Sewage lagoons – artificial ponds designed to treat human sewage -- are well-known birding hotspots where abundant birds and regionally rare species can often be found by birders regardless of their expertise. Despite their popularity, whether sewage lagoons benefit birds or merely offer convenience for birders remains unclear. Conventional wisdom and some evidence suggest that sewage lagoons provide more productive habitat for birds feeding on aquatic invertebrates than neighbouring natural areas, albeit with a heavier load of detergents and other pollutants. Here, we investigate the question of whether sewage lagoons provide ecosystem functionality that is good for birds, or are they merely good for birders? We approach this by comparing observations of birds in sewage lagoons with nearby natural areas, focusing on seasonal bird species richness.
We used monthly eBird ‘hotspot’ data from Jan 2010 to Dec 2019 and the locations of wastewater treatment (WWT) plants with lagoons throughout Ontario (n = 131) to examine seasonal bird species richness at WWTs and other nearby (<1500 m) hotspots (nonWWTs). Linear mixed effect models were used to evaluate temporal trends in species richness, controlling for seasonal fluctuations, differences between WWT and nonWWT localities, observation effort, and other potential confounders. Results showed that species richness was higher at nonWWT sites overall, yet WWT lagoons supported more species per unit effort during winter and shoulder seasons. This seasonal trend suggests that lagoons may provide favorable conditions such as warmer microclimates and abundant foraging opportunities during stopovers and colder months. These findings highlight the value of sewage lagoons during winter, yet caution is warranted before concluding they are “good for birds.” While they attract diverse species, sewage lagoons may function as ecological traps, exposing wildlife to pollutants and bacteria under the cloak of warmer temperatures and high food availability. Generally, they seem to be better for birders than for birds, particularly in the winter.