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

WRENDERINGS
Reports from the Field
Spring 1999

The Story of the Yellow Rail
The Tall Slender Round Softwood Structure Principle
More About the Sage Sparrows


The Story of the Yellow Rail

It isn't often any more that a species never before recorded in San Diego County shows up. But it happened again on 16 December 1998 when Dan Martin took his morning walk before dawn near the north end of Cuyamaca Street in Santee (square O12). He noticed a small bird in the middle of the street. Fearing it would be run over, he picked it up and placed it under a bush. On returning from his walk, he found it in the same place he had left it. Realizing this was not normal behavior, he picked the bird up and brought it home. He called his brother, who recommended grinding up some worms to feed it. Figuring this was perhaps a bit more than he could handle, he called Project Wildlife instead. One of the rehabilitators retrieved the bird, but it was not breathing properly and it died later that day. Two days later Meryl Faulkner received the specimen and called me. She knew the bird was not a Black Rail as initially identified and called me. From her description, I thought there was a good possibility the bird was a Yellow Rail and arranged to meet her as quickly as possible. That evening the transfer took place, in a plastic bag in front of a 7-11 store on Morena Blvd. Lo and behold, it was indeed a Yellow Rail, a first for San Diego County and only the second in southern California in over 50 years (another was picked up very similarly in Santa Barbara in 1996). I skinned it for the Museum's research collection, saving the maximum possible of the skeleton (also new to our collection) and extending one wing to show the white patch in the secondaries, much less extensive than illustrated in field guides. We never know how remarkable vagrants will come to light!

--Philip Unitt


The Tall Slender Round Softwood Structure Principle

Why do the Acorn Woodpeckers that occur in the Boulevard region of southeastern San Diego County in squares T26, T27, and U26 not occur in identical habitat in U27? Perhaps the answer lies in a corollary to Phil's "isolated thorny shrub" principle: the tall slender round softwood structure principle.

The fairly broad oak/grassland habitat along Jewell Valley Creek (incorrectly shown on maps as Boundary Creek; the real Boundary Creek drains Lake Domingo and does parallel the boundary) is found in both T26 and U27. My ranch in U27 includes approximately 1/2 mile of the Jewell Valley oak/grasslands. In nearly 30 years I have observed only a few transient stray Acorn Woodpeckers in U27. Since the beginning of the atlas project not one has shown its little red head to add to the head count in U27.

The Acorn Woodpecker has an unusual squirrel-like habit of food storage, making holes in dead wood of just the proper diameter and depth to accept an acorn. It has found that beating its brains out attempting to fabricate these holes into far east-county oaks is not productive. In an energy-balance study one would likely find that the energy consumed in the hole's excavation in hard wood exceeds the return from the stored acorn. Species survival dictates this is not a wise strategy.

There are several potential storage woods available in T26 and U27, but I suggest they may be unacceptable to Acorn Woodpeckers for the following reasons. Dead portions of cottonwoods are available, but though cottonwood is a softwood, when dead it is much harder than pine or sycamore. There are no sycamores. Sections of willow of sufficient diameter are available, but they decay quickly so are not suitable for long-term storage. Fence posts are numerous but present two negative aspects. They are not tall enough to keep the woodpecker out of four-legged predators' way, and they are not usually round. Hopping around a vertical corner of a rectangular pole is not easy. Roundness is an important criterion because by hopping around the pole an Acorn Woodpecker can quickly place the pole between itself and a perceived threat, fleeing by flying if necessary.

In the far east county it must find a substitute for the dead conifers it uses typically for granaries in the mixed conifer/oak woodlands to the west. A new free self-storage bonanza came in the form of the softwood telephone poles of the San Diego & Arizona Eastern Railroad. These were installed first in the early part of this century and later in the 1930s when the Rural Electrification Authority placed many potential storage facilities, pole by pole, leading followers into new habitats.

Thus, corollary 1 of the tall slender round softwood structure principle: in T26, T27, and U26 the Acorn Woodpecker is an introduced species. There are currently no poles near the oaks in U27, thus no Acorn Woodpeckers. The woodlands of U27 are the most easterly in this part of the county and end shortly after Boundary and Jewell Valley creeks merge, eventually to become Carrizo Creek. Only when power poles are put into the oak woodlands of U27 are Acorn Woodpeckers likely to follow into their most southeasterly range possible within San Diego County.

--Frank L. Unmack


More about the Sage Sparrow

Phil Unitt's article on the Sage and Black-throated Sparrows in the fall 1998 Wrenderings prompted me to recall some of my observations and speculations. I have studied the distribution and ecology of Bell's Sage Sparrow in San Diego County through extensive consulting work and research for my master's thesis on bird communities in fragmented coastal sage scrub. In his 1996 book J. D. Rising considered the Bell's, the dark sedentary coastal form, a species distinct from the pale migratory Sage Sparrows.

