Abstract:
Understanding the consequences of anthropogenic change for migratory species is challenging because although they have evolved to cope with environmental uncertainty, migrants still rely on predictable relationships within and among habitats to make informed decisions. Calidris shorebirds rely on ephemeral wetlands during northward migration through mid-continental North America, where favorable habitat conditions are annually and regionally unpredictable and increasingly altered by land-use change. During spring 2013 and 2014, we assessed Calidris habitat use in the Rainwater Basin (RWB) and the Prairie Pothole Region (PPR) at both local and landscape scales. Although anthropogenic change has altered the wetland landscape in both regions, the scope and scale of anthropogenic change is more pronounced in the RWB. Our results indicate that invertebrate abundance predicted occupancy, but not abundance, of Calidris shorebirds at wetlands in the RWB. Regionally, we find that habitat structure which predicts shorebird occupancy and abundance is similar in both regions, but wetlands in the PPR supported a higher abundance of Calidris shorebirds than wetlands in the RWB. Our results suggest that the overall availability of wetlands on the landscape limits shorebird abundance independent of individual wetland quality, thus management efforts should consider not only the structure and function of individual wetlands, but also entire networks of managed habitat across the landscape.
We also tested for variance in abundance estimates within and among wetlands by employing both visual point and flush surveys. We find considerable variation in predicted relationships between shorebird abundance and habitat attributes depending on method, observer, and site; variance also increased with area and vegetative characteristics of wetlands. Our results draw attention to potential weaknesses associated with traditional shorebird sampling approaches, as it is unclear whether errors in detection or shifts in habitat use account for variation among surveys. We urge further examination of sources of error in shorebird surveys in order to establish meaningful patterns relevant for the management of wetland habitat and the conservation of migratory populations.
