Hooded Merganser is a long-time favorite species of mine--at the age of eight, I made a watercolor copy of Jim Arnosky's Hooded Merganser painting in Watching Water Birds. I saw my first one on November 27th, 1999, at Belle Isle in Detroit, Michigan.
"Homies" are uncommon in southern California. Finding one is always a treat, but it won't send you scrambling to document and report it. A flock of five hens has been fishing at the neighborhood lake this week (a long-overdue addition to my neighborhood list), causing me to wonder about their range...
First, the general purpose eBird map. They are widely spread across the Lower 48, the Rocky Mountain-ish area comprising the weakest link.
Summer (Jun-Jul) distribution. Aha. Here it seems we have another coverage issue. I bet if platoons of high-intensity eBirders swept through Northern Canada, we'd have solid purple from Lake Superior to Churchill. Notably, the bird seems to be present in most of the places it winters with the exceptions of the Southwest and the far South.
And, winter (Dec-Feb). I'm struck by the concentration on the immediate East Coast.
And finally, the obligatory Southern California perspective. Most of the records (except for the Colorado River and the Imperial Valley) are within 75-ish miles of the coast. And interesting detail--there seem to be more concentrations slightly inland, in the foothills and mountains (e.g., north of Los Angeles). From San Diego County Bird Atlas: "..the Hooded Merganser avoids San Diego Bay and occurs in most other coastal wetlands only rarely...the Hooded Merganser is as likely to occur in the higher mountains as at most places along the coast."