Date of Award

2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Environmental Sciences

First Advisor

Scott R. McWilliams

Abstract

Migration is a physiologically and energetically demanding period in the life cycle of a migratory animal. Most migrant songbirds alternate between periods of nocturnal flight, when energy is used, and stopover, when energy is accumulated for the next flight period; indeed, songbirds spend most of their time and energy on stopover. Conversely, the habits of migratory bats remain largely enigmatic, but there is reason to expect that models of songbird migration can inform bat migration research. I investigated, at multiple scales, the migration ecology of songbirds and bats and the stopover ecology of songbirds along the Atlantic Coast of southern New England. My research comprised four components: acoustic monitoring to explore the regional spatiotemporal dynamics of (1) songbird and (2) bat migration along the Rhode Island coast and relate them to synoptic weather conditions, (3) an experimental manipulation of arrowwood (Viburnum recognitum and V. dentatum) fruits, a preferred resource of omnivorous migratory songbirds on Block Island, to evaluate how resource consumption depends on consumer abundance at the landscape scale and the abundance and distribution of that resource at a neighborhood (local) scale, and (4) a field experiment that manipulated Hermit Thrush (Catharus guttatus) body condition (fuel stores) during stopover to isolate and evaluate its influence on subsequent stopover movement behavior and departure decisions on Block Island.

Patterns of warbler and sparrow nocturnal flight call (NFC) detections largely supported our expectations in that NFC detections associated positively and strongly with wind conditions that influence the intensity of coastal bird migration and negatively with regional precipitation; increased during conditions with reduced visibility (e.g., high cloud cover); decreased with higher wind speeds, presumably due to increased ambient noise; and coastal mainland sites recorded four to seven times more NFCs, on average, than coastal nearshore or offshore island sites. Despite some potential complications in inferring migration intensity and species composition from NFC data, the acoustic monitoring of NFCs provides a viable complement to methodologies (e.g., radar) currently used to explore the spatiotemporal patterns of songbird migration as well as evaluating the atmospheric conditions that shape these patterns.

Coastal bat acoustic activity varied with regional wind conditions indicative of cold front passage and expected to induce a more coastal flight path, but associations with other atmospheric conditions from models of songbird migration were typically weak; bat acoustic activity also associated with various aspects of temperature. Predictive models of forthcoming regional bat activity have direct conservation implications given that migration figures prominently in wind turbine-related bat fatalities and the imminent expansion of wind energy into the nearshore and offshore environments of New England and the mid-Atlantic. Predictive models were reasonably accurate in anticipating nights of the highest and lowest bat activity, particularly for low frequency bats. Thus, these predictive models may provide a regional migratory bat activity context for future site-specific applications that, in turn, inform turbine operations and reduce adverse interactions and fatalities.

I conducted the first empirical and simultaneous test of the two primary predictions of contemporary models of plant-frugivore interactions within spatially explicit networks: (1) rate of fruit removal increases as densities of conspecific neighborhood fruits increase, and (2) fruit removal rate varies positively with frugivore abundance. Focal arrowwood plants in neighborhoods with high conspecific fruit density sustained moderately decreased fruit removal rates (i.e., competition) relative to those in low density neighborhoods, a result that agrees with most field research to date but contrasts with theoretical expectation. I suggest the spatial contexts that favor competition are considerably more common than the relatively uniform, low aggregation fruiting landscapes that promote facilitation. Patterns of arrowwood removal by avian frugivores generally varied positively with, and apparently in response to, seasonal changes in migratory frugivore abundance, but this effect varied with the distribution of arrowwood. My results underscore the importance of considering spatial context (e.g., fruit distribution and aggregation, frugivory hubs) in plant-avian frugivore interactions. Thus, contemporary theoretical models of plant-frugivore interactions, while quite useful, may not adequately characterize most empirical work to date, particularly in temperate systems that support seasonally abundant frugivores. As such, models of plant-frugivore interactions will benefit from the exploration of alternative or additional model parameters.

Fuel stores in a migrating songbird, manipulated during stopover, directly affected stopover movement dynamics and departure decisions; however, their influence on stopover dynamic was most pronounced later in fall migration. Precipitation and wind additionally modified stopover and departure behavior. My results demonstrate the importance of placing stopover behaviors in the context of relevant intrinsic (e.g., endogenous time program) and extrinsic (e.g., resource distribution and abundance, topography, atmospheric conditions) factors. The effect of fuel stores on migration speed may be more pronounced along migratory barriers like the Atlantic coast, as larger fuel stores resulted in shorter stopovers and a more direct migratory route. The pervasive influence of fuel stores on migrant stopover behavior underscores the central role of fuel acquisition in the dynamics, speed, and success of migration and the importance of quality stopover sites to migratory birds.

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