The least Bell's vireo is predominately an insectivore. During the early and mid portion of the nesting season most foraging occurs in the vicinity of the nest site, predominately in willow, Salix spp., Both high and low shrub layers are used as foraging substrate. These birds use non-riparian habitats occasionally and will travel an average of 15m to forage. Birds using non-riparian areas for foraging tend to have territories in the narrowest sections of riparian habitat (Kus and Miner, 1987).
A low, dense shrub layer is considered essential for nesting (Franzreb, 1989), and a large degree of vertical stratification is preferred. Willow are the most commonly used vegetation for this need. Plant species used for nesting and foraging include the California wild rose, Rosa california, and coastal live oak, Quercus agrifolia. Most nest sites are located near the edges of thickets. Nest height on average is 1m above the ground (Regional Environmental Consultants, 1988). Males are site tenacious and return to the same site to nest in succeeding years. Regional Environmental Consultants (1988) reported an average territory size of about 0.8ha.
As seen in figures 64 and 65, most of the current habitat is concentrated on Camp Pendleton and in the foothills of southern Orange County. This pattern will remain through Plans Build-Out, largely because of riparian zone management in Camp Pendleton.