Study links bird sampling density in the United States to historical race-based zoning policies

Study of 90 years of bird sampling in 195 cities across the United States finds significant disparities in historically marginalized communities

Data resources used via GBIF : 12,297,506 species occurrences
Sayornis phoebe
Eastern phoebe [Sayornis phoebe (Latham, 1790)] observed in Southbury, Connecticut, USA by altspen (CC BY-NC 4.0)

In the 1930s, U.S. zoning policies allowed the Home Owner's Loan Corporation (HOLC) to rate neighbourhoods based on racial composition and housing stock and guide mortgage loans on the perceived "safety" as real estate investments.

In this study, researchers obtained historical zoning information for 195 U.S. cities, which ranked more than 9,800 neighbourhoods as "A" (green), "B" (blue) "C" (yellow) or "D" (red: the source of the term "redlining"). They then downloaded the more than 12 million GBIF-mediated bird records observed in these neighbourhoods between 1932 and 2022.

Analyzing the combined data, the researchers found significantly different observation patterns across each HOLC ranking, with formerly A-graded areas having more than twice the sampling density of formerly D-graded areas. Likewise, survey completeness was lowest in formerly redlined areas, which also had the highest number of coldspots.

A further exploration of sampling across time showed that the ratio of sampling density between grades A and D grew from around 2.0 in 2000 to 2.7 in 2020, representing an increase in disparity of more than 35 per cent.

These disparate patterns varied slightly, however, depending on the source of the data. While the largest contributor of bird data in GBIF, eBird, tracked with the overall trend, iNaturalist, while only a comparatively minor contributor of avian data, showed notably more even distribution across all four HOLC grades.

Ellis-Soto D, Chapman M and Locke DH (2023) Historical redlining is associated with increasing geographical disparities in bird biodiversity sampling in the United States. Nature Human Behaviour. Springer Science and Business Media LLC. Available at: