Population Structure and Demography of the Least Bell’s Vireo and Southwestern Willow Flycatcher and Use of Restored Riparian Habitat

Science Center Objects

Riparian woodlands are highly productive ecosystems that support a disproportionately high fraction of regional biodiversity. They are also one of the most endangered terrestrial systems in temperate North America, and have been reduced to just 5% of their former extent in California and throughout the American southwest. These losses have been accompanied by steep declines in numerous plant and animal species, including the federally endangered Least Bell’s Vireo and Southwestern Willow Flycatcher, which are considered flagship species for riparian conservation. Dr. Barbara Kus conducts long-term research on multiple inter-related aspects of breeding and wintering ecology of these species, with a focus on response to management to reduce threats such as cowbird parasitism and habitat loss.

Riparian habitat supports more species of birds, as well as other wildlife, than any other habitat type in the southwest. This is particularly true in southern California, where riparian woodlands provide a literal oasis in an otherwise arid landscape. Riparian habitat is also one of the State’s most endangered habitats, with less than five percent of the woodlands remaining that were present at the time of statehood. It was inevitable that losses of this scope and magnitude lead to declines in many riparian species, bringing some, including the Least Bell’s Vireo (Vireo bellii pusillus) and the Southwestern Willow Flycatcher (Empidonax traillii extimus), to the brink of extinction.


A banded adult southwestern willow flycatcher at a nest. (Credit: Scarlett Howell, USGS, WERC. Public domain.)

Least Bell’s Vireo and Southwestern Willow Flycatcher, both migratory passerines dependent on riparian habitat for breeding, share many of the same threats. They both have declined in response to widespread habitat loss, as well as brood parasitism by the Brown-headed Cowbird (Molothrus ater). Since 1986, our team has conducted research on the distribution, habitat requirements, nesting ecology, wintering ecology, foraging behavior, singing behavior, demography and genetics of the least Bell’s vireo. Our studies of Southwestern Willow Flycatchers focus on habitat requirements, nesting ecology, and demography. A primary focus of this work is to identify mechanisms by which small populations of flycatchers are able to persist by examining adult and juvenile survival, local recruitment, immigration and dispersal. Through our research, we seek to document response to management and other factors that influence population growth, including cowbird management, urbanization, wildfire, invasive vegetation, drought and flooding.  The results of these studies provide information needed by resource agencies and managers to develop sound conservation and recovery plans for these species.

A major facet of our work on riparian birds considers the effectiveness of habitat creation and restoration as a way to reverse declines in habitat availability. While habitat creation has been shown to be successful in producing nesting habitat for some endangered species, at least in the short term, little information is available on the long-term viability of restored sites, or their ability to persist as self-sustaining ecosystems. We consider restoration broadly and have expanded our research over time to address new emerging issues. One line of investigation examines bird response to “traditional” restoration taking the form of planting cuttings and container stock of native plants to create habitat “from scratch”.  In these studies, necessarily long-term, we link vegetation development over time to establishment of the riparian bird community, using vegetation structure and bird communities in natural habitat as the standards for comparison. As habitat restoration evolved to take the form of exotic vegetation (such as giant reed and salt cedar) eradication, we initiated studies focusing on short- and long-term responses of endangered birds to this management practice. We later expanded our research to include endangered bird response to anthropogenic manipulation of vegetation within the context of phreatophyte control for flood management purposes. Collectively, the results of these studies provide resource agencies and wildlife managers with information on the value of restoration to wildlife beyond endangered species, as well as input on specific features of restored habitat that add to or detract from this value.

Looking for life in the ashes - The San Diego Union-Tribune 2014 - Read about how the 2014 fires affected bird habitat in southern California.