A .—Cowbird (Molothrus spp.) control is a major focus of recovery-oriented management
of two endangered riparian bird species, the Least Bell's Vireo (Vireo bellii pusillus) and
Southwestern Willow Flycatcher (Empidonax traillii extimus). During the past 20 years, annual
trapping of cowbirds at Least Bell's Vireo and Southwestern Willow Flycatcher breeding sites
has eliminated or reduced parasitism in comparison with pretrapping rates and, thereby,
signifi cantly increased seasonal productivity of nesting pairs. Enhanced productivity, in turn,
has resulted in an 8-fold increase in numbers of Least Bell's Vireos; Southwestern Willow
Flycatcher abundance, however, has changed li le, and at some sites has declined despite
cowbird control. Although generally successful by these short-term measures of host population
response, cowbird control poses potential negative consequences for long-term recovery
of endangered species. As currently employed, cowbird control lacks predetermined biological
criteria to trigger an end to the control, making these species' dependence on human intervention
open-ended. Prolonged reliance on cowbird control to manage endangered species
can shi a ention from identifying and managing other factors that limit populations—in
particular, habitat availability. On the basis of our analysis of these long-term programs, we
suggest that cowbird control be reserved for short-term crisis management and be replaced,
when appropriate, by practice
The primary objective of this protocol is to provide
a standardized survey technique to detect Southwestern
Willow Flycatchers, determine breeding status, and facilitate
consistent and standardized data reporting.
Recording from the January 26, 2022 SDMMP Management and Monitoring Coordination Meeting. Presentations from Dr. Barbara Kus (USGS) and Scarlett Howell (USGS): "Distribution,
abundance, and demography of Southwestern Willow Flycatchers in
San Diego County".
We have learned of many new breeding sites and territories since the early 1990s as a result of extensive survey efforts throughout the Southwest. In 1993, there were only 140 known territories distributed among 40 breeding sites. The current estimate (as of 2006) is 1262 territories located among 284 sites (but remember the earlier caution about lack of standard definition for "site").
Not all of the 284 known sites are surveyed every year. The total estimated number of known territories (1262) is based on the most recent survey at all sites and does not reflect sites that were actually surveyed in a given year. At 126 sites surveyed in 2006, there were 831 territories detected.
Most territories are found within small breeding sites (those sites with five or fewer territories). There are only six sites with 50 or more territories, though this comparison is confounded by lack of a standard definition of site.
Most territories are found within small breeding sites (those sites with five or fewer territories). There are only six sites with 50 or more territories, though this comparison is confounded by lack of a standard definition of site.
The states of California, Arizona, and New Mexico account for 88% of known flycatcher territories. Nevada, Colorado, and Utah collectively have 12% of the known territories. We have received no reporting from standardized Southwestern Willow Flycatcher surveys in Texas, and hence know nothing of the current status of the flycatcher there.
Southwestern Willow Flycatchers are distributed over a wide elevation range, with most from sea level to 1600 m, but a few sites (n=3) are located as high as 2500 m in elevation.
Southwestern Willow Flycatchers are distributed over a wide elevation range, with most from sea level to 1600 m, but a few sites (n=3) are located as high as 2500 m in elevation.
Fewer than half (43%) of territories are in native habitat and 28% are in habitats having a 50% or greater exotic component. A large percentage of the territories in native habitat occur at one site - the Cliff-Gila Valley in New Mexico. Over 90% of territories are in habitats where willow, saltcedar, or boxelder are the dominant tree species; flycatchers breed in boxelder-dominated habitats only in the Cliff-Gila Valley, New Mexico.
Fewer than half (44%) of sites are on federally-controlled lands and 28% are on private lands; these privately owned sites account for 36% of known territories. Approximately one-third (32%
Surveys for the endangered southwestern willow flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 31
August 2000. Eleven transient flycatchers of unknown subspecies were detected during
surveys, and two transients were captured in mist nets at MAPS (Monitoring Avian Productivity
and Survival) stations on De Luz Creek and the Santa Margarita River. Transients occurred in a
range of habitat types including mixed willow riparian, willow-sycamore dominated riparian,
and sandbar and mule fat scrub. All but one transient individual were sighted within 50 m of
surface water.
Eighteen southwestern willow flycatcher breeding territories were located. With the
exception of a new site at Lake O'Neill on Fallbrook Creek, all territories were along the lower
Santa Margarita River. The majority of territories (15/18) were located in mixed willow riparian
habitat. Resident flycatchers exhibited a bimodal distribution with regard to distance to surface
water, with 53 percent within 50 m, and the remainder from 150 to 575 m away from it.
