ou'd think gnatcatchers might have caught on to Robb Hamilton by now. After all, since 1999 the biologist has routinely ventured into their habitats across the reserve, tape recorder at the ready. He pushes play, and pre-recorded calls of the birds float into the air above the coastal sage scrub where they live. And sure enough, thinking their territory is being invaded, gnatcatchers - which stand all of four inches tall - begin popping up and calling back. This unusual game of peek-a-boo enables Robb - one of Orange County's top ornithologists - to maintain a detailed year-to-year accounting of the birds' population.

The same ploy works with the cactus wren, a bird twice the size of the gnatcatcher that lives in coastal prickly pear. Well, it works most of the time. "Both birds can be a little tricky," says Robb, an independent biological consultant who wrote the book, "The Birds of Orange County," with fellow ornithologist Doug Willick. "Some days they're so obvious - they're hopping around, screaming in your face - and other days, they can be very quiet." That's why Robb conducts two or three counts a year at each of the 40 gnatcatcher/wren monitoring sites on the reserve. Each site is at least 50 acres large; some are more than double that size. And while most sites support one or both species, others have none or are only occupied during certain years.

A time-consuming undertaking, to be sure. But Robb's ongoing research produces up-to-date data on the status of these birds and the habitat that supports them. Monitoring the gnatcatcher is especially important, he says, because it is listed as threatened under the federal Endangered Species Act. The cactus wren has no current federal status, but is considered a Bird Species of Special Concern by the state of California.

Fluctuations can and do occur in the birds' populations from year to year. Gnatcatchers, in particular, respond dramatically to changing weather conditions. Low rainfall in a season, for example, equals fewer insects, which equals less food for gnatcatchers, which equals fewer eggs laid. Heavy rains, meanwhile, can devastate the tiny, fragile birds as well, Robb explains.

Overall, Robb says, the reserve's populations of both birds are respectable and stable. Their status is considered one measure of the well-being of the reserve's ecosystems. So come March 2003, Robb will return to the reserve, tape recorder in hand, to see how many of his feathered friends are willing to stand up and be counted.