Colonies of Cliff Swallows on Highway Bridges: A Source of Escherichia coli in Surface Waters

Sejkora, Patrick, Mary Jo Kirisits, and Michael Barrett, 2011. Colonies of Cliff Swallows on Highway Bridges: A Source of Escherichia coli in Surface Waters. Journal of the American Water Resources Association (JAWRA) 47(6):1275–1284. DOI: 10.1111/j.1752-1688.2011.00566.x

Abstract:  Animals, such as birds, are a source of fecal indicator bacteria and pathogens in the environment. Our objective was to determine whether a colony of cliff swallows nesting underneath a bridge would yield a measurable increase in fecal indicator bacteria (specifically Escherichia coli) in the underlying creek. When the swallows were absent, dry-weather concentrations of E. coli upstream and downstream of the bridge (in Austin, Texas) were below the Texas contact recreation criteria. When the swallows were present, dry-weather geometric-mean E. coli concentrations increased significantly from upstream (43 most probable number [MPN]/100 ml) to downstream (106 MPN/100 ml) of the bridge. One exceedance and one near-exceedance of the Texas single-sample contact recreation criterion were observed during the swallows’ nesting phase. When the swallows were present, the downstream E. coli geometric-mean concentration in storm events (875 MPN/100 ml) was significantly higher than the upstream concentration (356 MPN/100 ml), suggesting that runoff flushes swallow feces from the ground into the creek. Although the loading of E. coli from cliff swallows nesting under bridges can be significant (e.g., dry-weather loading of 3.1 × 10⁸ MPN/day/nest), the zoonotic potential of the cliff swallow must be examined to determine the risk to human health from contact recreation in waters contaminated with cliff swallow feces.