Roosting birds link built structures to nutrient dynamics in human-altered coastal ecosystems

Coastal birds are important biogeochemical vectors, redistributing nutrients within and between coastal environments. These inputs can enhance primary production, alter food web dynamics, and support the productivity of ecologically and economically valuable species. However, coastal habitat loss has increased bird reliance on built structures for roosting and nesting, reshaping where nutrients are deposited in nearshore habitats. In Chapter 1, I investigate how bird use of built structures changes with water level in a developed coastal setting, using biomass distribution data from a year-long set of transit surveys. Across all birds and individual functional groups (gulls/terns, shorebirds, wading birds, cormorants/pelicans), the proportion of biomass using built structures increases with higher water levels, suggesting even greater use as water levels rise. In Chapter 2, I estimate bird-derived nitrogen (N) and phosphorus (P) inputs at three farms using hourly camera trap monitoring and complementary metabolically informed nutrient loading models. N loading is on the scale of 4-58 kg N y-1 across sites and models, with farms exhibiting a sharp nutrient input peak during fall (September-November) driven by staging and migrating birds. When scaled as site-specific fluxes (g N m-2 y-1), these N inputs represent notable localized contributions that likely exceed other N sources such as biological fixation and atmospheric deposition. In Chapter 3, I assess the ecological consequences of these N inputs during fall migration using a before-after-control-impact (BACI) experiment to quantify isotopic (δ15N/δ13C) enrichment and trophic propagation in oyster-associated organisms. Relative δ15N enrichment is evident across multiple trophic levels post-migration, with fixed macroalgae and oyster tissue providing two distinct pathways for bird-derived N to enter the ecosystem. δ15N enrichment does not, however, scale proportionally with N input, indicating that other factors govern rates of localized incorporation. Collectively, this work highlights how coastal birds adapt to and utilize built structure in developed coastal ecosystems, and how these behaviors influence nutrient dynamics and ecosystem function. As human activities continue to reshape coastal environments, understanding the ecological consequences of bird use of the built structures will be essential for managing productive habitats where humans and birds can coexist.