Environment : Notes about your surroundings.

Bye-Bye Birdies--Theories abounded among local birders earlier this season to explain the exceptionally low numbers of migrating birds passing through the county. Had the escalating destruction of wintering grounds in the tropics taken a sudden and devastating toll?

But a late rush of migrating land birds started early this month and reached a big peak a couple of weeks ago. On one day in Huntington Central Park, for instance, the avian display included a count of 125 yellow warblers and 100 to 150 Swainson’s thrushes. “It was really impressive,” said Doug Willick, who keeps track of rare local sightings for American Bird magazine. “That was encouraging after a very disappointing April.”

Along with the usual Pacific Flyway travelers have come a number of vagrants that strayed from other migratory paths. A number of rare fall vagrants that are very rare as spring vagrants in the county have been spotted in recent weeks. Some highlights:

Tennessee warbler (three different individuals), May 17, 20 and 24.

* Indigo bunting (two individuals), May 4 to 6 and 18.

* Northern waterthrush, May 3.

* Black and white warbler, May 29.

* Gray flycatcher, April 15.

* Summer tanager, May 20.

* Franklin’s gull, May 14.

On May 20, a spotted redshank was seen just south of Orange County on Camp Pendleton. The Asiatic shore bird, a rare vagrant in the United States outside of the Aleutian Islands, has never been seen before in Orange County.

Making several appearances in recent weeks on the San Joaquin Freshwater Marsh Reserve in Irvine has been an American bittern, a member of the heron family. “They didn’t use to be rare, but numbers in Southern California have really dwindled,” Willick said. At one time, the birds bred in Orange County.

While the passage of spring migrants through the county is now waning, the number of rare and unusual vagrants should pick up for the next several weeks. These birds are probably straggling, Willick said, because they are lost far from their usual migratory paths. The vagrants provide an added challenge to avid birders and can make for additions to life lists (a birder’s list of birds seen).

Quail Hill Update--The city of Irvine has already announced plans to preserve Quail Hill, the spot where thousands of migrating Canada geese spend the winter each year. Now it’s time to decide what will happen to the land behind Quail Hill.

On both the city and county plans it is part of William R. Mason Regional Park. It has not been developed, however, and is now open space and strawberry fields. The land is leased from the Irvine Co., and if the county wants to keep that arrangement it has to show improvements on the property by 1991.

The county pitched a plan to build a golf course and hotel on the property but met a less-than-enthusiastic response from the City Council. Some Irvine officials, meanwhile, would like to restore the area to something approaching a natural state, with native plants and trees and winding trails.

Plans to that effect are being drawn up by a city administrative aide.

New Checklist--UC Irvine’s Museum of Systematic Biology has published a new “Checklist of the Vascular Plants of Orange County, California,” the first time the list has been updated since 1968. Fred Roberts of the museum compiled the work, which will serve as the framework for the first-ever complete flora for the county, due to be published sometime after 1990.

The checklist includes all native, introduced, naturalized or escaped species of vascular plants that have become established or persist as part of the county’s flora. As presently known, the flora of Orange County includes 1,157 species of vascular plants with an additional 99 subspecies or variations, for a total of 1,256 taxa. Of the total, 806 are native to the county.

Copies of the checklist are available at several locations, including the Museum of Natural History and Science in Newport Beach.