Trends in the California Bird List:
Jehl (1980) Revisited

Twenty-seven years ago, Joseph R. Jehl (1980) analyzed trends in the growth rate of California’s bird list during the decades that followed the 1944 publication of Joseph Grinnell and Alden H. Miller’s ornithological benchmark, The Distribution of the Birds of California. He also defined four “source regions” for birds reaching California and compared their contributions to the state list by decade. This essay updates Jehl’s analyses and assesses more recent trends in the growth of the California Bird List.

From 1900 to 1959, the state list increased at a fairly steady average of 1.2 species per year (Figure 1), with an obvious lull that corresponded to American involvement in World War II (1941–1945) and a less clear-cut dry spell during and immediately after World War I (1917–1919; Table E). As Jehl noted, and as shown in Figure 1, this essentially unwavering rate increased to approximately 4 species per year during the 1960s and 1970s. According to the accounting of state firsts presented in Table E, the list grew by 4.7 species per year in the 1960s, 3.7 species per year in the 1970s, and 3.6 species per year in the 1980s. The essay following this one, “Birding in California, 1960–2007,” describes and analyzes this era of rapid growth in some detail. Between 1990 and 2006, the rate of growth dropped off to 2.4 species per year. This rate cannot be expected to rebound to earlier levels for any sustained period, and its continued decline must be considered likely.

Figure 1. Additions of native species to the California Bird List averaged 1.15 per year from 1900 to 1959; 4.28 per year from 1960 to 1984; and 2.36 per year from 1985 to 2006.

Here, we examine temporal variation in the contributions of the four “source regions” to the state list (see Table F and Figure 2), follow with a discussion of changes in our perception of rarity, and conclude by evaluating each region’s future prospects for generating additional California firsts. Note that our classification follows Jehl (1980) in considering the Blue-footed Booby and Brown Booby “Mexican” rather than “pelagic” species; for consistency, the Sandwich Tern is also considered to be “Mexican,” but species that breed well offshore, such as the Masked Booby, Great Frigatebird, and Bridled Tern, are retained as “pelagic” species. Note also that Alaskan-breeding alcids are considered to be Asiatic/Arctic.

Figure 2. Relative Contributions of Source Regions by Decade, 1900–2006 (Click on the image to see a larger version.)

Between 1900 and 1978, only a handful of pelagic species were added to California’s list. This minimal contribution later changed considerably, and ocean-going birds account for an impressive one-third (29%) of species added to the list between 1990 and 2006. To an extent, the northeastern Pacific Ocean represents the state’s only remaining “frontier” (see Shearwater 2004). Two species added since the late 1970s—Murphy’s and Cook’s Petrels—have proven to be of regular occurrence more than 40 nautical miles offshore, and others may be similarly regular far from land (for example, the Red-tailed Tropicbird in subtropical waters well west of the Channel Islands).

For decades, species that breed primarily in the eastern half of North America (including some with ranges that extend westward across Canada to Alaska) served as a steady source of novel species in California. From 1900 to 1969 almost half (46%) of all additions were from this source region. The well then began to run dry, as the share of new records originating from this region dropped to 30% during the 1970s, then to 11% during the 1980s, and finally to 8% between 1990 and 2006.

Mexico, the Southwest, and tropical America contributed fully one-third of California’s new species between 1910 and 1979, and this region could be counted on to provide 25–45% of additions during any given decade of this protracted period. Since 1980, however, the region’s relative contribution has fallen to less than one-fifth (18%), suggesting possible depletion of this long-productive pool of potential vagrants.

The contribution of species from the Asiatic/Arctic source region has been more volatile. Before 1970, less than a fifth (17%) of additions were of species whose normal range is in Siberia, Alaska, or arctic Canada, but since 1970 this region has accounted for more than a third (39%) of California’s new birds. This group accounted for most of the species added to the state list during the 1980s, including a unique convergence of records in northern Monterey County during the fall of 1988. Between 28 August and 9 October, three Siberian species were found within about 12 miles of each other. That event seemed rare enough at the time, but nearly two decades later the Terek Sandpiper, Long-toed Stint, and Gray Wagtail remain otherwise unrecorded in California. In fall 2006 two new species were documented, Ross’s Gull and Taiga Flycatcher, additions that further strengthened the Asiatic/Arctic dominance of the last few decades.

