Hi Lynne!
I've become a frequent birder at Plaster over the past couple years. It is indeed a nifty place to bird year round in fact.
There are a few options for where to park to get access to the area, but for quickest access to the birdiest spots, I'd recommend parking at this pin here:
39.944681, -105.026856
This is in the residential community area for Plaster Reservoir, but it's a general parking area and there's always space available and it's plenty safe. From there you can take a public connector sidewalk through the residences to the reservoir. You can also park on Plaster Circle itself right near where this path crosses it to get to the reservoir. That's fine too. It'd save you all of about 1-2 minutes, but the parking is in front of houses instead which you may or may not be as comfortable with.
Of particular birding note is the area just east of the reservoir, below the dam, where there are thickets of willows. There's a private golf course just to the north of there that abuts one of the thickets, but you can still view and circumlocute much of the thicket on what should be fairly obvious "deer trails" and sidewalks nearby.
Eric
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Eric DeFonso
Boulder County, CO