Sunset at Ellis Creek

In the official Petaluma Wetlands Field Guide, the letter “O” beside a bird’s photograph apparently means “occasional.”

That’s compared to “A” (abundant), “C” (common), “”U” (uncommon) and “R” (rare). Mallards, coots and black-?bellied plovers are, according to the guide, abundant here at Petaluma’s Ellis Creek Water Recycling Facility. Great blue herons, red-shouldered hawks and marbled godwits are common, long-billed dowitchers and California scrub jays are uncommon, and the chestnut-backed chickadee, the great-horned owl and the brown-headed cowbird are rare.

But the great-tailed grackle I am looking at right now is definitely “O,” a native species generally observed only occasionally throughout the season at Ellis Creek, which stands adjacent to Shollenberger Park in Petaluma. Great-tailed grackles are black with a faint purple hue (the males are, anyway, with females, according the guide, tending toward a “warm brown, darker below, slight sheen on back, wings,” with light eyes (the males’ eyes are bright yellow). As the name-implies, the great-tailed grackle has a long, wedge-like tail shaped something like a narrow slice of pizza. They tend to inhabit areas with open fields and water, and are equally comfortable in “urban areas.”

That makes Ellis Creek the perfect spot to hang out in, swooping around the numerous ponds, perching on rocks, tall grass and trees, and not minding the nearby industrial-looking infrastructure that identifies Ellis Creek as anything but the typical Petaluma park and open space. Though a favorite destination for local birders and walkers, this is a working wastewater-treatment facility, its various pools and ponds designed to let the reeds and rushes naturally filter out contaminants from the water already treated inside those buildings, on its way out to the Bay.

With Shollenberger Park next door, and frequently on the dry side - though still attracting a fair number of birds even during the summer - Ellis Creek has become a year-round guarantee of water for local and migrating birds. As such, it has also become a primary source of birdwatching fun for locals, who can often be seen here with binoculars, scopes, notebooks and - since its publication last year - copies of the Petaluma Wetlands Field Guide, written by Marian Parker and John Shribbs, and published by the Petaluma Wetlands Alliance.

Though the Ellis Creek facility, which also functions as a park and open space, does draw a number of joggers and cyclists, the place was designed more form nature-viewing and birdwatching than for recreation. This windy afternoon, not long before the gates close at dusk, a fair number of mostly-masked pedestrians wander about as the sun slides slowly behind the trees, casting long spidery shadows that seem to be following the walkers along the trails, all while the birds grow increasingly visible and active. Some of them appear to be coming home from a day’s adventures, including squadrons of Canada geese splash-landing in the ponds, and many are simply coming up and coming out for a snack or a hobnob with its fellow creatures of the night.

Within the space of an hour (6:15 to 7:15 p.m.), I’ve had the opportunity to look up the previously described grackle, along with dozens of its ornithological colleagues. There are the aforementioned Canada geese (“A,” page 50) in huge numbers on the trail and flying overhead, numerous duck species (“C” and “A”), mute swans (“C,” page 53), green herons (“O,” page 55), snowy egrets (“C,” page 56), and turkey vultures (“C,” page 56) by the dozens. As the sun bounces off the surface of the ponds, there have been a few high-trilling killdeer (“C”), some random gulls (“C”), doves (“C”) and hummingbirds (“C”), a western kingbird (“C”), a common raven or two (“O”), a western bluebird (“O”) and others I’m not fast enough a guidebook-flipper to identify.

For what it’s worth, on the non-bird side of the biological lifelist, an enormous black-tailed jackrabbit (C,” page 77) hop-scotches across the trail at one point, stopping half-way to glance up the trail toward the declining sun, as if to check the time before bouncing off into the brush.

The Ellis Creek Water Recycling Facility was officially opened in 2009, joining several other similar facilities around the world that use natural filtration and ultraviolet disinfection as tertiary treatment of wastewater. The whole facility treats just under 5 million gallons of wastewater daily. Though one could never tell from the side of the “finishing ponds,” some were designed to resemble butterflies or harvest mice, if seen from a bird’s eye view. Between the ponds and Petaluma River are acres of fields leading to even more acres of mudflats, altogether the perfect environment for birds, small mammals, insects and reptiles.

And for the record, not a bad place for visiting wildlife-lovers (mark them “C”), who will find, at any time of day, a rich jigsaw puzzle of ecological abundance at Ellis Creek. As I pack up my guidebook book - making a mental note to never go hiking in Petaluma without it again - the settling sunlight casts a wide, soft beam of orange across the trail, and I follow my ever-lengthening shadow back home.