The notion of what birds are “rare” is in constant flux, and not all of these 228 additions to the state list are regarded as rarities today. By examining Table E we can trace our collective learning curve and perhaps update our perspective on rarity.

Among those species added to the state list between 1900 and 1959, most (39 of 69; 57%) are now considered regular. Such routinely encountered species as the Flesh-footed Shearwater, Northern Parula, and Lapland Longspur almost certainly were overlooked in earlier years. Included in this group are the Gull-billed Tern and Inca Dove, whose first representatives recorded in California were among the vanguards of successful colonization. Among the 30 species that remain rare enough in the state to warrant continued CBRC review, only nine have been recorded fewer than 10 times, and only one, the Black-tailed Gull, has not been recorded again. Thus, a robust majority of species added to the state list between 1900 and 1959 are now known to be regularly occurring migrants or winter visitors, recent colonizers, or only marginally rare vagrants.

Knowledge of the status and distribution of birds in California took a giant leap during the 1960s. The state list grew by an astounding 48 species, and it is no less remarkable that, only four decades later, 17 of these additions no longer merit CBRC review. The 1960s saw the inaugural records of such regular migrants as the Broad-winged Hawk, Semipalmated Sandpiper, and Blackpoll Warbler, as well as first contact with four arriving colonists—the Black Skimmer, Little Blue Heron, Cattle Egret, and Great-tailed Grackle—species that are regular, if not common, today. Among the remaining 31 additions are many of the state’s more regular vagrants, but eight of these remain “top drawer” rarities recorded fewer than 10 times. The Wandering Albatross, White-tailed Tropicbird, and Kittlitz’s Murrelet have yet to reappear.

Since 1970, with the likeliest candidates for vagrancy to California largely out of the way, the 111 additions to the state list generally involve species that we still consider truly rare. Thirty-five species are represented by a single state record, 42 have been recorded fewer than 10 times, and most of the remaining 23 species average well under one record per year since their discovery. Committee review has been terminated for only four species added since 1970: the colonizing Barred Owl and Ruddy Ground-Dove, as well as Cook’s and Murphy’s Petrels, seabirds whose ranges were worked out only after deep-water investigations intensified during the 1980s. The Mottled Petrel, which has averaged nearly two per year since 1976, and the Galapagos/Hawaiian Petrel, which averaged more than one per year between 1992 and 2006, are among deep-water species whose ranges we still do not know in detail. The Manx Shearwater was first documented only in 1993, but by the end of 2006 the accepted records had surged to at least 94, a status change that might reflect recent colonization of the Pacific basin. Most recently, we have seen the Slaty-backed Gull vault from hypothetical occurrence in 2001 to a species with 16 accepted records at the end of 2006. Could other species be slipping under our noses in similar fashion?

As part of his review, Jehl (1980) asked Laurence C. Binford, Jon L. Dunn, Paul E. Lehman, Guy McCaskie, and Rich Stallcup each to predict ten birds that would next join the California list, a process that yielded a total of 26 candidate species. Since 1980, 66 new species have been accepted to the state list, 19 of which had been named by the expert panel (including Cook’s Petrel, added in late 1979). Two of the top six aggregate choices, the Wood Sandpiper (Tringa glareola) and Swainson’s Warbler (Limnothlypis swainsonii), remain unverified in the state (although as this book goes to press the CBRC is reviewing the state’s first well-documented Wood Sandpiper record, of a bird photographed 22–23 May 2007 at China Lake, northeastern Kern County). The following paragraphs look forward to what the future might bring.

The most obvious seabird candidates for addition to the state list include the Juan Fernandez Petrel (Pterodroma externa), Solander’s Petrel (Pterodroma solandri), Townsend’s Shearwater (Puffinus auricularis), Markham’s Storm-Petrel (Oceanodroma markhami), and Nazca Booby—birds of the Pacific Ocean that have been recorded near California. But recent years have taught us to expect the unexpected, with remarkable additions of such species as the Light-mantled Albatross, Great-winged Petrel, Bulwer’s Petrel, Parkinson’s Petrel, Cory’s Shearwater, and Ringed Storm-Petrel. Undoubtedly, the pool of pelagic species that might eventually reach California is both mysterious and deep.