The eighteen territorial males included ten confirmed pairs, five single males, and three
males of undetermined status. Nesting was documented for eight of the ten pairs, with each pair
attempting one nest. All nests were successful, and flycatchers fledged an average of 2.3 young
per pair. No instances of cowbird parasitism were observed. Pairs placed nests in five species of
plants, including black willow (Salix goodinggii), sandbar willow (S. exigua), stinging nettles
(Urtica dioica), blackberry (Rubus ursinus) and giant reed (Arundo donax).
Two resident males and one female were returning banded birds, including one male
banded as an adult in 1998 and one female banded as an adult in 1999. Four resident males and
one female were captured and color banded in 2000, and six nestlings in two nests were banded.
In addition, both of the transients captured at MAPS stations were banded. None of the
transients observed during surveys carried bands.
Surveys for the endangered southwestern willow flycatcher (Empidonax traillii extimus) were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 15 August 2004. Eighty-two transient flycatchers of unknown subspecies were detected during surveys. Transients occurred in a range of habitat types including mixed willow riparian, willow-sycamore dominated riparian, oak-sycamore dominated riparian, riparian scrub, and upland scrub. The distance from transient locations to the nearest surface water averaged 274 ± 374 m (N = 72).
Surveys for the endangered southwestern willow flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 15
August 2007. Seventy transient flycatchers of unknown sub-species were observed during Basewide
surveys. Transients occurred on 12 of the 16 drainages surveyed in 2007. No willow
flycatchers were detected at De Luz, Horno, Roblar, or Windmill Creeks. Transients occurred in
a range of habitat types including mixed willow riparian, willow-sycamore dominated riparian,
oak-sycamore dominated riparian, riparian scrub, and upland scrub. The distance from transient
locations to the nearest surface water averaged 340 ± 424 m (std, n = 70).
In 2007, the resident southwestern willow flycatcher population on Base consisted of 14
females and 12 males. However, because of within season flycatcher movement, 16 territories
were established. One male defended territories in two locations, separated by more than 1 km,
pairing with a female in the second location. Another male remained single during the entire
2007 breeding season. In total, 14 females formed pair bonds with 11 male willow flycatchers.
Two of the 11 paired males were polygynous with two females each. Based on movement data,
two additional males were suspected to be polygynous with neighboring females. All territories
were located in mixed willow riparian habitat. Poison hemlock (Conium maculatum) was
present in all territories. Distance to surface water averaged 168 ± 244 m (std, n = 16), with 69%
(11/16) of territories located within 100 m of water.
Nineteen nesting attempts by willow flycatchers were documented during the 2007
breeding season. Nesting was initiated in early June and continued into August. Forty-two
percent (8/19) of nests successfully fledged at least one flycatcher young. Predation accounted
for 73% (8/11) of nest failures. The other documented cause of nest lost was substrate failure.
The cause of failure for two nests was unknown. It is possible that they were depredated in the
egg stage or abandoned prior to egg laying, as they failed during the time eggs should have been
laid, but no eggs were observed in the nests. Of the 12 pairs whose nests were monitored, 67%
(8/12) fledged young. Seventeen fledglings were produced, yielding an estimated seasonal
productivity of 1.4 young per pair (17 young/12 pairs). No instances of brown-headed cowbird
(Molothrus ater) parasitism were observed. P
Surveys for the endangered southwestern willow flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 31
August 2002. One hundred and two transient flycatchers of unknown subspecies were detected
during surveys. Transients occurred in a range of habitat types including mixed willow riparian,
willow-oak-sycamore dominated riparian, willow-cottonwood dominated riparian, riparian scrub
and upland scrub. The distance from transient locations to the nearest surface water averaged
489  603 m (N = 101).
Eighteen southwestern willow flycatcher breeding territories were located. With the
exception of one territory at Lake O=Neill on Fallbrook Creek, all territories were along the
Santa Margarita River, including a new site near the Base hospital, and one downstream of the
concentration of birds between Rifle Range Road and Ysidora Basin. Ninety-four percent
(17/18) of territories were located in mixed willow riparian habitat. Exotic vegetation,
particularly and giant reed (Arundo donax), tamarisk (Tamarix ramosissima), and poison
hemlock (Conium maculatum) was present in all territories, and was dominant (% cover > 50) in
11% (2/18) of territories. Resident flycatchers exhibited a bimodal distribution with regard to
distance to surface water, with 60% within 70 m, and the remainder 100-900 m away from it.
