Page 930 - trc_centennial_deir201705

5.7 Biological Resources

summering on the Project site in July 2008. This group consisted of five immatures foraging
for grasshoppers in the Oso Canyon grasslands just west of Cement Plant Road. Project
implementation would result in the loss of habitats used by migrating Swainson’s hawks,
occasional non-breeding birds in summer, and potentially suitable nesting habitats in the
eastern portions (i.e., alfalfa fields and surrounding trees) of the Project site. The primary
threat to this species is the loss of breeding habitats; therefore, these impacts are considered
adverse but less than significant and no mitigation is required. As with other migratory birds,
however, this species may occur as a breeder in the future and MM 7-2 is included to conduct
pre-construction protocol surveys for the Swainson’s hawk to confirm absence prior to
disturbance.

The Project site is outside the known range and does not provide suitable habitat for the
coastal California gnatcatcher. Therefore, Project implementation is not expected to impact
this listed species and no mitigation would be required.

The tricolored blackbird is currently a Candidate State-listed species that receives the same
legal protection afforded to an endangered or threatened species. Its status was only recently
elevated, so protocol surveys were conducted for the first time in 2015 (BonTerra Psomas
2015c). The results of this focused survey documented relatively heavy use of the Project
site for foraging purposes by tricolored blackbirds nesting off site at Quail Lake, and to a
much lesser degree, Holiday Lake. In an effort to determine the Project’s potential effect on
tricolored blackbird nesting colony foraging lands of the region, an assessment of each of
four known colonies was conducted (BonTerra Psomas 2015c). Breeding colonies are known
to forage as far as five miles away from the nesting location. Therefore, the suitability of
foraging was assessed for all lands within a five-mile radius of each colonies nesting location.
Based on vegetation types, potentially suitable foraging habitat occurs on nearly 100,000
acres in the Project region. The Project would result in the loss of approximately 8 percent
of this nesting colony foraging habitat. Although this loss is considerable, the effect on
individual colonies may be more informative in determining overall effects on the regional
nesting population.

Due to the shifting of tricolored blackbird nesting locations from year to year, a subset of two
or three of the available nesting locations may represent the entire regional breeding
population. A substantial reduction of foraging lands surrounding this subset of nesting
locations could result in considerable reduction of nesting success. A review of the Quail
Lake and the on-site colony subset identified a potential post-Project reduction of
approximately 20 percent of potentially suitable foraging habitat at each location. In a given
year, if these two areas were used for nesting, the reduced availability of suitable foraging
land could result in substantial reduction in nesting success.

The indirect impacts described above may be considered potentially significant pursuant to
CEQA; and mitigation may be required to reduce the impact to a less than significant level.
Potentially suitable areas for enhancement and preservation include the pond area along the
northern boundary and Oso Canyon, as well as any other created water bodies as part of the
Project Drainage Plan, where feasible. Tricolored blackbirds were observed foraging heavily
within the Mitigation Preserve in Oso Canyon and, with some hydrology and habitat
enhancement in this area, it may be suitable to support a breeding colony in the future. An

R:\Projects\PAS\CEN\000306\Draft EIR\5.7 Bio_051117.docx 5.7-149 Centennial Project
Draft EIR