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Least Bell's Vireo Breeds in Restored Riparian at San Joaquin River National Wildlife Refuge

Least Bell’s Vireo Breeds in Restored Riparian
at San Joaquin River National Wildlife Refuge
2005 Final Report
Submitted to:
U.S. Fish and Wildlife Service
San Luis National Wildlife Refuge Complex
June 2006
Submitted by:
Julian K. Wood, Christine A. Howell, Geoffrey R. Geupel,,
PRBO Conservation Science
3820 Cypress Dr.
Petaluma, CA 94954
(707) 781-2555
PRBO Contribution # 1511
The historic nesting of Least Bell’s Vireos (Vireo bellii pusillus) on a restoration plot on
the San Joaquin River National Wildlife Refuge (Refuge) represents a monumental
success for CalFed, U.S. Fish and Wildlife Service (USFWS) and its partners in their
efforts to restore riparian habitat to benefit wildlife. The Least Bell’s Vireo has not bred
in the Central Valley for over 50 years and is rarely detected during the breeding season.
The distance to the nearest known source population, along the Santa Clara River, is over
400 km. In order to establish a breeding population in the Valley, Franzreb (1990)
proposed various reintroduction methods and considered natural dispersion highly
unlikely. A single detection of a Least Bell’s Vireo anywhere in the Valley during the
breeding season is a significant sighting but a nesting pair is unprecedented and was
wholly unexpected.
On 10 June a male Least Bell’s Vireo was detected singing during a point count survey of
the H8-9 restoration site. Soon after, a pair was found feeding two dependent young
(Table 1.) indicating they nested in the immediate vicinity. On 28 June, the pair’s
presumed second nest attempt was located in the same area in a three-year old arroyo
willow (Salix lasiolepsis) cultivated by River Partners as part of the restoration of
Hagemann’s Field (Figure 1). The nest was concealed by a dense stand of mugwort
(Artemisia douglasiana) also cultivated by River Partners. The second nesting attempt
was successful and produced four young bringing the total number of young fledged from
the Refuge pair to at least six.
The Least Bell's Vireo is one of four subspecies of Bell's Vireo recognized by the
American Ornithologist's Union (AOU 1957). It is the western-most subspecies,
breeding entirely within California and northern Baja California. Historically, the Least
Bell's Vireo was a common to locally abundant species in lowland riparian habitat,
ranging from coastal southern California through the Sacramento and San Joaquin valleys
as far north as Red Bluff, Tehama County (Goldman 1908, Linton 1908, Grinnell and
Miller 1944).
A decline in Least Bell’s Vireos was noticed by the early 1930’s (Grinnell and Miller
1944). The last confirmed breeding records for the Valley are from 1919 from Merced
and Stanislaus Counties, (MVZ 1919 and WFVZ 1919). Although nesting likely
continued after this date, records of Least Bell’s Vireo were uncommon by the 1960s.
State sponsored surveys in the 1970s failed to detect a single Least Bell’s Vireo in
northern California, leading to the conclusion that “no other California passerine has
declined so dramatically in historic times” (Goldwasser et al. 1980). The loss of
approximately 90% of riparian habitat and increased parasitism by Brown-headed
Cowbird (Molothrus ater) resulted in its extirpation from the Valley and dramatic
declines elsewhere in the state. By 1986, the population had declined to an estimated 300
pairs, with the majority occurring in San Diego County (Kus 2002). The Least Bell's
Vireo was listed as a state endangered species in 1980 and as a federally endangered
species in 1986. PRBO surveys throughout the Valley in the 1990s and 2000s failed to
detect Bell’s Vireo during the breeding season.
Figure 1. Map of planting schematic used by River Partners with PRBO point count locations and Least
Bell’s Vireo nest. Map by River Partners.