The resident flycatcher population included two non-territorial "floater" males, two
unpaired males, and 16 pairs. Nesting was documented for all 16 pairs, which produced 1-2
nests each. Fifty-three percent (10/29) of nests were successful, and flycatchers fledged an
average of 1.5 young per pair. No instances of cowbird parasitism were observed. Pairs placed
nests in five species of plants, including black willow (Salix gooddingii), arroyo willow (S.
lasiolepis), giant reed, tamarisk, and maiden's bower (Clematis ligusticifolia).
Fourteen resident males and seven females were birds banded previously at Camp
Pendleton between 1998 and 2001. Three birds (two males and one female) were immigrants
from the San Luis Rey River. In addition, a male color banded in Costa Rica during the winter
of 2001-2002 was resighted at the Base in July. Four resident males and four females were
captured and color banded in 2002, and 10 nestlings in five nests were banded. None of the
transients observed during surveys carried bands.
Surveys for the endangered southwestern willow flycatcher (Empidonax traillii extimus) were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 15 August 2006. Thirty-five transient flycatchers of unknown subspecies were detected during surveys. Transients occurred in a range of habitat types including mixed willow riparian, willow-sycamore dominated riparian, oak-sycamore dominated riparian, riparian scrub, and upland scrub. The distance from transient locations to the nearest surface water averaged 328 ± 515 m (N = 35)
We surveyed for Southwestern Willow Flycatchers (Empidonax traillii extimus; flycatcher) at 33 locations along multiple drainages in San Diego County, including portions of Agua Hedionda Creek, Cottonwood Creek, Escondido Creek, Los Penasquitos Creek, Otay River, San Diego River, San Dieguito River, San Luis Rey River, Sweetwater River, and Tijuana River. Resident flycatchers were only found on two drainages in San Diego County, at San Dieguito and San Luis Rey Rivers, with 99 percent occurring on the San Luis Rey River. Resident flycatchers were detected at 18 percent of survey locations (Bonsall, Cleveland National Forest, Rey River Ranch, San Dieguito, and Vista Irrigation District [VID], and VID Lake Henshaw). Resident flycatchers were documented for the first time at Lake Henshaw, the only new location surveyed that supported flycatchers. We detected a minimum of 80 resident flycatchers from 2015 to 2019, most of these were upstream and downstream from Lake Henshaw. Transient flycatchers were found at 42 percent of survey locations; 38 transient individuals were detected at Agua Hedionda Creek, Otay River, San Diego River, San Dieguito River, and the San Luis Rey River.
Over the course of this study, 11 locations historically occupied by resident flycatchers were resurveyed; only 5 were found to have resident flycatchers: (1) Bonsall, (2) Cleveland National Forest, (3) Rey River Ranch, (4) San Dieguito, and (5) Vista Irrigation District. The number of resident flycatchers declined from previous high counts at all five locations. Collectively, the number of resident flycatcher territories within the historically occupied area of the upper San Luis Rey River downstream from Lake Henshaw (Cleveland National Forest, Rey River Ranch, and Vista Irrigation District) declined 71 percent between 1999 (48) and 2019 (14); 42 percent of the decline occurred between 1999 and 2016, with an additional decline (50 percent) occurring between 2016 and 2019. In 2016, the distribution of flycatcher territories at the historically occupied area of the upper San Luis Rey River changed relative to the distribution in 1999: the proportion of territories at Cleveland National Forest and Rey River Ranch decreased to 36 percent each, while Vista Irrigation District increased to 29 percent, creating a more equal distribution of territories across the historically occupied area. By 2019, the distribution changed relative to 2016, with most of the territories spread equally bet
Surveys and monitoring for least Bell's vireos (Vireo bellii pusillus) and southwestern willow flycatchers (Empidonax traillii extimus) were conducted on the San Luis Rey River, San Diego County CA, between 1 April and 21 August 2006. Vireo surveys were conducted from Interstate 15 west approximately 6.5 km to Mission Road. Southwestern willow flycatchers were surveyed in the same area, as well as downstream between Sante Fe Road and a point approximately 1 km upstream on the San Luis Rey River (Guajome Regional Park).
Fifty-three territorial male least Bell's vireos were observed within the study area, 50 of which (94 percent) were confirmed as paired. Nine transient male vireos were also detected. Within the section of river consistently monitored since 2003, vireo numbers declined from 46 territories in 2005 to 31 in 2006. For the three years prior to 2006 the number of resident territorial males had remained relatively constant, varying from 40 to 46 territorial males.
