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Long-term Population Monitoring of the Least Bell’s Vireo, Otay Ranch Preserve

Since 2011, museum biologists have been surveying for and monitoring nests of the endangered Least Bell’s Vireo (Vireo bellii pusillus) along Salt Creek and Otay River, and controlling populations of the known brood parasite Brown-headed Cowbird (Molothrus ater) in accordance with the resource management plans for the Otay Ranch Preserve. Our biologists provide annual updates on the population size, distribution, and breeding status of the Least Bell’s Vireo at the Preserve with the goal of identifying and tracking changes over time and informing management decisions at the Preserve.

Over the past 10 years, we have documented that vireo numbers, nest success, and productivity have fluctuated, but population size has increased overall, from a low of 9 territories in 2011 and 2012 to the current high of 16 territories in 2020. The substantial fluctuations in the vireo population are mostly in response to rainfall and Brown-headed Cowbird (Molothrus ater) parasitism. Following high rainfall in 2019, food was plentiful, and vireos produced an above average number of fledglings (4.1 fledglings per pair), which in turn resulted in a higher number of vireos returning to breed in 2020. Following below average rainfall in 2014, vireos fledged less than one fledgling per pair (0.8 fledglings per pair). In years of high cowbird parasitism (2011, 2012, and 2020) when 35-45 percent of nests were parasitized, vireo productivity was greatly reduced as a result of decreased clutch sizes and low success of parasitized nests.

To minimize brood parasitism of the vireo, museum biologists have been implementing a Brown-headed Cowbird management program for the Preserve since 2012. While the most common control measure is to use cowbird traps to remove cowbirds from the area, we have experimented with using targeted mist netting as an alternative method for capturing cowbirds. Comparison of data from three years of cowbird trapping and two years of mist netting have shown that targeted mist netting resulted in a higher capture rate of female cowbirds than did trapping. Given that mist netting is more flexible than a fixed cowbird trap and costs almost half as much per captured female as trapping, we have shown that mist netting may be a useful alternative when a site is small and the vireo and cowbird populations are not large.

Moving on Up(land)

An interesting behavioral characteristic of the Otay River vireos is their frequent use of upland habitat. Least Bell’s Vireos are thought to prefer dense riparian vegetation for foraging and nesting. However, at the Preserve, more than half of all vireo territories are located in the benches along the river in upland habitats dominated by laurel sumac (Malosma laurina), Peruvian pepper trees (Schinus molle), and blue elderberry (Sambucus nigra subsp. caerulea).

Our biologists have documented similar use of atypical habitats on a separate project at MCAS Miramar, where vireos have been documented foraging and nesting in coastal sage scrub and chaparral vegetation. This suggests that Least Bell’s Vireos are more flexible in their habitat use, especially when riparian vegetation is limited, which may have larger implications for management of habitat for this species as well as the vireo’s ability to adapt to climate change. With Southern California predicted to become drier over time, riparian vegetation may become more restricted.

Preservation of adjacent uplands may become more critical to this species’ survival than previously thought, directly affecting management decisions in the Otay Ranch Preserve and beyond.