Riparian Restoration
The two successful broods of the Central Valley pair may be attributed to the quality of
the riparian restoration at the Refuge. Three years ago, as part of a CalFed-funded
restoration project initiated by USFWS, River Partners developed the San Joaquin River
National Wildlife Refuge restoration design, the largest riparian restoration effort ever
undertaken in California. Using recommendations from the California Partners in Flight
Riparian Bird Conservation Plan (RHJV 2004) which included recommendations from
PRBO, the planting design included native riparian vegetation such as mugwort,
California wild rose (Rosa californica), arroyo willow, and Valley oak (Quercus lobata);
plant species known to benefit riparian-associated birds. The restoration design also
integrated the Riparian Plan recommendation to promote a dense, shrubby understory, an
important component in the breeding habitat of Least Bell’s Vireo. Moreover, River
Partners planted vegetation in a mosaic-design with shrub patches interspersed with trees
under the assumption that plantings that are concentrated into clumps will more quickly
create productive patches of habitat for nesting birds than plantings uniformly spaced
over a large area (RHJV 2004).
PRBO Point count location
LBVI nest
Table 1. Timeline of significant events for Least Bell’s Vireo in 2005.
10 June
Male and female discovered feeding 2 dependent young by
Linette Lina and Julian Wood, PRBO biologists
11 June
Eric Hopsen, Refuge Manager, visits Vireo site with
Linette Lina
14 June
Kenneth Griggs, Refuge Biologist, Julian Wood, Linette
Lina, confirm sighting, John Trochet, Ornithologist, takes
acoustic recording
15 June
Special Use permit issued to PRBO to take photographs of
pair and juveniles. Two rolls slide film taken by Po-Hon
Liu, PRBO intern and delivered to USFWS.
25 June
Federal permit obtained by PRBO to monitor Vireos
28 June
Nest found on hatch day in arroyo willow with 2 eggs and
2 young
1 July
Nest checked. 4 young observed (nestling age=4 days)
5 July
Nest checked. 4 young observed (nestling age=7 days)
9 July
Nest empty. 2 young fed by male. Female and other 2
young not seen but presumed fledged
13 July
Male Vireo captured using playbacks and mist net and
banded with USFWS band by Barbara Kus, USGS with
assistance from Linette Lina, and Julian Wood
Presumably absent in the Central Valley for the past 50 years, Least Bell’s Vireo
numbers have increased tenfold in southern California due to the tremendous efforts of a
broad partnership of local, state and federal agencies involving habitat protection and
restoration, and Brown-headed Cowbird control. In 1998, the population size was
estimated at 2,000 pairs (L. Hays, USFWS, pers. comm.). Nesting Least Bell’s Vireos
have re-colonized the Santa Clara River (Ventura County) to the north, where 67 pairs
nested in 1998 (J. Greaves, pers. comm.), and the Mojave River (San Bernardino County)
to the northeast (Kus and Beck 1998). Prior to 2005, the northernmost reported sighting
of a mated pair was near Gilroy, Santa Clara County in 1997 although no nest was
confirmed (M. Rogers, pers. comm.).
Site Fidelity
Data collected for color-banded birds indicate that site fidelity is high among adults, with
many birds not only returning to the same territory, but placing nests in the same shrub
used the previous year (Salata 1983, Kus unpubl. data). This high site fidelity, in concert
with the recent population increases and large scale restoration efforts, provides hope for
Least Bell’s Vireo re-colonization of the Refuge and in time perhaps its historic breeding
range throughout the Central Valley. Re-establishment of San Joaquin and Sacramento
Valley populations is part of the draft recovery plan to de-list the Least Bell’s Vireo
(USFWS 1998). The recent documentation of Least Bell’s Vireos breeding at the Refuge
underscores the role that habitat restoration can play in conserving biodiversity. Even
highly disturbed ecosystems hold the potential for healthy habitat to recover and for
many dependent species to return.
Habitat requirements
The Least Bell’s Vireo is a riparian obligate during the breeding season and is often
associated with early successional riparian habitat that is structurally diverse (Kus 1998).
The Least Bell’s Vireo will inhabit different riparian vegetation types (e.g., cottonwood-
willow and oak woodland) and different vegetation age classes but are most often
associated with a dense understory layer (0.6-3.0 m) (Goldwasser et al. 1980, Franzreb
1989). Least Bell’s Vireos respond favorably to restoration efforts, particularly when
restoration sites are located adjacent to established riparian habitat. Restored riparian in
the coastal lowlands of southern California has the habitat structure to support breeding
Least Bell’s Vireos within 3-5 years following restoration (Kus 1998; Kus and Beck
1998). Least Bell’s Vireo habitat requirements are compatible with habitat at restored
Refuge sites with its cottonwood-willow dominated riparian vegetation and densely
planted shrub and understory layer (Table 2). This densely planted vegetation provides
cover for nesting (Table 3 and Figure 4).
