Surveys and monitoring for the endangered Least Bell’s Vireo (Vireo bellii pusillus; vireo) were done at the San Luis Rey Flood Risk Management Project Area (Project Area) in the city of Oceanside, San Diego County, California, between March 31 and July 20, 2020. We completed four protocol surveys during the breeding season, supplemented by weekly territory monitoring visits. We identified a total of 161 territorial male vireos; 145 were confirmed as paired and 4 were confirmed as single males. For the remaining 12 territories, we were unable to confirm pair status. Three transient vireos were detected in 2020. The vireo population in the Project Area increased by 26 percent from 2019 to 2020. Vireo populations increased across San Diego County, with a 39-percent increase documented at Marine Corps Base Camp Pendleton (MCBCP); a 58-percent increase at Marine Corps Air Station; a 78-percent increase on the Otay River; and a 7-percent increase in the population on the middle San Luis Rey River.
We used an index of treatment (Treatment Index) to evaluate the impact of on-going vegetation clearing on the Project Area vireo population. The Treatment Index measures the cumulative effect of vegetation treatment within a territory (since 2005) by using the percent area treated weighted by the number of years since treatment. We found that the Treatment Index for unoccupied habitat was more than five times that of occupied habitat, indicating that vireos selected less disturbed habitat in which to settle.
We monitored vireo nests at three general site types: (1) within the flood channel where exotic and native vegetation removal has occurred regularly (Channel), (2) three sites next to the flood channel where limited exotic and native vegetation removal has occurred (Off-channel), and (3) three sites that have been actively restored by planting native vegetation (Restoration). Nesting activity was monitored in 100 territories, 4 of which were occupied by single males. Hatching success was higher in the Channel relative to the Off-channel. We found no other differences between Channel, Off-channel, and Restoration nests in terms of clutch size or fledging success. There also was no difference in measures of productivity per pair between Channel, Off-channel, Restoration, and Mixed territories (territories that were classified as one site type but nesting occurred in another site type, or where multiple site types were used for nesting). Overall, breeding success and productivity were lower in 2020 than in 2019, with 69 percent of pairs fledgling at least one young and pairs fledging an average of 2.1±1.7 young.
To investigate whether the cumulative years of treatment had an impact on vireo reproductive effort, we looked at the effects of the Treatment Index on reproductive parameters. Results from generalized linear models indicated that treatment did not have an effect on vireo nesting effort or the number of vireo fledglings per pair produced in 2020.
Similarly, our analysis of nest survival for 2020 revealed no effect of Treatment Index on daily survival rate. Analysis of vegetation data collected at vireo nests from 2006 to 2020 revealed that vegetation at 1–2 meters (m) from the ground was the most important predictor of daily survival rate.
There were differences in nest-placement characteristics among site types and successful/unsuccessful nests. Channel nests were placed higher in the vegetation than Off-channel or Restoration nests. Host plant height, distance to edge of host plant, and distance to edge of vegetation clump were greater at Channel sites compared with Off-channel sites, but were not different from Restoration sites. Within sites, we found only one difference between successful and unsuccessful nests. At Off-channel sites, successful nests were placed higher in the vegetation than unsuccessful nests.
Red/arroyo willow (Salix laevigata or Salix lasiolepis) and mule fat (Baccharis salicifolia) were the species most commonly selected for nesting by vireos in all 3 site types. Vireos used a wider variety of species for nesting in Channel and Off-channel sites (7 and 10 species, respectively) compared to Restoration sites (3 species).
Ninety-three vireos banded before the 2020 breeding season were resighted and identified at the Project Area in 2020, all of which were originally banded at the Project Area. Adult birds of known age ranged from 1 to 9 years old. A total of 171 vireos were newly banded in 2020.
Twenty-eight adult vireos were banded with a unique color combination, and 143 nestlings were banded with a single dark blue numbered federal band on the left leg. Between 2006 and 2020, survivorship of males (67±10 percent) was consistently higher than females (59±11 percent). First-year birds from 2006 to 2020 had an average over-winter survivorship of 17±5 percent. First-year dispersal in 2020 averaged 2.9±2.9 kilometers (km), with the longest dispersal (13.5 km) by a female that was recaptured at Las Flores Creek, MCBCP. From 2007 to 2012, most returning first-year vireos returned to the Project Area, whereas from 2013 to 2017, the majority of returning birds dispersed to areas outside of the Project Area. In 2018, the trend shifted, and most first-year vireos returned to the Project area. This trend continued in 2020 with most first-year vireos returning to the Project Area; 77 percent of all re-encountered first-year birds returned to the Project Area and 23 percent dispersed to areas outside of the Project Area (upstream to the middle San Luis Rey River and to drainages on MCBCP).
Most of the returning adult male vireos showed strong between-year site fidelity to their previous territories. Eighty percent of males (45/56) occupied a territory in 2020 that they had defended in 2019 (within 100 m). Thirty-three percent of females (2/6) detected in 2020 returned to a territory that they occupied in 2019. The average between-year movement for returning adult vireos was 0.1±0.5 km.
We completed four protocol surveys for the endangered Southwestern Willow Fycatcher (Empidonax traillii extimus; flycatcher) at the Project Area between May 20 and July 20, 2020. No Willow Flycatchers were detected in the Project Area in 2020.
A total of 46 vegetation transects (526 points) were sampled at the San Luis Rey Flood Risk Management Project Area in 2020. Seventy-one percent (376/526) of points were in the Channel and 22 percent (115/526) were at Upper Pond. The remaining 7 percent (35/526) were at the Whelan Restoration site. Foliage cover below 1 m was higher at the Channel points compared to Upper Pond and Whelan Restoration. Higher foliage cover in the Channel was attributed to the higher herbaceous component. However, foliage cover from 1 to 3 m was higher at the Whelan Restoration site compared to both Upper Pond and the Channel. Average canopy height was similar at all three site types and was 4.4 m or less. From 2006 to 2020, total foliage cover declined above 1 m in the Channel, from 4 to 5 m at Upper Pond, and above 8 m at Whelan Restoration. Within the Channel, the steepest declines occurred between 2009 and 2013 and between 2014 and 2016. Since 2016, we observed an increase in percent foliage between 0 and 2 m within the Channel, but for other height classes, percent cover remained below levels detected before 2009. Changes in cover at Upper Pond and Whelan Restoration appeared to be driven by the loss of tall tree cover. The vegetation mowing and treatment activities, in combination with lack of precipitation (especially between 2012 and 2016), may have contributed to the decline in foliage cover observed from 2006 to 2020.
We sampled vegetation at 49 vireo nests and 49 random plots (“territory” plots) within territories in the Channel and Upper Pond following the 2020 breeding season. Vireos in the Channel selected territories with significantly more foliage cover above 2 m but less cover below 1 m relative to the available habitat. In contrast, Channel vireos selected nest sites within their territories with lower foliage cover above 3 m and were non-selective with regard to cover below 2 m. Vireos at Upper Pond generally were less selective with regard to territory and nest sites but tended to select territories with more foliage cover from 1 to 2 m and above 8 m, and they selected nest sites within their territories with greater foliage cover from 0 to 1 m.