Harbor Herons:
Sentinels of Health in the New York Harbor

by Bruce Stutz

No one is quite certain still as to why, in the early 1970s, herons began to return to nest on islands in the New York harbor. Great egrets, Snowy egrets, Black-crowned Night-Herons, Glossy ibis, along with cormorants and Black-backed gulls, hundreds of them, began appearing in spring, building their awkward stick nests in dense shoreline scrub or on barbed wire fencing, laying eggs and rearing chicks.

Word of the Harbor Herons, as they became known, spread among city birdwatchers who, starved for the sight of charismatic wading birds, ventured out with their binoculars to the remotest islets of the metropolitan archipelago, places not known for their natural beauty, some of them downright ugly.

The birds, however, seemed unconcerned about the aesthetics of their new breeding grounds and the colonies continued to expand. Had the cleaner waters brought them here? Would more of them come? What were they eating? Were the young healthy?

All of these questions became more important when, between 1997 and 2001, birds completely vanished from three islands that had previously hosted more than 1,500 nests. The birds had disappeared and no one knew why they had departed, or where they had gone.

Scott Newman, Conservation Medicine scientist for Wildlife Trust’s New York Bioscape Initiative, wanted to learn more about the disappearance. “These birds are sentinels of the health of New York harbor. They are top predators. They swim and fish in our waters. If their health is compromised by exposure to contaminants or pathogens, guess who’s having the same exposure?”

Newman was used to working in less than ideal conditions. Before he relocated to New York in 2002, he spent his first post-doc years along the northern California coast dealing with the effects of oil spills on birds. The circumstances in which the Harbor Herons nested were not as much a concern to him as where and what they were eating, since what adults fed their young determined the health of their chicks.

“The presence of nests doesn’t equal a healthy population,” Newman explains. “The birds can have nests with no eggs. They can have eggs that don’t hatch or chicks that don’t make it. A healthy population is one that sustains numbers of individuals that compose that population.”

Since the adult birds migrate south during the winter, their health might be affected by what they ate outside the harbor. The chicks, however, would be fed only what its parents could forage within a couple kilometers of the nest sites. Newman and his team decided to focus their studies on the chicks of the Black-crowned Night-Herons.

Heron chicks are gawky assemblages of unmatched parts - feet and bills too large for their sparsely feathered gray-brown bodies - crowned with a spiky burlesque of head feathers. They’re not difficult to examine, once you get to them; the hard part was making it to the nest through the matted vegetation without disturbing every bird around.

With a bird in hand, Newman and crew gave the chick what amounted to a complete medical workup, taking vital signs, measurements and a blood sample. (Occasionally, the nervous young birds would also favor the scientists with a surprise, but welcome, sample of stomach contents.) The birds were given a leg band to show on which island they were born, and gently returned to their nests.

What did the scientists find? For the most part, nothing unusual. The birds were being fed mostly fish and crustacean with a bit of rodent, bird or insect as well. The chicks were neither too thin nor light.

The blood samples and toxicology, though, had more of a story to tell. One result was a difference in white blood cell counts between one site, North Brother Island, and the two others tested. The North Brother chicks had a higher white blood cell count. Since white blood cells are the ones that fight off disease, were the North Brother chicks being exposed to a greater number of pathogens? Or, were contaminants such as PCBs, heavy metals, and DDE (a form of DDT) suppressing the chicks’ immune system at other locations making white blood cell counts lower at other locations?

The chicks on all the islands were already carrying pollutant loads, which, by all accounts, were in concentrations that should pose no immediate health hazards. The question, however, was what would happen as these chicks left the nest and their burden of pollutants continued to grow? Was a combination of environmental factors keeping new birds from returning?

This is, says Newman, “a web of life problem” made difficult to solve because so many of the particulars of these birds’ lives remain unknown. For instance, no one knows where - what rivers, what wetlands - the adult birds find their food. While most of the places where these birds breed are protected, the places where they forage are probably not protected. Are their feeding sites being polluted? Or being lost to development? Are the birds’ numbers limited by the quality or sufficiency of food?

“The New York estuary system is quite productive, with a variety of invertebrates and fish using the system throughout various life stages,” Newman explains. “However, there are other important interactions. One may be the seeming growing predominance of cormorants. Like the Bald eagle, the Peregrine falcon and other fish-eating birds, cormorants have responded well to the clean up of the harbor, a little too well some might say, as their numbers in the New York/New Jersey harbor have increased from 20 pairs to nearly 2,000 pairs within the past 20 years.”

This growing population of cormorants is a significant problem as the highly acidic cormorant guano kills vegetation and trees necessary for herons to nest. (Cormorants as ground nesters aren’t affected by the loss of trees.) As we’ve seen on the Great Lakes where cormorants have displaced herons by altering the habitat (denuding the islands of vegetation), herons leave in search of better nesting grounds.

“Cormorants are survivors,” says Newman. “And in other locations it’s possible for them to begin displacing the herons by altering the habitat. In New York Harbor, we need to study these interactions among bird species, and between birds and vegetation/habitat.”

These New York harbor islands – specifically North Brother Island, South Brother Island and Prall’s Island - have been a strong center of interest for ornithologists and scientists since the early 1970s. At that time, New York City Audubon began efforts to protect nesting sites and initiated an annual census of breeding herons, egrets and ibises in New York City. Wildlife Trust's study now adds the critical Conservation Medicine approach to the research.

The research continues into 2006. After a summer’s research, what does Newman have?

“We’ve established a baseline, a point of reference for our future studies. Birds are sensitive predictors of what direction the environment is taking - the canary in the mine -that will let us know what’s there long enough ahead of time to allow us to change course.”

In this case, the canary just happens to be a heron.