Table 2. Lest Bell’s Vireo nest and nest substrate description. Concealment recorded as % of nest
concealed by vegetation at one meter distance from nest
Substrate Primary concealment Secondary concealment Substrate
dbh (cm) Substrate
height (cm) Nest height
Salix lasiolepsis Artemisia douglasiana Salix lasiolepsis 4 450 84
Table 3. Least Bell’s Vireo nest concealment recorded as % of nest concealed by vegetation at one meter
distance from nest (from above, below and from each cardinal direction)
% of nest concealed
Above Below North South East West
40 0 0 30 0 90
Figure 2. Least Bell’s Vireo nest in Arroyo Willow (Salix lasiolepsis) with four 7-day old nestlings.
Photo by Linette Lina/PRBO
The Least Bell’s Vireo is one of four subspecies of Bell’s Vireo and is characterized by
dull grayish plumage. Sub-specific differences in plumage are most evident in fresh
plumage when other races show yellow and olive-gray wash (Pyle 1997). In worn
plumage these individuals of other races may resemble the grayer pusillus (P. Unitt pers.
comm.). The individuals at the Refuge were in worn plumage consistent with pusillus.
However, sub specific identification of these birds cannot be confirmed based on
plumage alone. The most convincing argument is the pair’s location within the former
breeding range of pusillus. The distance from the nearest arizonae source population and
the physical barriers between makes the possibility of arizonae extremely remote. The
nearest known source population of pusillus, although still far, is much closer (Ventura
County). There is also the possibility that these birds came from an undocumented
location closer to the San Joaquin Valley such as the Coast Range, especially as there are
several confirmed sightings from the south bay area (M. Rogers pers. Comm.).
Interestingly, one of the last probable occurrences of breeding Bell’s Vireos in the San
Joaquin Valley is from Corral Hollow, Stanislaus County, (only 26 km northwest of the
Refuge) where a dependent juvenile was collected in 1932 (MVZ 1932). The likelihood
of the Refuge Bell’s Vireos being of the pusillus race is very high considering the
combination of light gray plumage, nest location within former pusillus breeding range
and the extreme distance and physical barriers between source populations of other
subspecies (P. Unitt and B. Kus pers. comm.).
Acoustic documentation
On 14 June 2005, a recording of the Least Bell’s Vireo was made by ornithologist John
Trochet. Due to wind, other birdsongs in the area, and the quality of the recording
equipment used the sound quality was relatively low. A copy of the original tape was
made and delivered to USFWS (Jim Nickles, public affairs/media relations) by PRBO
biologist Andrea Pfeffer in July 2005.
Photographic documentation
Photographs of the Least Bell’s Vireo were taken in order to document its presence at the
Refuge and to provide material to educate the public (Figure 3). USFWS requested
PRBO intern and former professional wildlife photographer for Nikon, Po-Hon Liu, to
photograph the vireos. On June 15, Po-Hon Liu and PRBO biologist Julian Wood visited
the Least Bell’s Vireo site and shot two rolls of slide film using a 600 mm lens. The slide
film was delivered to USFWS for printing. Figure 3 shows the male Least Bell’s Vireo
in a cultivated arroyo willow (Salix lasiolepsis) at Hagemman’s Field 8.
Figure 3. Least Bell’s Vireo foraging in cultivated arroyo willow (Salix lasiolepsis).
Photo by Po-Hon Liu/PRBO
On 1 July and 5 July, additional photos of the nest were taken in digital format by Linette
Lina (Figure 2). All photos taken will be given to USFWS on CD.
