2023 Black-Crowned Night Heron Field Log

September 6, 2023

From Henry Adams, Wildlife Management Coordinator at the Urban Wildlife Institute and primary investigator of all things night heron-related at Lincoln Park Zoo.

March 16 – The herons are coming back! Today, we noted two adult herons and one juvenile hanging out at Pritzker Family Children’s Zoo. The return of this migratory species to their natal colony indicates the approach of spring!

April 3 – With warmer and sunnier weather in the area, we have seen an influx of birds at the colony: we counted 80 birds today. Pair bonds are starting to be formed. Typically, males will first establish a nesting site and offer it as a means of attracting a mate. Once bonded for the season, the pair will build their nest together, presenting each other with sticks and other building materials in tender, yet sometimes clumsy, displays.

Black-Crowned Night herons nesting

April 12 – There are already approximately 370 birds present at the colony…incredible! In looking back at the data, the last time there were this many birds present at the colony at this time of year was 2017.

I did a bit of research to discover that these years were both the start of El Niño-Southern Oscillation (ENSO) neutral periods, during which neither El Niño or La Niña climate phenomena are present. In 2017, an El Niño period was wrapping up around this time while now, we’re exiting a La Niña period, which both have potentially positive and negative impacts on midwestern ecosystems.

Perhaps this return to neutrality in 2017 and now in 2023 encourages quicker returns for the birds. Chicago also falls in an area of North America that is historically less impacted by both La Niña and El Niño events—maybe this increases its attractiveness as well. Hopefully we can find out more with the research we’re doing this year and in years to come!

April 19 – Today, we found the first heron eggshells of the 2023 breeding season on the ground. This comes nearly 10 days earlier than when eggs were first seen in 2022. With roughly 560 birds returned for the season and babies on the way, the colony is full and thriving!

April 26 – The Urban Wildlife Institute is collaborating with a team of scientists that includes Sarah Slayton, Dr. Mark Ward, and Michael Avara from the University of Illinois, Urbana-Champaign and Brad Semel from the Illinois Department of Natural Resources to investigate the urban ecology of the black-crowned night herons. We will be banding and collecting biological samples from herons to better understand the health, diet, and demography of this urbanized population that nests at the zoo.

A subset of adult herons involved in this study will be fitted with GPS transmitter units, which remotely provide detailed movement data. The research team will use that movement data to better understand how these birds select habitat, move throughout their annual migratory routes, and connect to other heron populations.

The structures you may see at Lincoln Park Zoo’s Nature Boardwalk, Marovitz Golf Course Pond, and other nearby locations throughout the summer are called walk-in enclosures, a tried-and-true method for safely capturing and sampling various bird species. Some of us spent today constructing these enclosures.

April 28 – Today, I counted 638 adult herons at the colony. This is more adult herons at this time of year than any other year on record! Typically, the colony reaches peak adult abundance a week and a half or two weeks into May, so we’re still anticipating more.

May 3 – During their heron count this morning, I heard a familiar and distinctive sound: the distinct “chip chip chip” of a baby heron! The baby, also called a “young of year” for research purposes, was huddled underneath its parent and was not visible.

This is the earliest that hatchlings have been documented at the Lincoln Park colony—in recent years, they’ve generally appeared two to three weeks into May. Night herons have an incubation period ranging 24-26 days. Clearly the birds have been laying eggs since well before April 19, when we detected the first eggs. Perhaps the warm spell between April 3 and 14 prompted some early clutches.

May 10 – The team and I documented 686 adult herons on this day, marking peak adult abundance for the 2023 breeding season. While this number of adult birds does not surpass 2022’s record of 750, this is still QUITE a lot of birds—220 over the colony’s annual average of 468 adults. This peak in adult abundance comes roughly a week earlier than 2022, which was somewhat expected given the colony’s early return.

May 11 – Since April, the research team has continuously conducted scouting trips to various heron foraging sites, such as Montrose Harbor, North Pond, and Nature Boardwalk, to see how the birds utilize said habitats and inform sampling. The team found that herons foraging at Montrose Harbor have developed a fishing technique that is a bit atypical of wading birds. Herons will perch on the edge of boat docks and concrete walls of the harbor, dive into the water in pursuit of fish (much like a gull or other sea bird), then quickly return to their perch. The team believes it may be able to exploit this behavior and use leg lassos to capture birds while in fishing mode.

May 15 – The research team attempted night heron captures at Montrose Harbor and the Marovitz Golf Course pond using a suite of techniques, including walk-in enclosures, mist nets, and leg lassos, but unfortunately to no avail. Many more capture attempts are yet to come.

