Increasing marsh bird abundance in coastal wetlands of the Great Lakes (2011–2021) likely caused by increasing water levels

Wetlands of the Laurentian Great Lakes of North America, i.e., lakes
Superior, Michigan, Huron, Erie, and Ontario, provide critical habitat for
marsh birds. We used 11 years (2011–2021) of data collected by the Great
Lakes Coastal Wetland Monitoring Program at 1,962 point count locations in
792 wetlands to quantify the first-ever annual abundance indices and
trends of 18 marsh-breeding bird species in coastal wetlands throughout
the entire Great Lakes. Nine species (50%) increased by 8–37% per year
across all of the Great Lakes combined, whereas none decreased. Twelve
species (67%) increased by 5–50% per year in at least 1 of the 5 Great
Lakes, whereas only 3 species (17%) decreased by 2–10% per year in at
least 1 of the lakes. There were more positive trends among lakes and
species (n = 34, 48%) than negative trends (n = 5, 7%). These large
increases are welcomed because most of the species are of conservation
concern in the Great Lakes. Trends were likely caused by long-term,
cyclical fluctuations in Great Lakes water levels. Lake levels increased
over most of the study, which inundated vegetation and increased open
water-vegetation interspersion and open water extent, all of which are
known to positively influence abundance of most of the increasing species
and negatively influence abundance of all of the decreasing species.
Coastal wetlands may be more important for marsh birds than once thought
if they provide high-lake-level-induced population pulses for species of
conservation concern. Coastal wetland protection and restoration are of
utmost importance to safeguard this process. Future climate projections
show increases in lake levels over the coming decades, which will cause
“coastal squeeze” of many wetlands if they are unable to migrate landward
fast enough to keep pace. If this happens, less habitat will be available
to support periodic pulses in marsh bird abundance, which appear to be
important for regional population dynamics. Actions that allow landward
migration of coastal wetlands during increasing water levels by removing
or preventing barriers to movement, such as shoreline hardening, will be
useful for maintaining marsh bird breeding habitat in the Great Lakes.