Target mist netting
On 13 July 2005, the male Least Bell’s Vireo was captured and banded with a blue
anodized USFWS band by Barbara Kus, Research Ecologist with USGS, who possesses a
permit for banding Least Bell’s Vireo (Figure 4). Dr. Kus is an expert on Least Bell’s
Vireo having worked with them for over 19 years and published numerous peer-reviewed
articles on the species. PRBO did not have the permit necessary to capture and band
Least Bell’s Vireo. We used two standard 12 m long soft polyester mesh nets one
stacked on top of the other. We used song and call playback to lure the male into the net.
During banding two body feathers fell from the bird and were saved by Dr. Kus for
potential future genetic analysis. Dr. Kus anticipates that genetic analysis will be
conducted in fall 2006 as she is still setting up her genetics lab.
Figure 4. Photo of PRBO and USGS biologists banding Least Bell’s Vireo male on 13 July 2005. Left to
right: Linette Lina- PRBO, Barbara Kus- USGS, Julian Wood- PRBO.
Photo by Po-Hon Liu/PRBO
Nest monitoring precautions
PRBO biologists took precautions in monitoring the activities of the Least Bell’s Vireo
by following the guidelines outlined in Martin and Geupel (1993) to minimize potential
human impacts on nest success. Examples include:
1) Only one PRBO biologist to visit active nest during a nest check.
Visitors or photographers not permitted to approach active nest.
2) In general, use a nest stick to check nests. This prevents human scent
from being left on or near a nest.
3) Never use flagging or other visible markers less than 10 m from a nest.
4) Check nests with nestlings near fledging date done at a distance using
binoculars to avoid force fledging.
5) Never make a dead-end path to the nest which could draw predators to
the nest.
4) Wait 10-15 minutes before approaching a nest after a cowbird or nest
predator has been sighted in the area.
5) Use long extendable mirrors to check contents without approaching
6) Take all possible care in minimizing disturbance to vegetation
concealing the nest.
Brown-headed Cowbird Impacts
Brown-headed Cowbirds are a major threat to Least Bell’s Vireos throughout their range
(Kus 1999), as well as to other vireo species and subspecies. Brown-headed Cowbirds
are obligate brood parasites that rely on unsuspecting host species to raise the Cowbird
young. Unsuspecting host parents will abandon parasitized nests or raise them at the
expense of the host young (host young may be ejected or die from lack of food). Least
Bell’s Vireos are frequent Cowbird hosts in Southern California and they experience high
reproductive failure of their own offspring in the presence of Cowbird young (Kus 1999).
Brown-headed Cowbirds are common at the Refuge and parasitize host species’ nests,
especially those with open-cup nests located in the understory. Through our nest-
monitoring efforts, we have detected eight open-cup nesting species that have been
parasitized by Brown-headed Cowbirds at the Refuge (Lazuli Bunting (Passerina
amoena), California Towhee (Pipilo crissalis), Common Yellowthroat (Geothlypis
trichas), Blue Grosbeak (Passerina caerulea), Spotted Towhee (Pipilo maculatus), Song
Sparrow (Melospiza melodia), American Goldfinch (Carduelis tristis), and Black-headed
Grosbeak (Pheucticus melanocephalus). Of the over 600 nests found on the Refuge from
2000 to 2005, a total of 542 were open cup nests susceptible to Brown-headed Cowbird
parasitism. Of these nests, 118 (22%) were parasitized. Of the parasitized nests only 20
(17%) successfully fledged one or more host young and 33 nests (28%) successfully
fledged Brown-headed Cowbird young.
Any future monitoring of Least Bell’s Vireo on the Refuge should include an assessment
of the potential threat to nest success by Brown-headed Cowbirds especially in restored
riparian habitat.
Recommendations for Least Bell’s Vireo monitoring
We recommend gathering data and information necessary to manage for the recovery of
the federally Endangered Least Bell’s Vireo in the vicinity of the Refuge. We outline the
following actions to properly monitor and manage for a successful Least Bell’s Vireo
population at the Refuge.
Develop a map of potentially suitable Least Bell’s Vireo Habitat.
Identify areas within the Refuge and vicinity that currently provide potential suitable
habitat for the Least Bell’s Vireo. Review existing aerial photography and conduct on-
the-ground surveys to identify areas that have the proper floristic and structural
components noted in this report or in published research on the species (e.g. Kus 1999,
USFWS 1998). Produce a map (GIS layer) indicating areas that provide potentially
suitable Least Bell’s Vireo habitat within the region and concentrate monitoring efforts in
those areas.