Nesting activity at 99 nests within 43 vireo territories was monitored. Thirty-six percent of nests were successful, fledging at least one vireo young, while 64 percent failed. Sixty percent of vireo nests whose contents were observed were parasitized by brown-headed cowbirds (Molothrus ater). Nest predation and cowbird parasitism accounted for 66 and 21 percent of failures, respectively. However, biologists "rescued" parasitized nests by removing cowbird eggs shortly after they were laid, allowing some to fledge young. Without "rescuing" it is likely that nest success would have been only 15 percent. In total, 86 vireo young fledged from 35 nests, and pairs fledged on average 2.2 young by the end of the breeding season.
Thirteen least Bell's vireos banded prior to the 2006 breeding season were resighted within the study area. All had been banded as nestlings on the San Luis Rey River. Eight of the thirteen possessed a unique combination of color bands or were recaptured during the 2006 season and therefore could be identified to individual. Two of the eight were banded as nestlings outside of the study area and dispersed 14 and 4.9 km into the study area. All other uniquely color banded vireos fledged from and dispersed within the study area. The extent of their dispersal ranged from 0.8 to 4.4 km. Five other adult vireos that had been banded as nestlings with a single federal band were target netted, but attempts to recapture them were unsuccessful. Two additional adult vireos w
Surveys for the endangered southwestern willow flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 31
August 2001. Twenty-one transient flycatchers of unknown subspecies were detected during
surveys. Transients occurred in a range of habitat types including mixed willow riparian,
willow-sycamore dominated riparian, willow-oak dominated riparian, and mule fat scrub. The
distance from transient locations to the nearest surface water averaged 124  127 m (N = 21).
Nineteen southwestern willow flycatcher breeding territories were located. With the
exception of one territory at Lake O=Neill on Fallbrook Creek, all territories were along the
lower Santa Margarita River. Ninety-five percent (18/19) of territories were located in mixed
willow riparian habitat. Exotic vegetation, particularly poison hemlock (Conium maculatum)
and giant reed (Arundo donax), was present in all but one territory, and was dominant (% cover
> 50) in 32% (6/19) of territories. Resident flycatchers exhibited a bimodal distribution with
regard to distance to surface water, with 42% within 60 m, and the remainder 150-500 m away
from it.
The resident flycatcher population included one unpaired male and 18 pairs (16 males, 18
females). Two males were believed to be polygynous, each mating with two females. Nesting
was documented for 17 of the 18 pairs, which produced 1-3 nests each. Fifty-two percent
(15/29) of nests were successful, and flycatchers fledged an average of 1.9 young per pair. No
instances of cowbird parasitism were observed. Pairs placed nests in seven species of plants,
including black willow (Salix gooddingii), arroyo willow (S. lasiolepis), stinging nettle (Urtica
dioica), mule fat (Baccharis glutinosa), elderberry (Sambucus mexicana), poison hemlock, and
giant reed.
One resident male and one female were birds banded previously at Camp Pendleton; the
male was banded as an adult in 1998, while the female was banded as an adult in 2000. Nine
resident males and eight females were captured and color banded in 2001, and 26 nestlings in 12
nests were banded. None of the transients observed during surveys carried bands.
Surveys for the endangered southwestern willow flycatcher (Empidonax traillii extimus) were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 24 August 2005. Thirty-five transient flycatchers of unknown subspecies were detected during surveys. Transients occurred in a range of habitat types including mixed willow riparian, willow-sycamore dominated riparian, oak-sycamore dominated riparian, riparian scrub, upland scrub, and non-native. The distance from transient locations to the nearest surface water averaged 108 ± 173 m (N = 35).
This report summarizes the results of least Bell's vireo (Vireo bellii pusillus) and
southwestern willow flycatcher (Empidonax traillii extimus) surveys conducted in 2002
along the San Luis Rey River in San Diego County, California. The primary objectives
of this study were to determine the number and location of least Bells' vireo and
southwestern willow flycatchers within the survey areas, and document reproductive
status of the southwestern willow flycatcher.
Surveys for the endangered least Bell's vireo (Vireo bellii pusillus) were conducted at the San Luis Rey Flood Control Project Area (Project Area) in the city of Oceanside, San Diego County, California, between 1 April and 15 July 2008. Three protocol surveys were conducted during the breeding season and supplemented by weekly territory monitoring visits. A total of 130 least Bell's vireo territorial males were identified; 117 were confirmed as paired, four were confirmed as single males, and nine were not confirmed as paired. Six transient vireos were detected during surveys.
