Black-Crowned Night Heron Field Log: 2022 Season

To further the conservation and ecological understanding of the state-endangered black-crowned night heron, researchers at the Urban Wildlife Institute are studying the wild colony that has been nesting at Lincoln Park Zoo and the surrounding park since 2010.  

Now, with more than 600 individuals, the Lincoln Park Zoo colony has grown to be the largest in the state of Illinois and has become a fixture of the zoo, nesting primarily over the red wolf habitat at the zoo’s Pritzker Family Children’s Zoo! 

Black-crowned night herons. Photo courtesy of Henry Adams.

Join the zoo’s Urban Wildlife Institute scientists as they dive into the 2022 monitoring season by following along with their field notes, below: 

March 21 – Black-crowned night herons began returning to Lincoln Park Zoo’s Pritzker Family Children’s Zoo. Volunteers spotted six individuals, and Urban Wildlife Institute researchers began the 2022 monitoring season on March 23. 

April 2 – Three hundred twenty-three adult black-crowned night herons were detected in the Pritzker Family Children’s Zoo, surpassing the maximum number of adult herons detected in a single day there during the 2021 breeding season. 

April 13 – A yellow-crowned night heron, another rare wading bird species in the state of Illinois, was spotted by Urban Wildlife Institute (UWI) researchers and members of the Chicago birding community hanging out in the black-crowned night heron rookery. 

A yellow-crowned night heron illustration. Illustrated by UWI scientist Henry Adams.

April 24 – Fallen eggshells were first detected in the colony. As is the case with many wading bird species, black-crowned night heron nests can be extremely precariously placed and are vulnerable to predators, such as other birds and smaller, arboreal mammals. As such, seeing fallen eggshells is not uncommon and is a great way to tell when laying has begun. The incubation period for these eggs is roughly 24–26 days, with both parents sharing incubation duties. 

May 10 – There was a huge boom in the adult black-crowned night heron presence at the zoo between May 4 and May 10, presumably because of warmer weather finally rolling in. On May 4, UWI researchers detected 447 adult herons and on May 10, there were 619. 

May 13 – UWI researchers detected 719 adult black-crowned night herons at the Pritzker Family Children’s Zoo, making this the largest that the Chicago colony has been since UWI began monitoring the population in 2010. While more adults does not necessarily mean more successful reproduction, this is an extremely exciting development for this urbanized, state-endangered species and demonstrates how cities can provide valuable wildlife habitat! 

May 16 – UWI welcomed Maurice Cowan, the zoo’s Dr. Schol’s Nature Boardwalk Intern. Maurice has been a lifelong visitor of Lincoln Park Zoo and has spent many years as a zoo volunteer and employee. He now returns to the zoo with a Bachelor of Science degree in Zoology and a passion for urban wildlife behavioral ecology! He will be helping out with the management and research of the night heron colony throughout the summer and conducting his own independent research at the Nature Boardwalk. Welcome, Maurice!

Here Maurice and Henry Adams (UWI’s Wildlife Management Coordinator) are bringing in a flock of decoy black-crowned night herons from the zoo’s collaborators at University of Illinois. These decoys will be used to help capture adult herons for a banding project that will be conducted later this summer.

May 17 – UWI researchers performed nest checks for a few accessible nests in the Pritzker Family Children’s Zoo, photographing the eggs of the herons for the first time. These checks are conducted using a Wi-fi-enabled borescope. A borescope is a small camera fixed to a 33-foot long wire that connects to a phone or tablet to display and capture images and video. We attach the scope to an extendable pole and use it to take a peek inside the heron nests!  

We make sure to only check nests two times a week and in favorable weather conditions, so as to not disturb the birds. Monitoring the number of eggs laid, number of eggs hatched, and number of juveniles that reach fledging age (roughly 15–20 days old) per nest is an effective way of evaluating reproductive success and can greatly inform endangered species management. 

Eggs spotted in a nest via a borescope. Photo courtesy of Henry Adams.

May 18 – Exactly 24 days after eggshells were initially spotted in the colony, UWI researchers detected the tell-tale chipping of two baby black-crowned night herons, the first young of the year for 2022. Under the protection of their parents and the masses of tangled branches and new spring foliage, these babies are difficult to see. Still, UWI researchers were able to spot one of the calling chicks with binoculars. 

May 23 – Seven heron chicks have been detected in the Pritzker Family Children’s Zoo, and adult abundance in the colony has begun to dwindle. This kind of phenology (i.e., biological timing) is typical of these birds: around mid/late May, chicks start hatching and adult black-crowned night herons begin spending less and less time in the colony, either because they’re actively foraging for their newly-hatched chicks or because they did not secure nesting territory for the season. Both sexes are responsible for incubation and feeding offspring, so they will alternate in guarding the nest and foraging. 

May 31 – Did you know that, much like owls, herons regurgitate masses of indigestible material? These masses, known as boluses, provide an excellent opportunity to remotely examine the diet of the herons. While conducting routine heron counts, UWI researchers examined a few of these boluses, finding mostly crustacean exoskeletons! This is a great sign that the herons and their offspring are consuming a nutritious and protein-rich diet even while living in a highly urbanized landscape.

UWI scientists spotted crustacean exoskeletons. This is a great sign that the herons and their offspring are consuming a nutritious and protein-rich diet. Photo courtesy of Henry Adams.

June 6 – UWI welcomes Nature Boardwalk Intern Lainey Gregory. She is a rising second-year student at the University of Chicago with a passion for researching health and epidemiology. She will be working with the black-crowned night heron colony and conducting her own research on how motion-activated cameras can be used to document wildlife behaviors that may contribute to disease transmission. 

June 7 – Exactly 21 days after the first young of the year were detected, the heron chicks have begun to fledge! On this day, UWI researchers detected 49 young of the year, aka hatch year juveniles. A number of them are beginning to explore the branches near their nests. 

June 13 – UWI welcomes Nature Boardwalk intern Kali Radatz. Kali is a rising senior at DePaul University and is very enthusiastic about urban ecosystems. In addition to her work with the heron colony, she will be studying the turtle community at Nature Boardwalk, documenting biodiversity and making population estimates. 

June 14 – In partnership with Dr. Michael Ward’s lab at University of Illinois, Urbana-Champaign and Brad Semel, endangered species specialist from the Illinois Department of Natural Resources, the heron banding project has begun. These bands will help document the movement of these birds throughout Chicago and see if they return to the colony year after year. Keep an eye out for banded night herons and help UWI researchers collect movement data by submitting your sightings on iNaturalist and tagging UWI @urbanwildlifeinstitute.  

June 17–27 – Heron banding continues with the UWI’s Team Heron, composed of Adams, Cowan, Gregory, and Radatz. 

The zoo’s hard-working interns. Photo courtesy of Henry Adams.

July 1 – Today, the UWI team observed 400 hatch year herons at Pritzker Family Children’s Zoo, marking the most reproductively successful year the colony has had since UWI began monitoring this population.

July 13 – The 2022 heron banding season has come to an end. Banding adult herons was unsuccessful this year, but keep an eye out for 15 banded juveniles before they start dispersing for the fall migration season. 

A black-crowned night heron illustration. Illustrated by UWI scientist Henry Adams.