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Davis, California

Sunday, March 3, 2024

Large egret and heron population threatens arboretum habitat

To avid joggers, nature enthusiasts and couples looking for a romantic afternoon stroll, the UC Davis Arboretum is known for its beautiful trails and collection of diverse plants and wildlife. However, recent visitors to the arboretum may have noticed – and likely smelled – a nastier side of this usually pristine garden.

In recent years, the arboretum has witnessed a rapid influx of four different species of egrets and herons, whose large nesting population poses a particularly unique risk to the arboretum’s treasured oak grove.

Excrement from these birds “coats the oak trees’ branches like white paint” and creates “acrid smells that permeate the air,” according to a 2006 arboretum newsletter.

While the smelly effects of the excessive bird “guano” may only be a nuisance to arboretum visitors, it actually poses a much more serious risk to the well-being of the oak trees.

Excessive guano coverings can deny trees access to enough sunlight for photosynthesis. In addition, guano contains enough toxic ammonia to cause defoliation.

Guano contains salts that can also enter the trees through the soil and pose a risk to the internal structure of the trees. This can permanently affect the tree’s well-being, said Ellen Zagory, director of horticulture for the arboretum in an interview with Dateline UC Davis.

Salts stunt the growth of the tree’s roots and can cause defoliation, which over several years could kill the branches, Zagory said.

Mana Hattori, junior specialist for the wildlife and fisheries biology department, said the effects of the excrement are visible to anyone who takes a trip through the oak grove.

“There are definitely visible effects on the trees,” she said. “A lot of trees have become deformed and the guano is pretty visible.”

Andy Engilis, curator for the UC Davis Museum of Wildlife and Fish Biology, said the large nesting population also poses a risk to the arboretum’s human visitors.

“A rookery creates a high amount of fecal matter which dries, concentrates and can become aerosolized,” Engilis said. “There is an increased risk to humans when such a rookery becomes established in a public place such as the arboretum. In the United States, heron rookeries have been shown to harbor salmonella and respiratory illnesses such as histoplasmosis.”

In order to fight the threats posed by the nesting birds, wildlife experts have begun to test several methods of discouraging the birds’ nesting. One such way is to shine a laser at the birds when they land in the trees.

“We are trying the method on non-nesting birds only during the predawn and post-dusk hours,” Engilis said. “The effectiveness of this will be evaluated for further application if deemed appropriate. The method has proven successful in other bird nuisance problems including other herons, crows and blackbird roosts. It is a commercially approved and accessible method.

Most UC Davis experts agree that removing the egret and heron populations from the arboretum does not pose a risk to the well-being of the birds.

“Their habitats are not endangered,” Hattori said “they can pretty much nest anywhere that they can find trees. They are known for nesting with other species so locating new habitats is not difficult for them.”

“Rookeries such as these are not uncommon throughout the state,” Engilis said. “Urbanization is not the problem and birds readily move to other sites if they find past rookery sites unappealing.”


ERICA LEE can be reached at campus@californiaaggie.com


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