Vol. 29 No. 4 (1998)

Interest in the coastal populations of the Black-tailed Gnatcatcher (Polioptila melanura) first diffused outside of the ornithological community in 1978. Notable for its odd vocalizations, localized distribution, and co-occurrence with a vegetation type then known as "inland sage scrub" (Thorne 1976), the coastal subspecies of the Black-tailed Gnatcatcher (P. m. californica) was judged by environmental planners to be sensitive because of its small range and vulnerability to habitat conversion (M. U. Evans, pers. comm.).

To conserve and manage endangered and threatened species effectively requires an understanding of their overall distribution as well as a more detailed understanding of individuals’ use of space. Nonmigratory birds' use of an area in the breeding season may differ from their use of it in the nonbreeding season.

Some passerine songs and calls have fairly specific functions (Kroodsma 1981, Catchpole 1982, Merilä and Sorjonen 1994). Other songs or calls may have more than one function, with the meaning varying with the context in which the song or call is given and received (Lein 1981, Catchpole 1982, Howes-Jones 1985, Nelson and Croner 1991, Slagsvold et al. 1994).

Efforts to conserve the California Gnatcatcher (Polioptila californica) have generally focused on maintaining large, viable tracts of coastal sage scrub habitat through regional land-use planning, rather than on delineation and protection of specific areas used by individual pairs (Atwood and Noss 1994, Reid and Murphy 1995).

Despite much recent interest in the systematics and demography of the California Gnatcatcher (Polioptila californica) (Atwood 1988, 1991; Phillips 1991; Mellink and Rea 1994), the molts and plumages of this species have not been described in detail, though treated briefly by Swarth (1902), Ridgway (1904), Woods (1949), and Dunn and Garrett (1987).

"Reconstructing past selective pressures can be a daunting task, as data are often sparse, unreliable, or inconsistently gathered. Nevertheless, museum collections provide an impressive but all-too-often untapped resource. Data housed in such collections provide an important window to the past and allow us to identify and explore changes in the ecology of a species or a community (Reznick et al. 1994, Smith et al. 1995, Schaffer et al. 1998)."

We intensively studied various aspects of habitat use, breeding biology, home-range requirements, and dispersal patterns of a population of the California Gnatcatcher (Polioptila californica) in southwestern San Diego County from 1989 to 1992. When this field effort was initiated, few quantitative demographic studies of the species were available to aid in making informed decisions regarding the conservation and management of this species (Atwood 1993).

The California Gnatcatcher (Polioptila californica) is a persistent nester, attempting up to seven nests after repeated failures (Bontrager 1991). I collected data on the dispersal and breeding biology of the gnatcatcher, particularly relating to nesting success and nest location, in Orange County, California.

"Bird populations and distributions are known to be influenced by environmental factors (e.g., Graber and Graber 1979; Cawthorne and Marchant 1980; Root 1988a, b, c; Arcese et al. 1992; Mehlman 1997). Nevertheless, 'whether and how most animal populations are regulated remains one of the principal unanswered questions in ecology' (Rodenhouse et al. 1997). Another major gap in our understanding concerns the scale over which species' populations fluctuate (Holmes et al. 1986)

The potential effects of fragmentation of natural communities by urban development are dramatically evident on the Palos Verdes Peninsula of Los Angeles County, California. Because of the long history of agricultural and, more recently, intensive urban development of the Los Angeles basin, tracts of natural scrub habitat on the peninsula have become increasingly isolated and smaller since the mid-1900s (Fink 1966, Gales 1974).

Dispersal is the means by which genetic and demographic exchange between subpopulations maintains the viability of the regional metapopulation (Fahrig and Merriam 1985; Lacy 1987; Merriam 1991). Our paper describes potential limitations of existing data on dispersal of juvenile California Gnatcatchers (Polioptila californica) and provides a landscape analysis of distribution data from areas of fragmented habitat

Historically, the California Gnatcatcher (Polioptila californica) ranged north to southwestern San Bernardino County, where it has now been nearly eliminated by urbanization.

Californian coastal sage scrub (CSS), which consists of dense stands of soft-leaved, drought-deciduous subshrubs 0.5–1.5 meters tall, has been extensively cleared for agriculture and urbanization (Westman 1981).

The California Gnatcatcher is widely known to be strongly associated with coastal sage scrub, yet it does not occupy all stands of this highly variable vegetation type (Kirkpatrick and Hutchinson 1976; Westman 1983; O'Leary 1990; Atwood 1993).