Monitor for the presence and abundance of Least Bell’s Vireos
Conduct point count surveys to detect the presence and measure abundance of Least
Bell’s Vireos in areas identified in the habitat suitability map as well as historic PRBO
point count locations throughout the Refuge. In addition, the area search method, a
nationally standardized time-constrained survey method, should be used to locate Least
Bell’s Vireos. This method allows the researcher to survey an area more thoroughly than
other methods there and is ideal for smaller habitat fragments. The area search and point
count methods are repeatable and can be used to collect information on other species such
as Brown-headed Cowbirds without compromising the search for Least Bell’s Vireos.
Monitor the reproductive success of breeding Least Bell’s Vireo.
Search for the nests of all Least Bell’s Vireos detected at any of the survey locations.
Once found, nests should be carefully monitored to determine nest outcome (successful
or unsuccessful, number of young fledged, Brown-headed Cowbird status, etc.). Nest
monitoring should follow behavioral approaches described in Martin and Geupel (1993)
that minimize disturbance to adults and nests.
Monitor potential Brown-headed Cowbird impacts on the Least Bell’s Vireo
and other riparian bird species.
Brown-headed Cowbirds can cause high reproductive failure in Least Bell’s Vireo
populations (Kus 1999). Therefore it is extremely important to determine the magnitude
and extent of potential Cowbird impacts on Least Bell’s Vireo in the vicinity of the
Refuge. Because of the limited number of potential Least Bell’s Vireo nests we
recommend using songbirds with similar nesting characteristics as a proxy for vireo nests
(e.g., similar nest configuration, construction, and placement relative). Likely candidates
for proxy species include Song Sparrow (Melospiza melodia), Spotted Towhee (Pipilo
maculatus) and American Goldfinch (Carduelis tristis) and other low open cup-nesting
species. The parasitism rates of proxy species can be used to infer the potential
parasitism rates of the Least Bell’s Vireo population. With this knowledge, different
Brown-headed Cowbird management scenarios can be assessed including removing
Cowbird eggs from Least Bell’s Vireo nests, implementing a Cowbird removal program,
or managing livestock feedlot placement in the landscape.
Conduct public outreach activities related to the Least Bell’s Vireo and the
importance of riparian habitats for wildlife.
Education and community outreach are critical components of riparian restoration efforts.
We recommend outreach efforts at 1) the community, 2) private landowners, 3) agencies
and stakeholders with riparian conservation interests and 4) local and national media.
In addition, we recommend planning a “State of the San Joaquin River” conference to share
information and communicate research findings among groups working on the San Joaquin River
with interests relating to riparian habitats, riparian restoration, and endangered species recovery.
Literature Cited
American Ornithologists' Union. 1957. Check-list of North American birds. 5th edition.
Port City Press, Inc. Baltimore, Md.
Franzreb, K. 1989. Ecology and conservation of the endangered Least Bell's Vireo. U.S.
Fish and Wildlife Service, Biological Report 89(1). 17 pp.
Franzreb, K. 1990. An analysis of options for reintroducing a migratory, native
passerine, the endangered Least Bell’s Vireo Vireo bellii pusillus in the Central
Valley, California. Biological Conservation 53:105-123.
Goldman, E.A. 1908. Summer Birds of the Tulare Lake Region. Condor 10:200-204.
Goldwasser, S., D. Gaines, and S.R. Wilbur. 1980. The Least Bell’s Vireo in California: a
de facto endangered race. American Birds 34:742-745.
Grinnell, J., and A. Miller. 1944. The distribution of the birds of California. Pacific Coast
Avifauna No. 26.
MVZ (Museum of Vertebrate Zoology). 1919. Bell’s Vireo (Vireo bellii) nest and eggs
collected by Joseph Grinnell 2 miles southwest of Lagrange, Stanislaus County.
Catalogue #1786.
MVZ (Museum of Vertebrate Zoology). 1932. Bell’s Vireo (Vireo bellii) dependent
juvenile collected near head of Corral Hollow, San Joaquin County by Gordon L.
