[cross-posting from OCBirds]
From visiting bird guide James Smith:
"We just had a juv Tricolored Heron flying north over a drainage on Adams Ave, Costa Mesa [around 8:30 AM]. Couldn't stop because of traffic. Just to clarify the Heron flew over the drainage on Adams between Ranger and Shanter Roads, apparently just outside Costa Mesa but near the town line."
Sounds like this was along the lower Santa Ana River north of Fairview Park, toward the 405 Fwy. I'm sure locals know how to best access it (I don't, sorry!).