This male Gadwall (on the right) was hanging out with a bunch of Mallards at the Healdsburg wastewater treatment plant. Perhaps he was transitioning from breeding plumage to eclipse plumage?
On June 15, there was a female Bufflehead (or possibly a subadult male - check out the picture) swimming on a large, deep pond at the Healdsburg wastewater treatment facility. According to Bolander and Parmeter's "Birds of Sonoma County, California: An Annotated Checklist and Birding Gazetteer" (revised in 2000), Buffleheads are quite rare in June in Sonoma County - there were only two June records as of 2000. Last year, there were two female or subadult Buffleheads at the wastewater treatment facility on May 27 (also unusual; Buffleheads normally leave Sonoma County by early May).
This is the second time I've seen a Caspian Tern at the Healdsburg wastewater treatment facility. The last time was April 19, 2011. Note the dusky tip on the bill.