The smallest of the four subspecies of Bell’s Vireo, the Least Bell’s Vireo (Vireo bellii pusillus), calls Southern California riparian woodlands home for half the year. As a neotropical migrant, the Least Bell’s Vireo flies nearly 2,000 miles every spring from Southern Mexico up to Southern California to find suitable riparian habitat to breed before migrating back south in July. This chatty songbird is identified by its tiny size, ash-grey-colored head, whitish body, faint eye-rings, and straight beak. The Least Bell’s Vireo depends on understory and shrubs near the ground where they like to make cup-like nests and inhabit dense riparian vegetation such as willows (Salix sp.), mulefat (Baccharis salicifolia), California wild rose (Rosa californica), cottonwoods (Populus deltoides), and sycamores (Platanus occidentalis). These plants provide a plethora of cover and food for the insectivorous Least Bell’s Vireo where they are found foraging in the thickets or along stream beds for their next meal.
Despite the Least Bell’s Vireo once being known as one of California’s most common birds, this small bird has faced large threats to its existence. Declining numbers of the Least Bell’s Vireo have been noted as early as the 1930s due to habitat loss from urbanization. Fast forward to the late 1970s and this bird had lost significant range as it was extirpated from Northern California and the Central Valley, leaving only 300 breeding pairs within Santa Barbara, Riverside, and San Diego counties.
The downfall of the Least Bell’s Vireo is mainly attributed to habitat loss and degradation, a common theme with threatened species. Riparian habitats in Southern California have decreased by about 90% of what was present in 1850. Causes of their riparian habitat loss include the construction of dams,channelization of rivers and streams, urbanization, agricultural development, invasion of non-native plants, and pesticides. Furthermore, the Least Bell’s Vireo’s decline is exacerbated by the increased human activity surrounding riparian habitat. They are sensitive to human presence, noise, and nighttime light, and, if present, will cause them to abandon the area. These birds depend on mid-successional riparian habitat for breeding, but with most of it lost, their populations and success as a species has dwindled. The remaining habitat is also severely fragmented, leaving only small, vulnerable subpopulations. Habitat fragmentation is defined as “the process during which a large expanse of habitat is transformed into a number of smaller patches of smaller total area isolated from each other by a matrix of habitats unlike the original”(Fahrig, 2003). Fragmented habitat, especially those with no buffer between riparian and urban areas, can make the Least Bell’s Vireo more susceptible to predation by feral cats and the Argentine ant (Linepithema humile). Argentine ants are a non-native, invasive ant species whose spread is believed to be elevated by urbanization (Suarez et al. 1998). These ants have been observed to be a predator of vireo nests where they co-occur. The Argentine ants will gradually build in numbers in the nest and attack by biting nestlings as they hatch.
Another serious threat the Least Bell’s Vireo has faced is brood parasitism by Brown-headed Cowbirds (Molothrus ater). Brown-headed Cowbirds are implicated in the decline of Least Bell’s Vireo populations because of their reproductive strategy. Female Brown-headed cowbirds will lay their eggs in the nests of other birds such as the Least Bell’s Vireo and let the vireo mother bird do all of the chick-rearing. This strategy is used by Brown-headed Cowbirds so they can save energy from building a nest and having to care for their own young. The cowbird will wait for the Least Bell’s Vireo to leave her nest and then will usually damage or remove one or more eggs and replace it with their own. When Brown-headed Cowbird eggs are laid in the Least Bell’s Vireo nests this leads to either the Least Bell’s Vireo abandoning the nest or the Brown-headed Cowbird hatchling indirectly killing the Least Bell’s Vireo chicks by starving them out. The Brown-headed Cowbird hatches earlier than the other eggs, is bigger in size, cries louder for food, and is able to raise its head higher so it gets more of the food from the vireo mother. Brown-headed Cowbirds are native to North America but have been in California for only 76 years. Since they are a newly introduced species, the Least Bell’s Vireos have not evolved a defense mechanism against them.
Action was taken to save the Least Bell’s Vireo from possible extinction in 1986 when it was federally listed as endangered under the Endangered Species Act (ESA). Under the Endangered Species Act, the Least Bell’s Vireo was designated critical habitat which encompasses a total of 38,000 acres that stretches from Santa Barabara to San Diego county. According to the US Fish and Wildlife Service (USFWS), critical habitat is “specific geographic areas that contain features essential to the conservation of an endangered or threatened species and that may require special management and protection. Critical habitat may also include areas that are not currently occupied by the species but will be needed for its recovery.” As a result of this designation, federal agencies and activities that involve a federal permit, license, or funding are prohibited from destroying or adversely modifying the critical habitat. Private landowners are not affected by this designation. According to the 1998 Draft Recovery Plan for the Least Bell’s Vireo, a population total of 4,200 pairs would be needed for delisting. The US Fish and Wildlife Service’s last review of the Least Bell’s Vireo in 2005 shows there’s an increase in the population and estimates that there are 2,500 breeding pairs in the wild. Under the ESA, Recovery Plans are made to outline measurable steps that need to be taken to delist a species. Along with providing the Least Bell’s Vireo critical habitat, one main measure mentioned in the Draft Recovery Plan is the reduction of Brown-headed Cowbird brood parasitism. Funded by mitigation requirements of the ESA, monitoring, trapping, and euthanasia of Brown-headed Cowbirds are often employed. Other measures include habitat creation, restoration, monitoring, research, and a river enhancement and management plan for the Santa Clara River.
In the face of challenges the Least Bell’s Vireo has encountered within the last century, efforts to restore their habitat and bring them back look promising. In 2005, a pair of Least Bell’s Vireo nested in a restored part of the San Joaquin National Wildlife Area, nearly 60 years after they had last been seen in San Joaquin County. At the Otay Delta restoration site located in San Diego county, it only took three years after the initial planting of riparian vegetation for the Least Bell’s Vireo to reappear. Furthermore, in 2017 Point Blue Conservation Science released a study concluding that restored habitat leads to increased bird abundance and species richness, which will continue to increase over time as the restored site matures and grows.