Bolander. Catalogue #60897.
Martin, T. E. & G. R. Geupel. 1993. Nest monitoring plots: Methods for locating
nests and monitoring success. J. Field. Ornith. 64: 507-519.
RHJV (Riparian Habitat Joint Venture). 2004. Version 2.0. The Riparian Bird
Conservation Plan: a strategy for reversing the decline of riparian associated birds
in California. California Partners in Flight.
Kus, B. E. 1998. Use of restored riparian habitat by the endangered Least Bell’s Vireo.
Restoration Ecology 6:75-82.
Kus, B. E. 1999. Impacts of brown-headed cowbird parasitism on productivity of the
endangered Least Bell's Vireo. Studies in Avian biology 18:160-166.
Kus, B. E. 2002. Least Bell's Vireo (Vireo bellii pusillus). In The Riparian Bird
Conservation Plan: a strategy for reversing the decline of riparian-associated birds
in California. California Partners in Flight.
Kus, B. E. and P. Beck. 1998. Distribution and abundance of the Least Bell's Vireo
(Vireo bellii pusillus) and the southwestern willow flycatcher (Empidonax traillii
extimus) at selected southern California sites in 1997. Prepared for California
Department of Fish and Game, Wildlife Management Division, Sacramento, CA.
Linton, C. B. 1908. Notes from Buena Vista Lake, May 20 to June 16, 1907. Condor
PRBO. 2002. Songbird Monitoring on the San Joaquin River National Wildlife Refuge:
Progress Report for the 2001 field Season. Unpubl. Rept., U. S. Fish and Wildlife
Service, Los Banos, CA.
Salata, L. 1983. Status of the Least Bell's Vireo on Camp Pendleton, California: report on
research done in 1983. Unpubl. Rept., U. S. Fish and Wildlife Service, Laguna
Niguel, CA.
USFWS (U.S. Fish and Wildlife Service). 1998. Draft Recovery Plan for the Least Bell’s
Vireo. Fish and Wildlife Service, Portland, OR. 139 pp.
WFVZ (Western Foundation of Vertebrate Zoology). 1919. 4 eggs collected by John G.
Tyler near Delhi, Merced County. Catalogue #33085.
... Wildlife Refuge (Wood et al. 2006) and other areas in the Central Valley (Howell et al. 2010), and high parasitism rates could hamper progress in their recolonization of these areas. Kus and Whitfield (2005) should be managed with parasitism thresholds <30%. ...
Full-text available
Brood parasitism by the brown‐headed cowbird (Molothrus ater) has caused population declines for conservation‐reliant songbird species such as the federally and state endangered subspecies least Bell's vireo (Vireo bellii pusillus). Although cowbird trapping has increased vireo populations, it is costly, and may prevent the development of parasitism‐avoidance behaviors and result in high rates of non‐target species captures. Therefore, wildlife managers have a compelling interest in evaluating multiple means of cowbird control. We conducted a study between 2016 and 2018 on the Santa Clara River (SCR) in Ventura County, California, USA, to test how the removal of cowbird traps would affect cowbird parasitism of the vireo. At our control site, cowbird trapping occurred in spring using modified Australian crow (Corvus spp.) traps. At our treatment site, we stopped this method of trapping and instead used target‐netting of individual cowbirds and a very short duration trap to capture specific individual cowbirds (i.e., contingency trapping). We also tested the impact of shortening the cowbird trapping period on nest parasitism rates, and used point counts to compare cowbird densities between sites. Additionally, we compared the costs of these techniques. A shorter trapping period still resulted in a large number of cowbird captures on the control site (n = 452 captures) with no observed brood parasitism (n = 67 nests). On the treatment site, cowbird parasitism rates varied from 0% to 14% (n = 75 broods). The target‐netting method was not effective for capturing female cowbirds, probably because of their low densities. The short‐term contingency trapping method effectively removed female cowbirds in the vicinity of vulnerable vireo nests. Cowbird traps captured >300 individuals of 9 different non‐target species each year on the control site, whereas target‐netting and contingency trapping resulted in only 8 captures of 4 different non‐target species. Costs of continuous trapping for 2 months versus point counts, nest monitoring, and contingency trapping were $21,100.00 versus $16,996.00, respectively. Based on these findings, we recommend that an adaptive cowbird management program to benefit songbird populations should consider using regular point counts and nest monitoring to provide data on cowbird impacts, in lieu of continued intensive trapping without monitoring. Thresholds for the density of cowbird females and the allowable level of parasitism for host species should also be established to guide an adaptive cowbird management program. Short‐term contingency trapping based on the thresholds should be considered, but restoration of native vegetation, especially in southwestern riparian areas, is also crucial and effective at limiting parasitism, and so should strongly be considered. Lastly, large numbers of non‐target species captures in cowbird traps are probably having consequential impacts to these species, and need to be monitored and minimized. At the Santa Clara River in Ventura County, California, shortening the trapping season by a month can eliminate brood parasitism of songbird nests and is less costly. Very short duration trapping worked well to capture individual cowbirds, but mist‐netting did not. Cowbird trapping should be used in a sparing and targeted fashion to avoid harming other species, and to make the best use of conservation funds.