The least Bell's vireo population at the San Luis Rey Flood Risk Management Project Area increased by 20% (22 territories) from 2007, to achieve the highest number of territories ever detected at this site. We evaluated the impact of ongoing channel vegetation clearing and giant reed (Arundo donax) eradication that has occurred in the river channel since 2005 on the Project Area vireo population by comparing vireos in the river channel (Channel), where vegetation treatment has occurred, with sites outside of the river channel (Untreated), where vegetation treatment has not occurred. While the total number of territories in 2008 at Untreated sites outside of the river channel rebounded to the same number as in 2006, the number of territories in the Channel, increased by 11 territories since 2006. Therefore, despite major habitat changes between 2005 and 2008 within the Channel, vegetation removal did not appear to have a negative impact on the abundance of vireos in 2008.
The majority of vireo territories (64%) occurred in habitat characterized as willow riparian. Sixteen percent of birds occupied habitat co-dominated by willows (Salix spp.) and cottonwoods (Populus fremontii), and 20% of territories were found in riparian scrub, dominated by mule fat (Baccharis salicifolia) and/or sandbar willow (S. exigua). Most vireo territories
(61%) were established in habitat where 50 to 95% of the vegetation cover was native species, 38% of the territories were in habitat vegetated almost entirely (>95%) by native species, and one territory was placed in habitat where 5 to 50% of the vegetation cover was native. The most common exotic species within territories was A. donax followed by black mustard (Brassica nigra), poison hemlock (Conium maculatum) and tamarisk (Tamarix ramosissima).
Nesting activity was monitored in 102 territories. Pair success was slightly higher fo
Surveys for the endangered least Bell's vireo (Vireo bellii pusillus) were conducted at the San Luis Rey River Flood Control Project area in the city of Oceanside, San Diego County, California, between 1 April and 15 July 2007. Three protocol surveys were conducted during the breeding season and supplemented by weekly territory monitoring visits. One hundred and eight least Bell's vireo territorial males were identified; all but three males were paired.
The vireo population in the project area declined by 9% (11 territories) from 2006, the largest drop observed in five years. Overall there was a net increase of five territories in the channel where exotic and native vegetation removal has occurred (Treated) and a net loss of 16 territories in the Untreated sites where vegetation removal will not occur (Untreated), suggesting that factors other than vegetation removal may have contributed to the 2007 decline. Factors contributing to the vireo decline may include drier conditions, reduced habitat quality, and human disturbance.
The majority of vireo territories (70%) occurred in habitat characterized as Willow Riparian. Ten percent of birds occupied habitat co-dominated by willows (Salix spp.) and cottonwoods (Populus fremontii), and 20% of territories were found in Riparian Scrub, dominated by mule fat (Baccharis salicifolia) and/or sandbar willow (S. exigua). Most vireo territories (63%) were established in habitat where 50 to 95% of the vegetation cover was native species while 37% of the territories were in habitat vegetated almost entirely (>95%) by native species. Giant reed (Arundo donax) was the most common exotic species within territories followed by black mustard (Brassica nigra), poison hemlock (Conium maculatum) and tamarisk (Tamarix ramosissima).
Nesting activity was monitored in 97 territories. Pair success from both treatments was comparable; 82% (50/61) of Treated pairs vs. 87% (27/31) of Untreated pairs were successful in fledging young from at least one nest. Nest success (number of nests fledging at least one young/total number of nests found) of pairs breeding in the channel (Treated) did not differ statistically from that of pairs breeding in the Untreated sites (49%; 60/122 vs. 59%; 32/54). Successful and failed nests within Treated and Untreated sites did not differ statistically in average nest height, height of the host plant, or the distance the nest was placed from the edge of the host plant. Eighty to ninet
A trapping program was implemented along the upper San Luis Rey River (uslr) and Pamo Valley (pv) in,San Diego County, California, to protect nesting southwestern willow flycatcher (Empidonax traillii extimus, flycatcher, swfl), least Bell's vireo ( Vireo bellii pusillus, vireo, lbvi), and their riparian cohabitants from brood parasitism by the brown-headed cowbird (Molothrus ater). Four traps were operated in each location (8 total) from 1 April to 30 June 2003. We removed 207 cowbirds (54 males, 38 females, and 0 juveniles at uslr; 61 males, 48 females, and 6 juveniles at pv). In addition, 330 individuals of 8 non-target species were captured (79 at uslr, 251 at pv), of which all but 6 (l.8%) were released unharmed.
Topical protection from cowbird parasitism allows targeted populations of host species to increase annual productivity and to grow between years, but does not affect the regional cowbird population (Griffith and Griffith 2000).
