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Non-Breeding Night Herons Used Home Ranges 18 Times Larger Than Breeders in Chicago

Black-crowned night heron beside the Chicago River in Chicago
Research · Ecology

GPS tracking at Chicago’s Lincoln Park Zoo revealed a sharp contrast between breeding and non-breeding black-crowned night herons (Nycticorax nycticorax). The colony is the largest remaining breeding site for the species in the Great Lakes region of the United States.

Accelerometers measure body acceleration and movement, allowing activity levels to be estimated even when the tagged birds are not being observed directly.

Researchers fitted 17 adult birds with GPS/GSM transmitters in 2023 and 2024. Three tags stopped sending data after only a few days, so home ranges were calculated for 14 birds: eight breeders and six non-breeders. Accelerometers provided activity data from 13 birds in 2024.

Non-breeders used home ranges about 18 times larger

Breeding status was closely associated with the size of the area used. The estimated total home ranges of non-breeders were about 18 times larger than those of breeders, and their core areas were about 25 times larger. Breeding birds used feeding sites an average of 3.62 kilometres from the colony and stayed within 10 kilometres. For non-breeders, the corresponding average distance was 21.31 kilometres.

Urban green space made up more than half of breeders’ off-colony locations

Urban green spaces—including parks, forest preserves, golf courses and cemeteries—accounted for an average of 54.2% of the locations recorded away from the colony for breeding birds. Lake Michigan, USA harbours, breakwaters and beaches accounted for 33.2%, while sections of the Chicago River accounted for 9.2%. Non-breeders also made frequent use of urban green spaces, rivers and creeks.

Breeders were more active at dawn and during the day

The activity sensors showed that both groups were active throughout the 24-hour cycle. Breeding birds were more active than non-breeders at dawn and during daytime, while the groups did not differ significantly at dusk or at night. The authors describe this flexible daily activity and broad habitat use as traits that help the species persist in a densely built landscape.

The study identifies nearby urban green spaces and water bodies as important feeding habitat for the colony. The authors recommend reducing aquatic pollution and improving enforcement of safe fishing practices because hooks and discarded line can injure or trap foraging birds.

Sources and image credit

  1. Slayton, S., Adams, H., Avara, M., Semel, B., Lardner, A., Lehrer, L., & Ward, M. (2026). The Windy City rookery: Movement and activity patterns of black-crowned night herons (Nycticorax nycticorax) in a human-dominated landscape. Ecology and Evolution, 16(4), e73310. https://doi.org/10.1002/ece3.73310Primary peer-reviewed open-access article first published 31 March 2026 under CC BY 4.0. GPS/GSM data from 14 black-crowned night herons in Chicago showed that non-breeders had total home ranges about 18 times larger and core areas about 25 times larger than breeders.

Image: Raed Mansour / Wikimedia Commons — wild black-crowned night heron at River Park beside the Chicago River in Chicago, photographed 7 September 2020; representative image from the study city, not a tagged study bird. · CC BY 2.0 · Source

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