Where Herons Dare
It's a short boat ride from the downtown dock to Ross Island. That's good because it's already raining as Carmen Thomas and Elaine Sproul shove off and motor downstream toward the middle of the Willamette River. For the two Oregon State University researchers it's the start of another work day that takes them from the riverside hubbub of Portland, Oregon, to the rain-soaked woods of an urban island.
They tie up the boat and begin picking their way through a mixture of stinging nettles and thick, tangled brush to a spot deep in the woods. The pair creep through the underbrush, careful not to disturb the activity high above. In the tall black cottonwoods an occasional swish of giant wings cuts through the big city drone, as great blue herons go about the business of tending their young.
The two have come here to study the birds--to see how they are coping with environmental contamination from municipal and industrial discharges, non-point source pollution, accidental oil and hazardous waste spills and agricultural runoff.
For two summers Thomas, working with field assistants Sproul and Chris Barclay, has spent 10 weeks eavesdropping on this treetop nursery, four others along the lower Columbia and Willamette Rivers and a sixth in the Puget Sound. The researchers have watched the comings and goings of the parents as they raised their young from birth to fledging.
At Ross Island, Thomas and Sproul climb to observation platforms 30 feet in the air. In all kinds of weather, they do four-hour stints on the camouflaged, temporary platforms. From their perspective above the underbrush, the two count chicks and record weather conditions, predators, disturbances and the behavior of the sometimes doting parents.
A few days later Thomas has moved on to Ridgefield National Wildlife Refuge in Washington along the Columbia. This time, swatting mosquitoes, she kneels inside a ground blind with her spotting scope aimed at "clacking" juveniles in nests overhead.
"I've always been interested in airborne animals," the master's student in wildlife science at OSU admits. "And herons--they're really fascinating." Her enthusiasm for the creature is genuine. It has to be when the birds and their nesting grounds become a consuming interest for the better part of two years.
Monitoring the birds from hatch to first flight was one part of the study. Chemical analysis of eggs and chicks was another. By comparing what they saw in the field with what they found in the laboratory, and tying it back to colony locations along the river, the researchers hoped to track the effects of contamination on heron reproduction and relate it to similar studies on another fish-eating bird, the threatened bald eagle.
In fact, it was concern over the condition of the bald eagle population that sparked researchers' interest in herons, according to Bob Anthony, leader of the Oregon Cooperative Wildlife Research Unit in OSU's Department of Fisheries and Wildlife. Anthony is Thomas' thesis advisor.
"Research in the 1980s showed high levels of contaminants in bald eagles on the lower Columbia River, and productivity (of eagles) was much lower than most healthy populations across the country," Anthony, a 20-year veteran of eagle research, recalls. Some eagle nests on this stretch of river have not produced any young for the past 19 years.
But problems with studying bald eagles include their limited number, their sensitivity to contaminants, and the fact that they nest in tall trees in hard-to-get-to places. Anthony wondered if there was another species they could monitor for environmental contaminants. The scientists were looking for an "indicator" species, kind of like a canary in a coal mine, that would tell them a lot about the quality of the environment shared by bald eagles.
For several reasons the research team looked to herons as that indicator species. Like eagles, they are a fish-eating species--predators exposed to higher levels of contamination accumulated from the food they eat. Herons are fairly common, with rookeries up and down the river. Herons on the lower Columbia and Willamette do not migrate, so the residue in eggs is from local contamination. And, they are relatively insensitive to pollutants and are able to reproduce while carrying high contaminant loads.
So in 1994, with a grant from the U.S. Fish and Wildlife Service, Carmen Thomas began the comprehensive research project, with Anthony as her advisor. It would be the basis for her master's thesis. During the next two years, she collected and chemically analyzed eggs and embryo liver tissue and observed heron behavior from birth to the fledgling stage of development, all in hopes of determining effects on herons and, ultimately, helping the bald eagle.
The first step: Select study sites. Herons pick a nesting site based on food sources, and they can adapt to various kinds of sites. The birds also tend to be devoted to sites they used in previous years. These can be spots like a quiet, mosquito-clouded riverside or an urban island with a noisy gravel dredging operation nearby. Both figured into the final mix of a half-dozen study sites.
While two sites along the Willamette, near Portland, do not have bald eagles living in the neighborhood, three others along the Columbia and one in Puget Sound do.
