There does seem to be quite a large number of singing male White-collared
Seedeater around the Dairy Mart Ponds, Tijuana River Valley. I have heard
as many as six singing just within a short distance of the ponds and
walking east across Dairy Mart Road. The species seems to be quite well
established as an introduced exotic. As others have mentioned a favorite
territory is on the east side of the "middle" pond among the willows - best
found early in the morning.
I happened to photograph this singing male last weekend at that exact
location. The appearance of male individuals that I have seen here is
remarkably uniform, all with puffed out cinnamon rumps, grayish backs, no
wing bars, small white check on the base of primaries and similar in song.
Some photos and a link to the song on my blog here
[Non-text portions of this message have been removed]