Date: 5/6/26 11:16 am From: Samuel Bressler via groups.io <pacificgoldenplover...> Subject: [OrangeCountyBirding] California Bird Atlas Survey of Chino Hills State Park
Hi all:
On Sunday, May 3rd, approximately 10 Southern California birders conducted a semiformal survey of 8 different atlas blocks in and adjacent to Chino Hills State Park (six blocks in Orange County, two blocks in San Bernardino County. The goal of the survey was to increase coverage for the atlas, log some new breeding species, and provide a snapshot of bird diversity for the Chino hills in early May. In that we were successful. A survey summary can be seen below, presented as before-after comparisons of effort hours (confirmed species) for each block
Yorba Linda CW (Olinda Oil Trail, Olinda Ridge)
42.83(21) - 47.02(22)
Yorba Linda CE (Gilman Loop, Tank Rd, Olinda Village)
2.77(1) - 10.37(23)
Prado Dam CW (Bane Canyon)
25.03(7) - 29.28(7)
Yorba Linda SE (Telegraph Canyon)
17.32(12) - 23.73(15)
Prado Dam SW (San Juan Hill)
0.63(0) - 1.62(3)
Prado Dam SE (Lower Aliso Canyon, Green River Golf Club)
0.00(0) - 3.00(9)
Black Star Canyon NW (Lower Coal Canyon, Santa Ana River)
11.63(3) - 13.97(4)
Black Star Canyon NE (Upper Coal Canyon)
1.10(1) - 3.53(2)
In total we totaled over 95 bird species, including almost all of the expected breeders and most expected migrants, which is very solid. Some notable species include:
*Bell's Sparrow* : Black Star Canyon NE, an expected species in Coal Canyon
*Black-chinned Sparrow* : both Black Star Canyon blocks, as well as one in Prado Dam CW. The latter record was of a male-female pair, and suggests the possibility of breeding, which to my knowledge has never been confirmed in Chino Hills proper.
*Grasshopper Sparrow:* Prado Dam CW, two countersinging males. Several singing birds have been reported in the San Bernardino portion of the park this spring, but unfortunately we weren't able to turn up any in the OC section (someday).
*Lawrence's Goldfinch:* Yorba Linda CW. Pair in suitable habitat (the only blooming fiddleneck along the entire trail).
We also found some cool vagrants:
*Green-tailed Towhee:* Yorba Linda CE
*White-winged Dove:* Yorba Linda CE
*Yellow-headed Blackbird:* Prado Dam SE
The most glaring miss was *Lark Sparrow*. The Orange County breeding bird atlas (from data collected in the late '80s) confirmed nesting of this species in at least two atlas blocks within the state park. We were not able to find any whatsoever. I don't know why this species has seemingly declined in the park, but it does track well with their absence in comparable tracts of otherwise suitable habitat (such as Limestone).
By contrast, *Rufous-crowned Sparrow* , which in the previous atlas was only listed as "probable" in one block and "possible" in a second block (outside Coal Canyon), was reported by us in four different blocks (outside Coal Canyon), including a Pair in Suitable Habitat in Yorba Linda CE. I have no idea why these birds would seemingly be doing so much better in the region compared to forty years ago, except that perhaps the habitat has shifted away from oak savannah towards chaparral and coastal sage scrub?
The presence of only one *Oak Titmouse* could potentially be explained because eastern Telegraph Canyon (in Prado Dam CW) was not surveyed. But I still would have expected more in Western Telegraph Canyon.
Also notable to me was the relative dearth of *Bullock's Orioles.* Given that most of us were surveying at least partially in oak-riparian woodlands (Telegraph Canyon, Bane Canyon, etc.), and generally away from the wildland urban interface, it seems notable to me that Hooded Orioles outnumbered Bullock's 31-10. Possibly something to keep an eye on in future years.
To close on a brighter note, the tally of over two dozen Bell's Vireos is wild in the best way and supports that ongoing conservation of this endangered species continues to be successful.
Anyway, that's enough for now, here's ( https://ebird.org/tripreport/509112 ) the trip report if you haven't already seen it. Reminder that trip report tallies will be skewed by double coverage and out/return trips, but I found it to be highly interesting regardless. I'll try to arrange something like this again next year, hopefully with a bit more organization and manpower. In the meantime, there are still a few blocks in OC with no atlas coverage, and many with low coverage, so I encourage everyone to get out there and find some nesting birds!