Because the California Gnatcatcher (Polioptila californica) is one of the focal species for conservation of coastal sage scrub being planned under the state of California's Natural Community Conservation Planning (NCCP) program (Atwood and Noss 1994; Reid and Murphy 1995), the species' response to large-scale loss of habitat is an important factor that may influence both the design of habitat reserves and the development of future habitat management strategies.

Year-to-year variations in temperature, precipitation, and food supply affect the survival and reproduction of organisms. Annual fluctuations in birth rate and survival are often expressions of this environmental variability.

That California Gnatcatchers (Polioptila californica) within the United States are dependent on coastal sage scrub habitats is well established (Atwood 1993; USFWS 1991, 1993). Our effort in this paper is not to question whether these habitats are necessary on a large scale, but rather to raise the question of whether they are always sufficient, at both local and larger scales.

Creation of coastal sage scrub has become an important component of long-term management for the California Gnatcatcher (Polioptila californica) and other sensitive species in southern California. We describe successful gnatcatcher nesting in four-year-old restored coastal sage scrub in coastal Orange County, California, in 1995; the birds continued to nest there from 1996 to 1998. Our intent is to demonstrate that restored coastal sage scrub has been used by gnatcatchers, not to recommend specific restoration procedures.

Public lands are increasingly being relied upon to provide the natural open space necessary to conserve California's rich natural heritage. Simple inclusion of land in parks is often assumed to provide the habitat and protective mechanisms required to ensure the long-term viability of natural populations.

Being aware of sage scrub and California Gnatcatchers (Polioptila californica) along its highways, the California Department of Transportation (Caltrans), District 11, contracted with Regional Environmental Consultants (RECON) to identify potential habitat patches and survey for the gnatcatcher in these areas.

Conservation of biodiversity is best attained by establishing habitat reserves that are designed for multiple species and sustain entire ecosystems (Noss 1983, Hunter et al. 1988, Scott et al. 1988, Brussard 1991). Most existing habitat reserves have been established primarily on the basis of the ecological requirements of only one or a few species.

In southern California, the California Gnatcatcher (Polioptila californica) is a year-round resident in coastal sage scrub (Atwood 1993). This habitat—reduced to 10 to 30% of its former extent due to conversion to human use—supports approximately 100 other animal and plant species considered rare, sensitive, threatened, or endangered by California or federal wildlife agencies (Atwood 1993, McCaull 1994).

There are numerous examples of interspecific feeding in birds (Skutch 1961, 1987; Shy 1982; Welty and Baptista 1988:362). Skutch (1960) described a female Tropical Gnatcatcher (Polioptila plumbea) in Costa Rica that abandoned helping her mate build a nest in order to tend to a brood of Golden-hooded Tanagers (Tangara larvata) in a nearby nest. However, there are no published accounts of this kind of behavior in the California Gnatcatcher (P. californica), except in cases involving the brood-parasitic Brown-headed Cowbird (Molothrus ater) (e.g., Braden et al. 1997). This note documents one such event.

Within its current range in the United States, the California Gnatcatcher (Polioptila californica) frequents low scrub on the coastal side of the Peninsular and Transverse Ranges in southwestern California. The closely related Black-tailed Gnatcatcher (Polioptila melanura) generally inhabits the Colorado and Mojave Deserts within its range in southern California. Atwood (1988) discovered that both species coexist in limited areas of northeastern Baja California. This note reports a new site of sympatry (co-occurrence) of the two species on the U.S. side of the border.

Two aspects of watching birds are that they move, and that their appearances can be highly variable. Even the best field guides—whether using paintings or photographs—are limited by having to represent everything through a small number of two-dimensional, static images. Given these inherent restrictions, books can still do a very good job—especially if one takes the time to read the text!

Nonetheless, the potential of video footage to show moving birds and to portray more plumage variations than even a specialized field guide suggests that the medium will become increasingly common in the field of bird identification.

This new edition of Colin Harrison’s classic work, Nests, Eggs, and Nestlings of North American Birds (hereafter Nests), originally published in 1978 but out of print for several years, is a window into the world of the nest. Paul Baicich has helped revise the guide, which covers all birds breeding north of the U.S.–Mexican border.

Probably nowhere else in the world do hybrids constitute such a large proportion of the total gull population as along the west coast of North America. Consequently, birders in this area have little choice but to familiarize themselves with the appearance and variability of hybrid gulls if they are to seriously attempt the identification of scarce and vagrant gulls.

Compiled by Jack W. Schlotter and Philip Unitt