B. caliginosus is characteristic of humid coastal areas whereas B. vosnesenskii is very widely distributed and is abundant at drier interior sites (where caliginosus is absent)
Flight season very long, as B. vosnesenskii can be found flying in mid-winter in central and southern California.
Please see this forum topic
for a discussion regarding the differentiation of these two similar species.
In many places in California, especially inland and to the south, B. caliginosus can be eliminated from consideration based on range as it has never been recorded there and should not occur (wrong habitat, climate, etc). Please consult Thorp et al. for details about their ranges.
To the north, especially at the coast, both species (B. caliginosus and B. vosnesenkii) could occur and ID is very difficult at best from images alone as these can be difficult to separate even with specimens. (Comment by John S. Ascher).