The following report summarizes the results of sensitive species surveys conducted for the Southwestern Willow Flycatcher (Empidonax traillii extimus), Least Bell's Vireo (Vireo bellii pusillus), Southwestern Arroyo Toad (Bufo microscaphus califomicus), and Southwestern Pond Turtle ( Clemmys marmorata pallida). Surveys were performed on the Pine Creek, Bauser Creek, and Cottonwood Creek
drainages within the Pine Creek and Bauser Canyon Wilderness areas of the Cleveland National Forest. The purpose of these surveys was to determine both species presence, population number, and breeding status, as well as habitat conditions within the survey areas.
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii
extimus) were conducted at Marine Corps Base Camp Pendleton, California, between 24 June
and 12 August 2011. Surveys in 2011 began later than usual (late June vs. 15 May) because
of contract delays. Consequently, data collected in 2011 may not be directly comparable to
other years and represents a minimum population estimate. Drainages containing riparian
habitat suitable for flycatchers were surveyed one to three times.
No transient flycatchers of unknown sub-species were observed during Base-wide
surveys.
The devastating effects of FD-KSHB on native riparian habitat in the Tijuana River Valley is of
great concern to land managers in San Diego County, especially as many new infested sites
continue to be identified throughout the region. These plant communities serve as critical
breeding habitat for species such as the endangered least bell’s vireo (Vireo bellii pusillus;
LBVI) (Kus 2002) and southwestern willow flycatcher (Epidomax traillii extimus; SWFL) (Craig
and Williams 1998). Our work points to a path forward to make informed decisions on best
approaches to management in the short- and long- term. In this project, our objectives were to
1) determine KSHB distribution with respect to key least Bell’s vireo and southwestern willow
flycatcher habitat; 2) develop a predictive model for which specific native habitats are most
vulnerable to FD-SHB invasion and impacts, based on an understanding of the evolutionary
ecology of the fungus and beetles, beneficial endophyte distribution, a landscape analysis of
vegetation and surrounding avocado groves, and environmental conditions; 3) evaluate
biological control potential of beneficial endophytic fungi and bacteria; 4) assess the efficacy of
application methods of entomopathogens suppressive to ISHB; 5) evaluate water-based latex
paint as a means to track ambrosia beetle activity on infested trees; 6) evaluate efficacy of
pesticides in suppressing beetle populations on critical host species; and 7) evaluate pesticide
residues on critical host tissues.
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 16 May and 28 July
2016. One hundred and seven transient Willow Flycatchers of unknown subspecies were
observed during Base-wide surveys. Transients occurred on 12 of the 16 drainages surveyed in
2016. No Willow Flycatchers were detected at Cristianitos Creek, Horno Canyon, Roblar Creek,
or San Onofre Creek. Transients occurred in a range of habitat types including mixed willow
(Salix spp.) riparian, riparian scrub, and willow-sycamore (Platanus racemosa) dominated
riparian vegetation. Exotic vegetation including poison hemlock (Conium maculatum) and
black mustard (Brassica nigra) was present in the majority of flycatcher locations.
We surveyed for Southwestern Willow Flycatchers
(Empidonax traillii extimus; flycatcher) along the upper
San Luis Rey River near Lake Henshaw in Santa Ysabel,
California, in 2021. Surveys were completed at four locations:
three downstream from Lake Henshaw, where surveys
occurred from 2015 to 2020 (Rey River Ranch [RRR],
Cleveland National Forest [CNF], Vista Irrigation District
[VID]), and one at VID Lake Henshaw (VLH) that has been
surveyed annually since 2018. There were 78 territorial
flycatchers detected at 3 locations (RRR, CNF, VLH), and
1 transient flycatcher of unknown subspecies was detected
at VID. Downstream from Lake Henshaw, five flycatchers,
including three males and two females, were detected at
RRR and CNF. In total, three territories were established,
consisting of two pairs and one male of undetermined
breeding status. At VLH, we detected 73 flycatchers, including
32 males, 38 females, and 3 flycatchers of unknown sex.
In total, 43 territories were established, containing 38 pairs
(22 monogamous pairings, 7 confirmed polygynous groups
consisting of 7 males each pairing with 2 different females,
and 1 suspected polygynous group consisting of 1 male and
2 females), and 5 flycatchers of undetermined breeding status
(2 males and 3 flycatchers of unknown sex). Brown-headed
cowbirds (Molothrus ater; cowbird) were detected at all four
survey locations.
Flycatchers used five habitat types in the survey
area: (1) mixed willow riparian, (2) willow-cottonwood,
(3) willow-oak, (4) willow-ash, and (5) sycamore-oak.
Eighty-seven percent of the flycatchers were detected in
habitat characterized as mixed willow riparian, and 94 percent
of the flycatchers were detected in habitat with greater than
95-percent native plant cover. Exotic vegetation was not
prevalent in the survey area.
