sudden increases in temperature associated with the passage of cold fronts. For bats engaging in site residency through the fall, the proportion of night-hours in which bats were in a resting state (and possibly torpor), increased with colder temperatures and the progression of the fall season. My study demonstrated that bats may be at risk to offshore wind turbine collisions off the mid-Atlantic, but that this risk might be minimal if most bats are migrating toward the interior landscape rather than following the coast. Nonetheless, if flight over large water bodies such as Chesapeake and Delaware bays is a viable proxy for over-ocean flight, then collision risk at offshore wind turbines may be somewhat linked to atmospheric, seasonal timing, or other effects, and therefore some level of predictable and manageable with mitigations options such as smart curtailment.