Full-text available
Surveys were conducted for least Bell's vireos ( Vireo bellii pusillus) and southwestern willow flycatchers (Empidonax traillii extimus) at selected southern California drainages between 2 April and 29 August 1997. A total of 566 km of river within 17 drainage systems in San Diego, Riverside, and San Bernardino Counties was evaluated for its potential to support vireos . and flycatchers, and 275 km of riparian habitat within these watersheds was surveyed on foot following census protocol for both species. Habitat structure, degree and nature of disturbance, and adjacent land use was described for each study site. Three hundred and one territorial least Bell's vireos were located on 15 drainages. Approximately one-third of these occurred on Santa Ysabel Creek in San Diego County. Vireo presence was documented for five sites lacking prior data on occurrence, and populations at all but the Coachella Valley sites were larger than previous surveys during the 1980's and 1990's. Forty color-banded vireos were detected during surveys, revealing a high level of genetic mixing among populations. Willow flycatchers were far less abundant and less-widely distributed than were least Bell's vireos. Five transient individuals, and two transient pairs, were observed passing through the study area. Just five breeding pairs of southwestern willow flycatchers were located: four on Santa Ysabel Creek, and one on Temecula Creek. One of two nests located contained two brown-headed cowbird (Molothrus ater) eggs. Disturbance varied within and between drainages, with invasion by exotics such as Arundo donax and Tamarix sp. posing major threats to habitat availability for both vireos and Vll flycatchers. Research and management to develop effective and alternative means for controlling cowbird parasitism, facilitate processes leading to population expansion, generate basic demographic data for willow flycatchers, and eradicate exotic vegetation from riparian habitats, are priority needs for these two endangered species.
Full-text available
The Least Bell's Vireo (Vireo be/Iii pusillus) is an obligate riparian breeder brought to the brink of extinction in the last 50 years by habitat loss and Brown-headed Cowbird (Molothrus ater) parasitism. Although cowbird removal programs have effectively reversed the species' decline and promoted a rangewide population rebound, limitations on the timing, duration, location, and scope of trapping efforts permit parasitism to continue in some vireo populations. We quantified the impact of cowbirds on vireo productivity during a 9-yr study of one such population at the San Luis Rey River, California, where trapping of adults and removal of cowbird eggs from vireo nests were conducted annually to control parasitism. Cowbird parasitism occurred in every year of the study, and extended throughout the entire breeding season (April-August). Nineteen to 43% of nests (N = 667), and 19-56% of pairs (25....66), were parasitized at least once during a given year. On average, 29% of parasitized nests (N = 207) were abandoned before cowbird eggs could be removed, and parasitism was responsible for up to 29% of all nest failures. Of 139 nests from which cowbird eggs were removed, 99% remained active, and half eventually fledged vireo young. However, reduced clutch size and hatch rate in these "rescued" nests resulted in the production of up to four times fewer young per nest, and half as many young per egg, than non-parasitized nests. Nevertheless, nest monitoring and cowbird egg removal enhanced annual productivity by 11-44% over that expected in the absence of monitoring and removal. While monitoring and egg removal are effective tools in reducing impacts of parasitized nests, it is essential that appropriate trapping protocol be implemented to prevent access of cowbirds to nests and thus eliminate the impacts that monitoring cannot control. Long-term plans for management of cowbirds and their hosts should emphasize controlling landscape level factors influencing cowbird abundance as opposed to reliance solely on localized trapping programs.
