San Diego Natural History Museum--Your Nature Connection[San Diego County Bird Atlas Project]

The quarterly newsletter for Bird Atlas volunteers
Summer 1998

In this Issue
Exploiting—and Extending—the Isolated-Thorny-Shrub Principle

Notable Observations

Reports from the Field Rockwrenderings: It's a desert out there—Or is it?
El Niño in the Anza-Borrego Desert
Caught in the Stormówaterbirds down on Inland Lakes

Focus On...
Western and Cassin's Kingbirds

News and Updates
A New Team: The Bird Atlas and the Zoological Society
Blockbuster Weekends
Wingding Things
Mark your Calendar

Exploiting—and Extending—the Isolated-Thorny-Shrub Principle

Before our San Diego County Bird Atlas effort began, few birder in the area had much experience searching for nests or observing breeding behaviors. Several have asked me how to go about doing this. In my experience, seeing the adults carrying food items or nest material, and haring the young begging for food, are the most obvious clues. But another method that often works will is what I call the isolated-thorny-shrub principle: looking for the vegetation that offers the best defense against predators an searching for nests in it. The method works best in the Anza-Borrego Desert, in open creosote bush scrub with isolated thorn shrubs or other species, such as palo verde, ocotillo, Lycium, or cacti. In square G29, Five Palms Spring, Ginger Rebstock and Karin Forney discovered and exploited this principle with outstanding results, as you can read elsewhere in this issue of Wrenderings.

The dominant plant species over much of the Anza-Borrego Desert, creosote bush and burro bush, with their open, slender branches or low habit, don't' offer sites that can protect nests from the prying eyes and hungry mouths of predators. In areas where denser, thornier shrubs, occur only scatteringly, the resident birds, even though very sparse, seek out these thorny shrubs for their nests. On 4 April, Jack Schlotte and I drove through some of the bleaker regions of the Anza-Borrego Desert, to hit some atlas squares that remain unadopted. Along Palo Verde Wash in square G28, just south of the border of the Ocotillo Wells ORV area, we saw an isolated shrubby palo verde that looked like a prime candidate for application of the principle. As we walked up to it, a LeConte's Thrasher—our only one for the day—slipped silently out the back and scurried away. A glance in the shrub revealed the nest, and tolerating some pain from the thorns got us close enough to see the four brilliant blue eggs. As we toured down Borrego Sink Wash in squares G26 and G27 during our Anza-Borrego blockbuster weekend on 18 April, Franc Unmack and I used the principle to good advantage, yielding us three nests of the Loggerhead Shrike, two of the Phainopepla, and a fledgling Mockingbird.

But the isolated-thorny-shrub principle needn't be confined to open desert. In almost any habitat, there will be some feature that offers better protection against predators than others, and it is a good bet that birds will recognize this quality too. While looking (unsuccessfully) for Clapper rails in the Paradise Creek marsh in National City in March, Ingri Quon, Pete Famolaro, and I repeatedly noticed a pair of Loggerhead Shrikes. There were several Myoporum trees and lots of chaparral broom and other shrubs that could conceal a nest. But a couple of Lycium bushes offered the added appeal of exceptionally rigid sharp-tipped stems. And sure enough, that was where the shrikes had put their nest, containing four brown-spotted beige eggs. For this extra attractiveness, a plant need not be as thorny as a cactus, just thornier than the other plants nearby.

The isolated-thorny-shrub principle can be applied even more broadly. In grassland, check out any isolated shrub capable of concealing a nest. In chaparral, look for isolated oak trees. In woodlands, look for dead trees that may offer more natural cavities or be easier for woodpeckers to excavate. I encountered an outstanding example of the last situation this spring West Sycamore Canyon on Marine Corps Air Station Miramar, where Tammy Conkle and I are making pint counts of birds to complement a banding station. The canyon floor is lightly wooded with live oaks and sycamores, themselves both very attractive to many nesting birds. But it also has two huge dead eucalyptus trees, side by side, with the bark sloughing off in great sheets. The trees have the nest of the canyon's pair of Red-tailed hawks. Every time the hawks leave their nest, they are harassed by a pair of Cassin's Kingbirds, themselves building a nest on a ledge of detached bark. Farther down the tree, at least one pair of House Finches has found a nest site under a slab of bark. And near the ground House Wrens have found a suitable cavity. Meanwhile, two pairs of Nuttall's Woodpeckers contest ownership of what I have nicknamed the "apartment house tree."

So during the breeding season, try thinking like a bird and target the most secure convenient nest sites.

--Philip Unitt

Wrenderings Archive | Bird Atlas Introduction