There were 15 nests incidentally located during surveys:
1 was successful, 2 were seen with eggs or nestlings on the
last visit, 9 failed, and the outcome of the remaining 3 nests
was unknown. Three of these nests were parasitized by
cowbirds. There were 13 juveniles detected at VLH during
surveys; no juveniles were detected at RRR or CNF.
Of the 10 banded flycatchers detected during surveys,
7 were resighted and confirmed to be adults that held
territories in previous years. Three flycatchers with a single
dark blue federal band, indicating that they were banded as
nestlings in the former demo
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii
extimus) were conducted at Marine Corps Base Camp Pendleton, California, between 15 May
and 26 July 2013. Forty-five transient Willow Flycatchers of unknown subspecies were
observed during Base-wide surveys. Transients occurred on 10 of the 16 drainages surveyed
in 2013. No Willow Flycatchers were detected at De Luz Creek, Horno Canyon, Newton
Canyon, Piedra de Lumbre Canyon, Roblar Creek, or Windmill Canyon. Transients occurred
in a range of habitat types including mixed willow (Salix spp.) riparian, willow-sycamore
(Platanus racemosa)- dominated riparian, oak (Quercus sp.)-sycamore dominated riparian,
and riparian scrub.
A trapping program was implemented along the upper San Luis Rey River (uslr) and Pamo Valley (pv) in San Diego County, California, to protect nesting southwestern willow flycatchers (Empidonax trail/ii extimus, flycatcher, swfl), least Bell's vireos ( Vireo bellii pusillus, vireo, lbvi), and riparian cohabitants from brood parasitism by the brown-headed cowbird
(Molothrus ater). Four traps were operated in each location (8 total) from 1 April to 30 June 2001. We removed 284 cowbirds (68 males, 42 females, and 1j 11veniles at 11slr; 103 males, 60 females, and 10juveniles at pv). In addition, 290 individuals of 8 non-target species were captured (47 at uslr, 243 at pv), of which all but 2 (0.7%) were released unharmed.
Topical protection from cowbird parasitism allows targeted populations of host species to increase annual productivity and to grow, but does not affect the regional cowbird population (Griffith and Griffith 2000).
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 31 July
2017. Twenty-one transient Willow Flycatchers of unknown subspecies were observed during
Base-wide surveys. Transients occurred on 5 of the 16 drainages surveyed in 2017. No Willow
Flycatchers were detected at Aliso Creek, Cockleburr Canyon, Fallbrook Creek, French Creek,
Horno Canyon, Piedra de Lumbre Canyon, Pilgrim Creek, Roblar Creek, San Mateo Creek, San
Onofre Creek, or Windmill Canyon. Transients occurred in a range of habitat types including
mixed willow (Salix spp.) riparian, riparian scrub, willow-sycamore (Platanus racemosa), and
sycamore-oak (Quercus agrifolia) dominated riparian vegetation. Exotic vegetation including
black mustard (Brassica nigra) and poison hemlock (Conium maculatum) was present in the
majority of flycatcher locations.
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 31 July
2010. Twenty-five transient flycatchers of unknown sub-species were observed during Basewide surveys. Transients occurred on 6 of the 16 drainages surveyed in 2010. No Willow
Flycatchers were detected at Cockleburr Canyon, De Luz Creek, Fallbrook Creek, French Creek,
Horno Canyon, Newton Canyon, Piedra de Lumbre Canyon, Pilgrim Creek, Roblar Creek, or
Windmill Canyon. Transients occurred in a range of habitat types including mixed willow (Salix
spp.) riparian, willow-sycamore (Platanus racemosa) dominated riparian, oak (Quercus sp.)-
sycamore dominated riparian, riparian scrub, and upland scrub. The distance from transient
locations to the nearest surface water or saturated soil averaged 245 ± 240 m (std, n = 25).
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 28 July
2015. Eighty-nine transient Willow Flycatchers of unknown subspecies were observed during
Base-wide surveys. Transients occurred on 10 of the 16 drainages surveyed in 2015. No Willow
Flycatchers were detected at Aliso Creek, Cockleburr Canyon, De Luz Creek, French Creek,
Roblar Creek, or Windmill Canyon. Transients occurred in a range of habitat types including
mixed willow (Salix spp.) riparian, riparian scrub, willow-sycamore (Platanus racemosa)
dominated riparian, willow-cottonwood (Populus fremontii) dominated riparian, and upland
scrub.
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii
extimus) were conducted at Marine Corps Base Camp Pendleton, California, between 15 May
and 28 July 2012. Twenty-nine transient flycatchers of unknown subspecies were observed
during Base-wide surveys. Transients occurred on 8 of the 16 drainages surveyed in 2012.
