2024 Black-crowned Night Heron Field Log

June 6, 2024

Lincoln Park Zoo becomes the nesting site for a colony of state-endangered black-crowned night herons every spring and summer since 2016. Their presence at Pritzker Family Children’s Zoo has allowed researchers to learn more about these birds than ever, and efforts to understand the behavior and migration patterns of the night herons continue every year. The following comes from Henry Adams, Primary Investigator of the Chicago Black-crowned Night-Heron Project and Wildlife Management Coordinator at Lincoln Park Zoo’s Urban Wildlife Institute.

March 7 – Members of the black-crowned night heron team met up at Chicago’s Big Marsh Park. The team hopes that to attract the birds to this site for foraging and breeding using night heron decoys, call boxes, and decoy nests to mimic the sights and sounds of an active site. Black-crowned night herons historically nested in Big Marsh and other habitats throughout the Lake Calumet region until the late 2000s, but abandoned these areas due to habitat degradation.

Enormous restoration efforts have greatly increased the quality of these habitats, and it is a goal of the heron team to encourage the Chicago-based colony to nest in Calumet and similar ex-urban wetland habitats once again to more evenly distribute the Illinois breeding population of herons across multiple habitats, which will help to better protect this endangered species.

March 13 – Perhaps because of the unseasonably warm weather, a few black-crowned night herons made quite an early return to the colony! Five adult and two juvenile night herons are spotted hanging around in the trees at Pritzker Family Children’s Zoo. This is now the earliest return these birds have made; the previous earliest return date on record was March 14, 2018. But this return is short-lived. Presumably due to a return to seasonably cool temperatures, these birds are no longer present by March 17.

March 26 – Roughly two weeks after their initial false return, 13 adult night herons are spotted hunkered down in the coniferous trees at the zoo. Before the weather starts warming up and our deciduous trees begin leafing out, early returning herons are mostly seen in these conifers. We theorize that this helps the birds stay insulated during cooler spring temperatures.

April 11 – The night heron team begins constructing the remote-activated walk-in enclosures that will later be used to capture adult birds.

The team will bait each enclosure on a daily basis with a bucket full of water and smelt to lure the birds into the enclosure. Once they are captured, they will be banded and rereleased for monitoring.

April 12 – Two yellow-crowned night herons are spotted in the colony, an exciting appearance for the Chicago birding community, as the yellow-crowned birds—cousins to the black-crowned variety—are uncommon in the Chicago area.

black-crowned night herons nesting

April 24 Members of the night heron team are trained by the zoo’s veterinary staff, led by Dr. Kathryn Gamble, to take blood samples from birds.  A single blood sample can help evaluate a bird’s stress hormone levels, the kinds of pathogens a bird may have come into contact with, and the bird’s general diet, greatly helping us understand the health of the heron population.

April 25 Not only were 400 adult herons counted during this day’s census, but the team also found eggshells on the ground, indicating the birds had officially begun incubating eggs. It’s likely the birds began laying the eggs at least a week prior to this discovery.

May 9 – The first heron chicks have hatched!

May 13 – During today’s census, baby herons are heard clicking away in the treetops—and, with 695 adults counted in total, the team documents what appears to be peak adult abundance for 2024.

May 14 – The team has its first successful capture and sampling of an adult night heron of the 2024 field season, dubbed A36 for the bright white alpha-numeric band placed on its left leg.

When banding these birds, researchers place an aluminum United States Geological Survey (USGS) band on the right leg and a white band with black lettering on the left. If you see a banded heron out in the wild, you can report your findings to the research team using this survey.

May 21 – It is not uncommon for Lincoln Park Zoo researchers to come across baby herons that have fallen out of the nest.

This is a frequent occupational hazard for baby herons. While Lincoln Park Zoo researchers prefer to take a hands-off approach with the colony, there are criteria in place to determine what qualifies a juvenile heron for rehabilitation. A juvenile heron may be considered for rehabilitation if they 1) are too young to thermoregulate without their parent (less than five weeks old) and have no major injuries or 2) are old enough to be out of the nest and have a treatable injury.

Today, researchers come across two individuals that qualify for rehabilitation. They are quite young, two and three weeks old respectively. After a consultation with zoo veterinary staff the chicks are transferred to our colleagues at Willowbrook Wildlife Center where the chicks will receive expert care for the next few months and will be banded prior to their release.

June 4 – The team captures and samples its 10th adult night heron of the season! This is a fantastic research milestone and the team looks forward to seeing what further headway can be made in the remaining 3.5 weeks of the adult sampling field season.

Meet the Chicago Black-crowned Night-Heron Project Team

  • Henry Adams, Primary Investigator and Wildlife Management Coordinator at Lincoln Park Zoo’s Urban Wildlife Institute
  • Sarah Slayton, University of Illinois, Urbana-Champaign Masters student
  • Brad Semel, project PI and Endangered Species Specialist with the Illinois Department of Natural Resources
  • Michael Avara, Field Coordinator & Laboratory Manager at UI, Urbana-Champaign
  • Amy Lardner of the Chicago Black-crowned Night-Heron Project
  • Brian Ellis of the Illinois Audubon Society
  • Research Technician Julia Knauz
  • Research Intern Samuel Cabindol
  • Liza Lehrer, project advisor and Assistant Director at Lincoln Park Zoo’s Urban Wildlife Institute
  • Dr. Seth Magle, project advisor and Director at Lincoln Park Zoo’s Urban Wildlife Institute
  • Dr. Michael Ward, project PI and Professor and Levenick Chair in Sustainability at UIUC
  • Jo Fesset, Executive Director of the Illinois Audubon Society



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