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Nesting Herons and Egrets Endanger a Rare Oak Grove: UC Davis Officials Face a Puzzler As Guano From Protected Birds Boosts Soil Acidity - and Drives Off Visitors.

Posted on: Sunday, 7 May 2006, 15:04 CDT

By Edie Lau, The Sacramento Bee, Calif.

May 7--Birds and trees usually go together like fish and water, but in this environmental conflict with a twist, night-herons and egrets might be killing a prized oak grove in Davis.

The grove, formally known as Shields Oak Grove, is at the secluded west end of the University of California, Davis, arboretum. It contains 89 kinds of oaks collected from around the world since the 1960s.

Now filled with mature trees, the grove has become a magnet for leggy wading birds. The birds are nesting and roosting in such abundance that their droppings are turning the soil salty and acidic, possibly threatening the health of the trees.

Last summer, 2,400 adult birds took up quarters in the four-acre grove, producing more than 850 chicks, campus biologists estimate.

By season's end, the ground beneath the trees was white with guano, its musty odor, sharp with ammonia, almost intolerable to the human nose.

A nearby gazebo used for weddings had to be closed to public use.

"It's called 'White Flower Garden,' " said Kathleen Sokolofsky, the arboretum director. "Well, it was a little too white last year to do any weddings!"

As is their habit, the night-herons and egrets migrated to other roosts in the winter. With the return of spring, they are coming back.

Amanda Castaneda, a research assistant at the UC Davis Museum of Wildlife and Fish Biology, counted 35 nests in mid-April, 60 at the end of the month.

Guano is beginning to coat tree leaves, the breeze wafting a pungent scent.

The case of birds vs. oaks poses quite a dilemma for the arboretum.

"We're UC Davis and we want to do the right thing with the trees and the wildlife," Sokolofsky said. "In some cases we've heard, (the birds) actually kill the trees and go to a new site. The question for me is, do you let it get to that point?"

She is not apt to. Originating as a cluster of native oaks growing along the old north channel of Putah Creek and dating to the turn of the last century, the grove has grown into a nationally recognized collection of trees from California and beyond.

Some of the acorns from which the oaks grew were collected from around the state four decades ago for the research of John Tucker, a UC Davis botanist who served from 1972 to 1984 as arboretum director.

Arboretum staff members acquired exotic species through a seed exchange with other institutions.

The trees number 365 today; their origins include Portugal, Spain, Israel, Mexico, Guatemala, Honduras, Iran and Big Bend National Park in Texas.

"These trees are analogous to a group of Dutch paintings in an art collection," Sokolofsky said, noting that the arboretum recently received two federal grants from the Institute of Museum and Library Services totaling $106,000 to care for the oaks.

In 2000, Tucker, now retired, donated $250,000 to establish an endowment to maintain and enhance the grove for education. Arboretum officials planned to showcase the grove by extending trails to entice the public beneath its canopy.

About the same time, the first black-crowned night-herons began moving in.

Egrets followed. Campus wildlife biologists identified them as snowy egrets, cattle egrets and great egrets, all birds with showy white plumage. Plumage was their downfall a century ago: People killed the birds to supply the millinery industry with feathers.

Laws eventually were adopted to protect the birds, enabling their populations to rebound. To this day, the Migratory Bird Treaty Act of 1918 prohibits interfering with the birds when they nest.

Night-herons and egrets, which are closely related, are colonial by nature, meaning they nest together, sometimes quite densely. In Shields Grove, those trees that drew birds last year - not all did - hosted an average of 25 nests each, said Ellen Zagory, the arboretum director of horticulture.

The problem in Davis is not common in California, but other states have struggled with "nuisance heronries."

Ray C. Telfair II, a wildlife biologist retired from the Texas Wildlife and Parks Department, spent much of his career studying heron nesting and roosting behavior. Of 53 heron nesting spots documented in Texas, Telfair has classified 33 as nuisances.

The "nuisance" tag is a social, not biological, description. The 20 cases that aren't a nuisance are "in locations where nobody's complained about them," Telfair said.

A colony has the power to rearrange the landscape. On a pair of islands in a man-made waterway called Cedar Creek Reservoir south of Dallas, Telfair watched over 30 years as native trees gave way to an exotic tree, the chinaberry, brought in as twigs with berries by nest-making egrets and herons.

Many of the native oaks, elms and ash could not tolerate changes in the soil chemistry caused by the abundant bird guano, Telfair said, while the chinaberry did just fine.

Intolerant trees can die quickly. Telfair has seen four species of oaks in Texas succumb in one or two years after bird colonies were established. But he cautioned against assuming that all oaks are equally vulnerable.

"You cannot extrapolate by saying, if post oaks (in Texas) are intolerant, then your California oaks are also going to be intolerant," he said. "It might not be so. There are all kinds of variables."

Davis arboretum officials are monitoring their oaks nervously. A couple of coast live oaks that are especially popular with the birds appear somewhat frazzled, their crowns partially defoliated.

Staff members also are concerned about visitors getting nailed by droppings or picking up pathogens that may lurk in the guano.

"People come here and picnic," said Zagory. "You wouldn't want kids to put their hands on the ground and in their mouths."

As chicks begin to hatch, there also may be the unsettling scene of baby birds falling from their nests and dying.

"That was part of the odor last year," Sokolofsky said.

"You'd see the ground squirrels come drag them into their holes," Zagory added.

"Nature unfolding," Sokolofsky called the scenario, with a dry chuckle. "But not the kind of picture people want to see."

Dale Steele, program manager for species conservation recovery at the state Department of Fish and Game, speculated that night-herons and egrets are drawn to the arboretum in numbers because their historical habitat - forested banks of rivers and streams - is shrinking. "There are fewer places for them to go, so the better spots get more attention," Steele offered.

Mindful that in an environmentally sensitive community such as Davis the birds will have as many champions as the trees, the arboretum staff is not given to precipitous action.

The one step it has taken so far is to remove remnants of last year's nests and twigs from beneath the trees to reduce the amount of nesting material.

When summer ends and the birds move on, the arboretum will thin the grove, removing about 25 of the 365 trees to give the remaining trees more space to grow.

"We'll take a scientific approach," Sokolofsky said. "One thing at a time, and see how it works."

...and a black-crowned night-heron nest in Shields Oak Grove in the UC Davis Arboretum. The birds return each spring but may be wearing out their welcome. Sacramento Bee/Bryan Patrick

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Copyright (c) 2006, The Sacramento Bee, Calif.

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Source: The Sacramento Bee

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