2023 Black-Crowned Night Heron Field Log

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.