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A program that successfully discouraged egrets and herons from nesting in the campus’s Shields Oak Grove will continue this year, with slight variations.

When the first black-crowned night-herons arrive (could be any day now), they will be greeted with disturbing but harmless fireworks and laser lights, said Emily Griswold, assistant director of horticulture for the arboretum and a member of the arboretum oak and wildlife management team.

No more distress calls

To reduce the staff workload and to focus on the most effective methods to discourage nesting, this year the wildlife management team will operate with reduced hours and will not be playing recorded bird distress calls.

The annoyances will persist, at dusk and dawn, as long as the big birds continue landing in the oak trees. Based on the annual pattern seen in the nine years since the first nests were built, the team expects to have to shoo away two overlapping waves of birds — night-herons, snowy egrets and great egrets followed by cattle egrets.

The Shields Oak Grove was established at the arboretum’s west end in 1962. The big, wading birds began using the trees as a rookery — a place to nest and raise offspring — in 2000. Each year, their nests became more numerous and spread over a larger area of the grove. In 2006, there were 631 nests; in 2008, there were 866.

The Shields grove became a roost for non-nesting birds after the breeding season. While the rookery numbers steadily rose, the roost numbers fluctuated widely, for unknown reasons.

What began as an interesting wildlife spectacle became a serious threat to the oak trees’ health, largely because of the toxic qualities of the birds’ excrement, or guano, Griswold said.

Complete defoliation of a branch over several years kills the branch. Dead branches can lead to weak and dying trees — not an option for the oak grove, which is a resource for scholars and conservationists, as well as recreational visitors.

In past years, in the hope of a sustainable balance for both trees and birds, the arboretum oak and wildlife management team has tested a variety of nesting deterrents: structural pruning of trees; removing some redundant and crowding trees; removing the remnants of previous years’ nests; and tying shiny Mylar streamers and balloons to the treetops.

The rookery kept growing despite these efforts, so in 2009 a new approach was taken. The team used laser lights in combination with bird distress calls broadcast from loudspeakers, human presence, and small noisemaking fireworks.”

The strategy worked, and no nesting occurred.

As in past years, the plan is that if any birds are resolute enough to lay eggs, the deterrent program will stop, in order to prevent any chick deaths, Engilis said.

More information:

SHARE YOUR OAK STORIES

No, not the ones having to do with egrets and herons. But stories about how oaks have played meaningful roles in your life, in the arboretum or anywhere else — in California and around the world. For example, did you grow up playing in a big old oak tree in your back yard? Did you get married beneath an oak? Does your family or culture have special traditions associated with oaks?

The arboretum is creating educational exhibits and programs about the trees in the Shields Oak Grove, and would like to hear stories about how oaks have played meaningful roles in people’s lives — in California and around the world.

Please share your stories via e-mail to aboretum@ucdavis.edu by March 31. Be sure to include your full name and phone number. Emily Griswold, assistant director of horticulture, said the arboretum will use the stories in print, online and in audio or video recordings in education programs.