In the wake of several decades during which eastern North America stood as the most productive source region for vagrant bird species in California, only a handful of likely candidates remain unrecorded in the state. Among these are the Swallow-tailed Kite (Elanoides forficatus), Great Black-backed Gull (Larus marinus), Acadian Flycatcher (Empidonax virescens), Carolina Wren (Thryothorus ludovicianus), Eastern Bluebird (Sialia sialis), Swainson’s Warbler, Eastern Towhee (Pipilo erythrophthalmus), and Henslow’s Sparrow (Ammodramus henslowii). Thus the trend for this source region can be expected to continue downward.

A fairly high number of as-yet unrecorded Mexican, Southwestern, and tropical species seem to have good potential to reach California. Occurrence in Arizona may serve as a useful predictor of this potential, but note that California hosted the United States’ first Streak-backed Oriole, and has single records of Nutting’s Flycatcher (Arizona has two), Couch’s Kingbird (Arizona has none), and the Golden-cheeked Warbler (Arizona has none). Mexican and Southwestern species known from Arizona but not California (as accepted vagrants) include the Northern Jacana (Jacana spinosa), White-eared Hummingbird (Hylocharis leucotis), Berylline Hummingbird (Amazilia beryllina), Plain-capped Starthroat (Heliomaster constantii), Tufted Flycatcher (Mitrephanes phaeocercus), Aztec Thrush (Ridgwayia pinicola), Crescent-chested Warbler (Parula superciliosa), Tropical Parula (Parula pitiayumi), Slate-throated Redstart (Myioborus miniatus), Flame-colored Tanager (Piranga bidentata), and Yellow Grosbeak (Pheucticus chrysopeplus). California’s first Magnificent Hummingbirds, recorded in 2003 and 2004, bring to mind other partly migratory species that breed in Arizona, such as the Olive Warbler (Peucedramus taeniatus) and Botteri’s Sparrow (Aimophila botterii). Any of these species, but especially those occurring in lowland subtropical thorn forest (e.g., the starthroat and grosbeak), are candidates to cross the Sonoran Desert into California. The 1925 specimen of a Fan-tailed Warbler (Euthlypis lachrymosa) from northwestern Baja California (Grinnell 1928) suggests what is possible. The Colorado River Valley could prove to be fertile ground for state firsts from the Southwest, including species from Arizona that are generally regarded as sedentary. Considering that Canyon Towhees (Pipilo fuscus) reside within several miles of the river, and Eastern Meadowlarks (Sturnella magna) within several dozen miles (Corman and Wise-Gervais 2005), might better winter coverage along the border yield a stray on this side of it? Does the remarkable record of a Bridled Titmouse (Baeolophus wollweberi) within a couple miles of the state line at the Bill Williams Delta (Rosenberg et al. 1991) hint at a more extensive range of Colorado River possibilities?

Viable candidates from Asia and the arctic are in ample supply. Species recorded on the Pacific slope between Oregon and Alaska include the Bean Goose (Anser fabalis), Eurasian Kestrel (Falco tinnunculus), Eurasian Hobby (Falco subbuteo), Far Eastern Curlew (Numenius madagascariensis), Great Knot (Calidris tenuirostris), Temminck’s Stint (Calidris temminckii), Dusky Thrush (Turdus naumanni), Redwing (Turdus iliacus), and Siberian Accentor (Prunella montanella). One highly migratory passerine, the Bluethroat (Luscinia svecica), breeds in Alaska but has yet to be found farther south along the Pacific coast. Perusal of a list of Asiatic migrants recorded in Alaska greatly increases the source pool of potential vagrants (see Gibson and Kessel 1992, Tobish 2000). With such species as the Greater Sand-Plover, Ross’s Gull, Ivory Gull, Brown Shrike, Lanceolated Warbler, Taiga Flycatcher, Red-flanked Bluetail, Stonechat, Eyebrowed Thrush, Gray Wagtail, and Little and Rustic Buntings already safely under California’s belt, it would seem downright timid to doubt the potential for a Common Ringed Plover (Charadrius hiaticula), Black-tailed Godwit (Limosa limosa), Fork-tailed Swift (Apus pacificus), Common House-Martin (Delichon urbicum), Yellow-browed Warbler (Phylloscopus inornatus), Siberian Rubythroat (Luscinia calliope), or Pechora Pipit (Anthus gustavi) to reach the Golden State. Time will tell.