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A primary objective of riparian restoration in California is the creation of habitat for endangered species. Four restoration sites in San Diego County were monitored between 1989 and 1993 and evaluated for their suitability as nesting habitat for Vireo bellii pusillus (Least Bell's Vireo), a state and federally endangered obligate riparian breeder. Vegetation structure at each site was quantified annually and compared to a model of canopy architecture derived from Least Bell's Vireo territories in natural habitat. Vireo use of restored habitat was documented through systematic surveys and nest monitoring. By 1993, only one site in its entirety met the habitat suitability criteria of the model, but portions of each site during all years did so. Differences between sites in the time required to develop suitable habitat—well-developed layered vegetation from the ground to under 8m in height)—were attributable largely to variation in annual rainfall. Vireos visited restoration sites to forage as early as the first growing season, but they did not establish territories or nest there until at least part of the site supported suitable habitat as determined from the model. Placement of territories and nests coincided with patches of dense vegetation characteristic of natural nesting areas. Occupation of restored sites was accelerated by the presence of adjacent mature riparian habitat, which afforded birds nest sites and/or foraging habitat lacking in the planted vegetation. Vireos nesting in restored habitat achieved success comparable to that of vireos nesting in surrounding natural habitat, and there was no evidence that productivity was reduced in created areas. These findings indicate that creating nesting habitat for this target species is feasible and suggest that the critical components of vireo nesting habitat have been captured in both the design and quantitative assessment of restoration sites.
At one time considered common, the least Bell's vireo (Vireo bellii pusillus) was distributed throughout the Central Valley and other low elevation riverine systems in California and Baja California, Mexico, but now has been extirpated from the majority of its breeding range. Habitat loss from agricultural, urban, and commercial developments, flood control and river channelization projects, livestock grazing, and other activities has severely restricted the vireo's range. Limited reproductive success of the vireo as the result of nest parasitism by the brown-headed cowbird (Molothrus ater) in concert with habitat loss has resulted in a decline in the overall vireo population to about 300 breeding pairs. Extensive suitable riparian habitat for the least Bell's vireo must be secured and protected so that the population can increase and be maintained in perpetuity. Additional information on vireo population ecology, genetics, and biological requirements should be obtained and assessed to maximize reproduction. Demographic data will aid in determining the reproductive rate that is necessary to sustain the subspecies on a perpetual basis. High quality, early successional stage riparian woodland is essential for providing adequate nesting habitat. In areas with active cowbird control programs, vireo productivity apparently has been enhanced. An expansion of such efforts and a long-term commitment to cowbird control and vireo nest monitoring to remove cowbird eggs and young will be essential. Methods to reintroduce vireos into the Central Valley and other areas within the historical range that are presently unoccupied should be evaluated.
Now primarily restricted to small, disjunct populations along low-elevation riverine systems in southern California, USA and Baja California, Mexico, the least Bell's vireo Vireo bellii pusillus numbers approximately 350 breeding pairs in the United States. Large-scale reductions in the distribution and numbers of vireos have been attributed to habitat loss and modifications as well as nest parasitism by the brown-headed cowbird Molothrus ater. The least Bell's vireo was determined to be an endangered species by the US Department of Interior, Fish and Wildlife Service in 1986.Before the vireo can be considered recovered, it must be re-established in the Central Valley of California, where once it was considered a common breeding species. To date, no successful reintroductions of native, migratory passerines have been reported. This paper explores and analyzes alternative strategies specific to re-establishing a migratory passerine (capture/release, autumn capture/spring release, cross-fostering, and captive breeding), assesses the alternatives with respect to reintroducing the least Bell's vireo into the Central Valley, and recommends the capture and release approach. Successful reintroduction and re-establishment of a native, migratory passerine, such as the least Bell's vireo, would have implications for the conservation and management of other endangered and threatened avian species.