Vireos in general switch from seemingly relying strictly on insects
for food to include berries and other soft fruits by the end of
summer (which end varies by species and location). I've seen Least
Bell's in CA feed on elderberry, poison oak, squaw-bush and possibly
others. On one occasion, I observed a family of Least Bell's (2
adults, 4 juveniles) about 4 weeks post-fledging all eating poison
oak berries, and have seen other similarly old(er) juveniles (with or
without adults present) eating elderberries. This usually occurs late
in their season, end of June to early July and August. Another aspect
of behavior I've observed in Least Bell's is that they will take
their young high into sub-canopy at and following fledging, while
Warbling will bring theirs down; Cassin's seem to remain mid-canopy
to high shrub level throughout their time in the bush. It may be that
the intermediate levels of vegetation provide them the best foraging,
or there may be an escape mechanism at work -- going to where they
are least expected by their particular predators (thought that seems
more complicated than merely food and water needs). In addition, lots
of higher nesting birds seem to make themselves readily visible as
the season progresses by moving downward, probably related to food
and water, as well as heat avoidance (it can be hot up there in the
hot wind), and since they are not finding as many soft, watery larvae
by the end of summer, they would be seeking what's available - Jim Greaves, MT