No Willow Flycatchers were detected at Aliso Creek, Cristianitos Creek, De Luz Creek,
Horno Canyon, Newton Canyon, Roblar Creek, San Onofre Creek, or Windmill Canyon.
Transients occurred in a range of habitat types including mixed willow (Salix spp.) riparian,
willow-sycamore (Platanus racemosa) dominated riparian, oak (Quercus sp.)-sycamore
dominated riparian, and riparian scrub.
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 1 May and 31 July
2018. Sixty-six transient Willow Flycatchers of unknown subspecies were observed during
Base-wide surveys. Transients occurred on 7 of the 16 drainages surveyed in 2018. No Willow
Flycatchers were detected at De Luz Creek, Fallbrook Creek, French Creek, Horno Canyon,
Newton Canyon, Piedra de Lumbre Canyon, Roblar Creek, San Mateo Creek, or Windmill
Canyon. Transients occurred in a range of habitat types including mixed willow (Salix spp.)
riparian, riparian scrub, and willow-sycamore (Platanus racemosa) dominated riparian
vegetation. Exotic vegetation including poison hemlock (Conium maculatum) and black mustard
(Brassica nigra) was present in the majority of flycatcher locations.
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 15 May and 28 July
2009. Thirty-five transient flycatchers of unknown sub-species were observed during Base-wide
surveys. Transients occurred on 10 of the 16 drainages surveyed in 2009. No Willow
Flycatchers were detected at Aliso Creek, Cristianitos Creek, Fallbrook Creek, Horno Canyon,
Piedra de Lumbre Canyon, or Roblar Creek. Transients occurred in a range of habitat types
including mixed willow (Salix spp.) riparian, willow-sycamore (Platanus racemosa) dominated
riparian, oak (Quercus sp.)-sycamore dominated riparian, riparian scrub, and upland scrub. The
distance from transient locations to the nearest surface water averaged 451 ± 593 m (std, n = 35).
Introduction
In an effort to both protect nesting Southwestern Willow Flycatchers as well as mitigate for potential indirect impacts from off-site grazing on Forest Service lands, the U.S. Forest Service has funded a yeai-ly Brown-headed Cowbird trapping program along the upper San Luis Rey River since 1992. The purpose of this trapping program is to reduce the threat and impact of brood parasitism to the endangered Southwestern Willow Flycatcher (Empidona:x trailli extimus).
The Brown-headed Cowbird (Molothrus ater) is an obligate brood parasite which lays its eggs in the nests of other bird species and is dependent upon the host to incubate their eggs and rear their young. Averaging 6"-7" in length, the Brown-headed Cowbird is a medium sized songbird with sexually dimorphic plumage. Adult males are dark brown to gloss black with a brown head and neck. Females are slightly smaller than males and dull tan to light brown with indistinct streaking on the breast. Originally restricted to the midwest region of North America, the Brown-headed Cowbird expanded in both range and abundance following the settlement and alteration of natural habitats, particularly with the increase in agriculture and livestock production. Reaching California in the late 1800's, this species was first documented breeding in San Diego County in 1915 (Unit 1984), and had become well established within southern California by the 1930's (Rothstein 1994, Willett 1933). Songbird species or populations which had not evolved with the cowbird and have no experience with parasitism may be subject to significantly reduced reproductive success. Brood parasitism combined with other impacts, such as habitat loss and fragmentation can lead to declines in songbird species, especially those with an already limited population and distribution.
Cowbird trapping has proven to be an effective method in the conservation efforts of sensitive-
. songbird populations throughout the United States, and was initially utilized in the recovery efforts of the Kirtland's Warbler in Michigan (Mayfield 1977). Subsequently, cowbird trapping has become an important tool in the management of several other sensitive songbird species, including the Black-capped Vireo, Least Bell's Vireo, and Southwestern Willow Flycatcher.
Surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)
were conducted at Marine Corps Base Camp Pendleton, California, between 20 May and 29 July
2014. Ninety-seven transient Willow Flycatchers of unknown subspecies were observed during
Base-wide surveys. Transients occurred on 9 of the 16 drainages surveyed in 2014. No Willow
Flycatchers were detected at Cockleburr Canyon, Cristianitos Creek, Fallbrook Creek, French
Creek, Newton Canyon, Roblar Creek, or Windmill Canyon. Transients occurred in a range of
habitat types including mixed willow (Salix spp.) riparian, willow-sycamore (Platanus
racemosa)- dominated riparian, oak (Quercus sp.)-sycamore dominated riparian, and riparian
scrub.
