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

Bird Atlas Trivia--Take 3

Now that the San Diego County Bird Atlas' final winter season is winding down, we can take a look at the big picture of our winter results, and entertain ourselves with some more bird atlas trivia.

How many species were recorded in San Diego County in winter in the past five years? Answer: 331 native and long-established introduced species (Domestic Pigeon, House Sparrow, and European Starling), plus 20 newly introduced species and escapees of varying status. These new exotics run the gamut from the "Wild" Turkey and Red-crowned Parrot, seemingly becoming entrenched, to the Ring-necked Pheasant and Spotted Dove, once established but now dying or died out, to the Mute Swan, Black-throated Magpie Jay, and Cardinal, just getting a toehold, to the contentious Black-backed Oriole in Smuggler's Gulch and the mysterious Darter (the Old World representative of the Anhinga) that Rich Breisch found at Barrett Lake (S18) on 2 February 2001.

Illustration of American Coot

What were the commonest and rarest winter species? Answer: If you add all individuals reported over the project's five winters, the most abundant was the American Coot, with over 192,000 reported way out in first place. Runners-up were the House Finch (over 137,000), White-crowned Sparrow (over 131,000), and Yellow-rumped Warbler (over 129,000), the only other species with over 100,000 individuals reported. Though these figures give us an interesting perspective, they can't be taken at face value. For example, if one of our participants visited a coot-infested lake regularly, reporting the same individuals time and time again through the winter, the figures could be inflated in comparison to locations where coots are less common or in comparison to terrestrial species. Nevertheless, it comes as no surprise to confirm that the American Coot is an abundant winter visitor in San Diego County.

The rarest species? There are several of which only a single individual was reported wintering over the past five years, such as the Wood Stork at the Wild Animal Park, the Tufted Duck at Famosa Slough, the Masked Booby and Magnificent Frigatebird at La Jolla, the Marbled Murrelet off the Silver Strand, the Dusky-capped Flycatcher in Greenwood Cemetery, the Gray Catbird and Grace's Warbler at Point Loma, the Prairie Warbler at Carrizo Palms, the Philadelphia and Bell's Vireos in Coronado, and the Bronzed Cowbird trapped in the Tijuana River valley. The Yellow Rail picked up in Santee deserves mention because it was not only the only one recorded during the atlas period but also the first and only one for the county. My vote for the rarest species, though, goes to the Belcher's Gull at the Tijuana River mouth in the winter of 1997-1998. Not only was this bird the first and only for San Diego County, it was the first and only for California and the first and only within a radius of thousands of miles, getting the difficult endorsement of the California Bird Records Committee as a natural vagrant.

What was the most widespread species in winter? Answer: The Common Raven, recorded in 475 of 476 covered squares. Runners up were the House Finch, recorded in 471 squares, Bewick's Wren, recorded in 463, and the Red-tailed Hawk, recorded in 460. The most interesting of these is Bewick's Wren, which proved to disperse far more widely into sparse desert scrub than realized previously. Now we see that species' status in the Imperial Valley in a much better perspective. Bewick's Wren is a regular if uncommon winter visitor in the Imperial Valley, and almost all the specimens collected there are of the dark chocolate-backed coastal subspecies charienturus, not the pale grayish inland subspecies eremophilus as one might have expected. Even though Bewick's Wrens prefer dense low brush, sparse desert scrub is no barrier to them.

Which square exceeded its target list by the biggest percentage? This prize goes to H25, Casa del Zorro, target number 44, species count 91, 107% greater than anticipated. I hadn't figured on the artificial ponds in this square, which attracted so many waterbirds and got thorough coverage on Anza-Borrego Christmas bird counts, as well as by adopting observers Herb Young and Mary Mosher.

Map of winter species diversity
  Winter Species Diversity
  Darkest: 95-188
Dark: 75-95
Medium: 61-75
Light: 40-61
Lightest: 40-61
White: No data

Which square ended with the most winter species? V10, Imperial Beach, takes the cake here with 184 species. Contributing factors to this were the importance of the salt marsh in the Tijuana estuary, the riparian woodland in the Tijuana River valley, and the mudflats and saltworks at the south end of San Diego Bay, plus intense coverage each year on the San Diego Christmas bird count. Runners up were K11, Kit Carson Park (including the upper end of Lake Hodges at high water) with 163 species, N7, Los Peñasquitos Lagoon, with 162, and J12, Wild Animal Park, with 151, the only other squares with over 150 winter species.

Which square ended with the fewest winter species? The cellar is occupied by square C26, Santa Rosa Mts. NW, which yielded only 11 species despite 20 hours of effort by John and Beverly Hargrove and Herb Stone. The area is extremely difficult, requiring a hike of about a mile and a half just to reach the southwest corner of the square. In the rain shadow of two ranges of mountains, it is extremely dry and bleak. From the ridge of the Santa Rosa Mountains near Rabbit Peak you can look down and see a few pinyon trees in the northeast corner of the square, but getting to these would entail another 5 miles of hiking from the southwest corner of the square. It's a nearly vertical drop of over 2000 feet from the area I've covered by means of the helicopter.

The square with fewest species that actually reached its threshold was M28, Fish Creek Wash W, with 16 species, including the Greater Roadrunner on the basis of its tracks alone. Seven other desert squares near the Imperial County line had only 17 to 19 species. Some of these might have had more had we covered them during the El Niño winter of 1997-1998-had I known how critical that year would be, I would have been pushing desert blockbusters every weekend!

What was the greatest number of winter species seen in one square on one day? Perhaps not surprisingly, Christmas bird counts yielded the high scorers in this category. In fact, square V10, Imperial Beach, took this prize every year. The top was 132 species in this square on 18 December 1999. Away from Imperial Beach, the "big day" winners were L7, San Elijo Lagoon, with 110 species on 22 December 2000 (during a Rancho Santa Fe Christmas bird count) and J12, Wild Animal Park, with 107 species on 2 January 1999 (during an Escondido Christmas bird count).

We can map the number of species per square, "species richness" in the jargon of ecology. In winter, the pattern that leaps out is that water makes all the difference. Natural or imported, water available to birds overwhelms more subtle factors influencing winter bird diversity.

Map of breeding species diversity
  Breeding Species Diversity
  Darkest: 78-106
Dark: 66-78
Medium: 52-66
Light: 34-52
Lightest: 14-34

If we look at the results for the breeding season, though, we see a more interesting interplay of factors. Yes, water is still all-important. But rather than lagoons, lakes, and ponds overwhelming other factors, we see strips of riparian woodland emerging as the key feature maximizing species diversity. In northwestern San Diego County, along and between the Santa Margarita and San Luis Rey rivers, we see an especially broad zone of high diversity of breeding birds, identifying the "Mesopotamia" between these rivers as San Diego County's "Fertile Crescent." Conversely, we see the effects of habitat loss and fragmentation clearly with reduced numbers of breeding species in central San Diego. Does not this pattern sound the alarm warning against the spread of urbanization up the San Luis Rey valley from Oceanside through Bonsall and Pala?

--Philip Unitt

Spring 2002 Wrenderings | Wrenderings Archive | Bird Atlas Introduction