May 24 – Our team documented the first fledging young-of-year herons in the colony exploring outside of the nest. Black-crowned night herons typically take four weeks to fledge and six weeks to become flighted. Clearly, even though hatch year birds weren’t detected until early May, some began hatching in late April.

May 26 – The team and I counted 50 hatch-year and 594 adult herons in the colony today. Usually by this time of year, the adult herons begin leaving the colony to begin their pre-migration dispersal and foraging period. Once the young-of-year start to become grown enough, adults will spend more and more time away from the colony. Herons and other colonial nesting wading birds use this pre-migration dispersal period to rebuild their muscle mass and fat stores in preparation for fall migration.

June 7 – I traveled with colleagues from UWI to Washington, D.C. to attend the 2023 International Urban Wildlife Conference. I presented on the history of night heron urban ecology in Chicago, results from the 2022 field season, and the research team’s ongoing efforts.

June 9 – After multiple unsuccessful capture attempts at the Marovitz Golf Course and Nature Boardwalk, the research team has switched its focus to the night heron colony at Pritzker Family Children’s Zoo. Almost immediately, adult and juvenile herons alike acclimated to the two walk-in enclosures we set up, enjoying the fish provided inside the enclosures. The birds clearly felt safer exploring novel objects at their breeding versus their foraging grounds. Hopefully this will turn the team’s luck with capture around.

June 12 – Today was a magical day: In addition to banding and sampling three hatch-year herons, the research team successfully captured, sampled, and deployed a GPS transmitter unit on its first adult black-crowned night heron, dubbed A23 or Abe. After capture via walk-in enclosure, A23 was measured, sampled, and fixed with an aluminum leg band issued by the United States Geological Survey. He also got a white alpha-numeric band for easier identification at a distance and a GPS transmitter. The transmitter was secured using Teflon ribbons that looped over the head and then under each wing, creating a backpack of sorts.

When this “backpack” was completed, the transmitter sat securely between the bird’s shoulder blades. A23 was immediately released after the sampling process, which took roughly 15-20 minutes. A23 is the first night heron from Illinois to be GPS-tagged and joins the ranks of only a small handful of black-crowned night herons to be tracked throughout the duration of their annual cycle.

You can see what A23 is up to via the Animal Tracker app. Hosted by the Max Planck Institute, this free-to-download app provides a publicly accessible platform to view the movements of scores of animals from a variety of taxa across the globe. All researchers have to do is register the animals they track with the platform, et voilá! You can find Abe by simply searching for “black-crowned night heron” within the app.

June 27 – The research team celebrated its second and third adult heron captures and GPS deployments! A24 and A26 join the study and, like A23, can be visualized on Animal Tracker.

June 28 – The team and I counted 486 hatch-year birds at the colony, marking peak hatch-year abundance for the 2023 breeding season. This breaks 2022’s record of 400 documented hatch-year birds by 86, making 2023 the most successful breeding year on record. This is enormously encouraging to see within an urban breeding colony of these endangered birds. While it is impossible to say for certain, my speculation is that this success may be attributed in some way to the infrequency of heavy spring storms relative to previous years. Perhaps this resulted in fewer hatchlings being knocked out of the nest, especially at earlier ages when they are unable to self-thermoregulate.

July 6 – The vast majority of adult night herons have flown the coop, making it increasingly challenging to deploy GPS transmitters. With only 30 of the nearly 700 adult herons remaining in the colony, today the research team decided to deploy a few transmitters on hatch-year birds, who, at this point in the breeding season, are well-flighted and of adult size.

On this evening alone, the team deployed three GPS transmitters on hatch-year birds and banded and sampled an additional two. With six transmitters successfully deployed, in A23, A24, A26, A32, A22, and A35—and an additional 10 hatch-year birds banded—the research team decided to close the 2023 field season as a great success. We can’t wait to see where the birds will go!

July 16 – All GPS units are online and the data is flowing! A couple of the GPS birds, specifically A23 and A35, have traveled up to Wisconsin. While it may appear to be counter-intuitive for a migratory bird to move north during the fall, this is not unheard of during the pre-migratory dispersal period. It’ll be exciting to see where these birds ultimately decide to spend the winter. Additionally, the remaining hatch-year birds at the colony continue to spread their wings and explore away from the colony, foraging and passing time at the Lincoln Park Zoo’s Waterfowl Lagoon as well as Nature Boardwalk. It is expected that the hatch year birds will be largely dispersed by early August.


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