Ecology and Vulnerability

Common Tern

Coastal shorebirds, waterbirds, waterfowl, and seabirds face multiple climate change impacts, with increasing evidence for phenological mismatches and changes to habitat availability, quality, and distribution. Recent studies suggest that the severity of impacts varies with life history traits, phenological plasticity, and type of migration.

Arctic-breeding, long-distance migrant birds such as Pale-bellied Brant (Branta bernicla hrota), Rufa Red Knot (Calidris canutus rufa), and Purple Sandpiper (Calidris maritima) rely on syncing migration with environmental conditions and food resources. The degree of existing or potential phenological mismatch due to climate change depends on the type of migration. For example, long-distance, non-stop migrants such as Pale-bellied Brant, rely on cues at their spring staging areas; these areas may become out of sync with spring onset in the northern breeding areas where warming temperatures are higher (latitude-dependent) and vary regionally (Clausen and Clausen 2013). Short-distance migrants are more likely to remain in sync with seasonal environments because they move across shorter distances with relatively similar conditions (Clausen and Clausen 2013). Species that make migratory stopovers are more likely to acclimate to environmental cues and adjust migratory patterns accordingly (Clausen and Clausen 2013).

Across all taxa, species that are more specialized in resource use will be more vulnerable to changing prey and habitat availability compared to generalists. While not unique to just migratory birds, there are widespread concerns for fitness consequences resulting from phenological mismatches in food resources, particularly during breeding and migration, for many coastal bird species, (Catry et al., 2013, Nightingale et al., 2018, Taillie and Moorman 2019, Tucker et al., 2019, Bratton et al., 2022). Moreover, strong site fidelity during the chick rearing period and a limited radius to hunt for food increases the vulnerability of coastal birds to changes in local environmental conditions and human disturbances including wind energy development, recreation, and fishing (Gibson et al., 2018, USFWS 2020a, Bratton et al., 2022).

Coastal birds are threatened by “coastal squeeze” of habitat from sea level rise, whether that be on their Arctic tundra breeding grounds of Red Knot and Pale-bellied Brant (Clausen and Clausen 2013, Clausen et al., 2013a, Smith et al., 2020), rocky intertidal foraging habitat of Purple Sandpiper (Nightingale et al., 2018), salt marsh and tidal wetland habitat of Eastern Black Rail or Black Skimmer (Adam 2002, Pontee 2013, Hunter et al., 2015, Roach and Barrett 2015, Tallie and Moorman 2019, Tattoni et al., 2020), or sandy beach nesting, foraging, or roosting habitat of Piping Plover, Red Knot, American Oystercatcher, Black Skimmer, Common Tern, or Least Tern (Seavey et al., 2011, Doody 2013, Sims et al., 2013, National Wildlife Federation and Manomet Center for Conservation Sciences 2014, Burger and Niles 2014, Ivajnšič et al., 2017, Maslo et al., 2018, Tattoni et al., 2020). 

Coastal beach habitats that were previously able to migrate across the landscape in response to changes in sea level, are now impeded by coastal modifications and face additional habitat loss and degradation from the direct and indirect effects of climate including erosion, and invasive plants (Taillie and Moorman 2019). Coastal marshes can adapt to rising sea levels through vertical accretion or marsh migration, where conditions allow, but adjacent forests where the saltwater resistance of mature trees limits the pace of migration can lead to net losses, particularly for high marsh habitats (Field et al., 2016, Taillie and Moorman 2019). As sea levels rise at an accelerating rate, these coastal habitats often are narrowing in spatial extent and are at high risk of becoming completely inundated at some locations due to interacting and amplifying climate and non-climate stressors. 

Increased storm frequency and intensity across the region can create or enhance the early successional beach habitat used by shorebirds and waterbirds for short periods of time (Schulte and Simons 2015 & 2016, Robinson et al., 2019, Walker et al., 2019, Zeigler et al., 2019). In some areas, active management of coastal wetland vegetation and natural disturbance regimes can increase the availability of high-value food or habitat resources, and partially offset reductions in resource availability or increase the carrying capacity of habitats (Clausen et al., 2013a, Livolsi et al., 2021; Maslo et al., 2018 & 2019, Robinson et al., 2019, Walker et al., 2019, USFWS 2020a). However, the cumulative effects of coastal squeeze and increased storm impacts on habitat availability, suitability, and distribution across the geographical ranges of coastal birds is of growing concern. Spatially explicit projections of where coastal squeeze is happening the fastest overlayed with SLR projections are useful to identifying local areas to prioritize actions such as assisted migration and habitat restoration.

Community composition of coastal birds in the Northeast is generally shifting towards warmer-breeding coastal bird species and resulting in declines of breeding habitat availability and increased interspecific competition due to the influx of Arctic- and southern-nesting shorebirds into the area (Anderson et al., 2023). 

Disease and pathogens are another emerging and indirect threat of concern that can result from climate-related increases in storm and precipitation frequency and intensity. For example, heavy rainfall and hurricane events in Tampa Bay, Florida, during the 2016 breeding season for Black Skimmers resulted in repeated sewage overflows into the estuaries adjacent to a breeding colony (Shender et al., 2022). Resulting Salmonellosis killed 39% of the Black Skimmer fledglings in the colony because harmful pathogens persisted in beach sands, allowing the substrate to harbor reservoirs of bacterial pathogens (Whiley et al., 2018, Shender et al., 2022). While this example is outside of the Northeast, it identifies potentially hazardous conditions to monitor in Northeastern habitats as the region continues to experience wetter warmer conditions.

Shifts in Phenology

Similar to Roseate Terns, Common Terns are threatened across their Northeast range by changes in prey availability during the critical pre-breeding, post-migration period (Staudinger et al., 2019; Bratton et al., 2022). Common Tern adult and chick diets are more generalized than other terns that breed in the Gulf of Maine; however, their diets are still reliant on a few prey groups including sand lance, herring, and hakes (Yakola et al. 2021; Bratton et al., 2022; Legett et al., 2023). 

Breeding timing is an important driver of fitness in many bird populations but identification of specific environmental drivers of seabird breeding phenology is lacking for most populations (Keogan et al., 2022). For seabirds in the North Atlantic (including Common and Roseate Terns), breeding phenology has been shown to respond idiosyncratically to local environmental conditions including sea surface temperatures (Keogan et al., 2022). A lack of evidence for a shared variance in intraspecific breeding phenology across populations and phenotypic plasticity in laying timing in Africa and Europe may be a sign of climate resiliency in this species (Dobson et al., 2017; Keogan et al., 2022).

Changes to Morphology or Physiology

In a study conducted outside the Northeast region, the most important environmental predictors for Common Tern nest distribution in the face of SLR were habitat type and elevation, with habitat diversity and distance to anthropogenic disturbance being secondary predictors (Ivajnšič et al., 2017). Projected increases in frequency and intensity of heavy rainfall events and storms are also of concern in effecting affecting nest inundation and failure (Seavey et al., 2011, Ivajnšič et al., 2017).