Updated 6/19: Some female Red-winged Blackbirds have resurfaced and at least one fledgling is out of the nest and up in the shrub tops. As bird expert Peter Rauch pointed out in a Comment, the bird with the puffy eyebrows in the photo to the left is a fledgling, not a female as I had thought. A nice fat one, too.
Some other birds, such as Double-crested Cormorants and Eared Grebes, sport bushy headfeathers when their reproductive hormones flow. But that’s not the case here. The bushy brows here are a marker of recent emergence from the egg shell.
In past years, the breeding season for these birds has ended right around the Summer Solstice. Looks like this season, although smaller in scale than some in the past, and starting later, is concluding right around the usual time.
Updated June 21: I checked the northwest corner of the park today, where the blackbirds do their thing, and it was empty. Neither sound nor sight of any blackbirds. I did see one bird on the south side of the ridge, voicing high pitched distressed-sounding peeps repeatedly while fluttering from one bush to another. Was that a female or a juvenile who missed the migration?