The distribution of any species is usually governed by several factors, often operating synergistically, if at different scales. In other words, although the individual factors may be evident, the resulting distributions may not make sense until the interplay of the factors is understood.

Though restricted to shrubby habitats, Bell's Sparrow, as Phil mentioned, tolerates a fairly broad range of shrublands, from coastal sage scrub to various types of chaparral. Nevertheless, the birds aren't distributed uniformly through the vast tracts of seemingly suitable habitat in central San Diego County. My observations suggest that vegetation structure may be a factor in their distribution. The sparrows prefer relatively low, dispersed shrubs. This suite of characteristics ("physiognomy" to plant ecologists) appears to favor the species' low nest placement and ground foraging. More than chaparral, "low" and "open" describe sage scrub, which, as we all know, has been extensively replaced and fragmented by human activities on the lower coastal slope. Historical records show Bell's Sparrow was once widespread on the mesas now covered by the city of San Diego, down to Ocean Beach.

Chaparral has the same physiognomy as sage scrub for several years as it recovers from a fire. I theorize that the large-scale distribution of Bell's Sparrow shifts over long periods of time in response to fire that results in low, uniform shrublands. Near the coast the opportunity for such shifting is now curtailed by the extensive loss of shrublands.

Topography may contribute to the suitability of an area for Bell's Sparrows. South-facing slopes, which often support lower, more open scrub, may be more favorable for it inland. In his 1984 Birds of San Diego County Phil suggested the birds prefer level or gently sloping topography, though they also inhabit steep slopes. From his study in burned and unburned chaparral around Pine Valley he suggests that soil type may contribute too, the birds being more numerous in chaparral stunted from growing on red magnesium-rich gabbro soil. In the short term, the distribution may be complicated by the birds' philopatry--their tendency to remain in their established territories rather than to shift suddenly to a new burn. Perhaps the distribution generated by the atlas project can be compared with records of fires maintained by the California Department of Forestry and U.S. Forest Service.

Unlike most other sedentary birds of the shrublands, Bell's Sparrow becomes more social in the nonbreeding season, often forming loose flocks. This may make the distribution more patchy in the nonbreeding than in the breeding season, when pairs maintain territories. Even during the breeding season, though, territories seem clumped, perhaps because of social interaction--yet another complicating factor.

In the research for my master's thesis, I found Bell's Sparrow to be the most sensitive to habitat fragmentation of 31 nesting species in southwestern San Diego County. The smallest fragment of habitat in which I have found the species is 160 hectares (about 400 acres). What subtle aspect of the bird's ecology requires it to seek only vast tracts of shrubs may never be known, but I venture to say that habitat fragmentation is the most important single factor in its distribution on the lower coastal slope. Of course, fragmentation must be judged by the birds. I recently found Bell's Sparrows in rather small patches of sage scrub amid a mosaic of grassland and weedy vegetation on the Fallbrook Naval Weapons Annex. Apparently the birds do not perceive the grassy areas between the scrub patches to be barriers like housing developments. I suggest that the effect of fragmentation depends heavily on the nature of the intervening vegetation.

The fragmentation hypothesis works well in explaining the now disjunct distribution of Bell's Sparrow in the coastal lowland. The bird is restricted to the last large blocks of brushland. Its prospects in this area are dubious since many of these areas are subject to further development--some in the wake of the passage of propositions K and M on last fall's ballot.

Realizing the atlas' full potential to clarify the distribution of this critical indicator species will rely on plotting the locations of the birds on field maps--Bell's Sage Sparrow is one of the "asterisked" species for which this information is requested. Noting also where locations are on discernable "islands" of shrubland will be most helpful. Even if the degree of isolation of a shrubland is not obvious from the ground, mapped locations can later be compared with the fine habitat maps available through the Natural Community Conservation Planning program. Please note the major plant species, average shrub height, percent shrub cover, and evidence of past fire. Such information will help test the habitat factors I've suggested, providing tools for planning and management.

As spring arrives, listen for the males' ethereal, tinkling song and watch for females flitting inconspicuously away from nests in the bases of low bushes. Incubating birds flushed at close range first drop to the ground, emerging from the shrubs a few feet from the nest. So to find the nest, trace the trajectory of the flushing bird backward a few feet. Limit your time at the vulnerable nest to only a few seconds. Bell's Sparrows nest at least twice in favorable years, first in March and early April, later in May.

--John Lovio

birdatlas@sdnhm.org

Redhead chick sketch by Nicole Perretta

Spring 1999 Wrenderings | Wrenderings Archive | Bird Atlas Introduction