Rain Birds: Green Stormwater Infrastructure as Avian Migratory Habitat

Abstract

Background, Question and Methods Cities are often located on migratory flyways, and the urban stopover site may be a critical bottleneck in the lives of migratory species. These stopover sites can be composed of novel anthropogenic land cover types and configurations, with built elements as well as open space types. Green stormwater infrastructure (GSI) elements, including constructed wetlands, detention/retention ponds, bioswales, and rain gardens are designed for stormwater function, but may support resident or transitory wildlife. Research into the habitat value of rain gardens is scarce, but previous work on bird diversity in rural and urban contexts has supported the importance of patch area, vegetation structure, and plant diversity and origin. Some have found landscape-scale characteristics can also be predictive. Remotely-sensed data and GIS have become ubiquitous tools to replace labor-intensive methods of measuring habitat characteristics. What are important predictors of bird occurrence in urban GSI sites during spring migration? During spring 2018, standardized area searches were performed for bird species in Ann Arbor, MI, USA, on GSI sites (n=37). Discrete-return LiDAR data was used to measure vegetation structure (including Foliage Height Diversity via the Shannon-Weaver Diversity index for 0.5’ return height bins) and classification of 4-band ortho-imagery to measure land cover within multiple scales around each site. Generalized linear models using Bayesian methods were built to analyze predictors of bird species richness, abundance, and abundance weighted by conservation value. Results and Conclusions 3407 birds were recorded using the GSI sites, comprising 97 total species, including 22 of moderate or high conservation concern according to the Partners in Flight. The best model for landbird species richness included three positive significant explanatory variables (with mean-standardized parameter estimates: Site Area (0.175), % Shrub Cover (0.09), and Foliage Height Diversity (0.335), while the effect of Plant Species Richness was inconclusive. These results support that classic theories regarding bird occurrence in response to habitat structure prevail in novel ecosystems within urban settings. The explanatory power of foliage height diversity is a confirmation of vertical vegetation structure as an important determinant in bird diversity, as well as the utility of multiple-return LiDAR as a method to measure FHD. This is an important point in the urban context, where simplification of structure is a common practice, and shows that novel habitat types formed by GSI can support biodiversity close to where people live.