On the Willamette, Thomas focused on heron colonies at Molalla State Park and on Ross Island. And along the Columbia, she studied colonies at Fisher Island near Longview, Washington; Bachelor Island in the Ridgefield National Wildlife Refuge in Washington; Karlson Island near Astoria, Oregon; and Samish Island in Puget Sound. Because of its history of limited contamination, Samish Island served as the reference site for the project.
The process of monitoring the birds began with collecting eggs. Thomas collected eggs each spring for two years from all six sites. But at three sites along the Columbia, the herons shared their nesting grounds with bald eagles. Because the researchers hoped to establish a correlation between the two species, they made a special effort to collect eggs at these sites during the same years that another research team was taking samples of bald eagle eggs.
Nest selection criteria went beyond shared locations with eagles. The scientists chose heron colonies with large, established populations. Colonies with 50 or more nests, where a sample of 10 to 15 eggs each year likely would not be missed. While eagles lay only one to two eggs a year, a heron "clutch" numbers three to five eggs every year.
Herons nest in tall trees. "Some in black ash, some in Sitka spruce, but typically you'll find them in cottonwoods on the Columbia and Willamette Rivers," Thomas says. To collect heron eggs she enlisted the help of some professional tree climbers and a couple of biologists trained to climb trees. She also collected some herself. "I couldn't very well ask them to do something I wouldn't do," she says.
During the two field seasons Thomas collected 140 eggs. About half of the eggs ended up in the incubator at a U.S. Fish and Wildlife Service lab, labeled and cataloged by site. Hatching rates varied from site to site.
Thomas took liver samples and tested them for cytochrome P450-1A1 (CYP450), an enzyme that detoxifies poisonous compounds called dioxins. Similar enzymes are present in birds, mammals and fish. Thomas wondered if dioxin concentrations detected in eggs were inducing or elevating CYP450 levels in Columbia and Willamette River heron embryos. While other studies have shown a relationship, her findings did not.
A laboratory working on contract performed chemical analyses on the other half of the egg collection. Lab technicians looked for toxins called organochlorines (OCs), polychlorinated biphenyls (PCBs) and dioxins, 150 compounds and 19 trace elements in all. OCs are pesticides. PCBs are compounds found in electric transformers, some plastics, lubricants and flame retardants. And dioxins most often are associated with pulp and paper mill chlorine bleaching processes and trash burning. Dioxins are the most toxic of the substances the labs searched for.
Scientists' concern over these compounds goes beyond mere presence to something called biomagnification. Although organochlorines, like the pesticide DDT, a derivative DDE, and PCBs have been banned since the early 1970s, years of use have left residues in river sediments and in the food chain. These toxins are absorbed and stored in the fat tissue of fish and birds and pass the poison along up the food chain. With each link the load gets bigger. What starts out as a trace in a fish can reach dangerous levels in fish-eating birds like herons and eagles at the top of the food chain.
When the reports came back, Thomas tracked which toxins were where, and in what quantity. Her analysis pinpointed an unexpected hot spot, Portland's Ross Island.
"Eggs from Ross had the highest mean concentrations of all OCs we tested," Thomas says. "Total PCB concentrations were also significantly higher at Ross." Although eggshells were thinner at all sites, shell thinning at Ross was two to three times greater than at Bachelor and Karlson. Concentrations of DDE and total PCBs were above detection limits in fish at all sites, but total PCB concentrations were up to 31 times greater at Ross."
"Ross Island was somewhat of a surprise," Anthony says. "We thought the highest contaminants would be in the lower Columbia estuary where contaminants settle out and accumulate."
Despite the elevated levels of contaminants, eggshell thinning, chick deformities at four of the six sites, and a correlation between concentrations of TCDD (a dioxin) and nest failure, hatching and fledgling mortality rates were comparable to other healthy heron populations. The numbers show that contaminants are not impairing heron reproduction on the Columbia and Willamette Rivers.
"The contaminants are affecting individual birds, but not the colony," Thomas concludes. But what about species with similar exposure and a higher sensitivity? They could suffer adverse effects, she warns.
Thomas is convinced that herons are a good indicator of contamination in aquatic ecosystems. They clearly have shown the need for regulation of pollution discharge into the two rivers. Are herons a good indicator species for bald eagles? Thomas and Anthony think the potential is still there, but it may take more study. "We'd need to look at more areas, repeat this kind of study at maybe eight to 10 across the country," says Anthony.
As often happens with research, a single project can generate as many questions as answers. And Thomas' heron work is no exception. Her project ended with fledglings leaving the nest. What happens after the herons learn to fly is not known. Do contaminants affect the birds at a later stage in life and reduce their survival rates?