Common Loon, aka Black-billed Loon or Great Northern Diver

Recently we spent 2 windblown hours birding at Bodega Bay, Sonoma County, CA. There a good number of birds gathered in the cove between Puerto Bodega and Spud Point, mostly massing close to the shoreline in the lee of the roadbed and riprap. Best of the bunch was an adult Common Loon in alternate (breeding) plumage, floating quietly. The loon never dove, extended its neck or spread its wings, but still provided good viewing for some digiscoped photos. Other species in the flotilla included Western and Clark's Grebes, Greater Scaups, Ruddy Ducks and 2 basic plumage Red-throated Loons.

Palmer (Handbook of North American Birds, Vol. 1 - Loons through Flamingos) reported the older common names for Common Loon as "Black-billed Loon" and "Great Northern Diver". The Common Loon is very similar in appearance to the slightly larger Yellow-billed Loon, Gavia adamsii, so the adjective Black-billed seems more appropriate than Common. "Common" tells us little as to how the species sets itself apart from the other 4 loon species, and especially from the Yellow-billed. "Common" can apparently refer to relative abundance compared to other local loon species, or to "common to both western and eastern hemispheres."

The photos show most of the important field marks of the Common Loon, especially the big blocky black head,  the huge deep black bill and the attractive checkered pattern of spots on the back (scapular feathers). In this loafing bird with retracted neck we can't really appreciate the shape of the neck collar, which widens toward the back of the neck (Yellow-billed's collar is widest on the side), and we can barely seen the small collar on the front of the neck below the chin, seen only as a few white spots. This photo also shows clearly how the collar tapers toward the front, and is incomplete, sort of like the breast band of the Snowy Plover.

The second photo shows the loon swimming just in front of a group of Greater Scaups, suggesting an impressive size. The Common Loon is indeed a big bird. The average length is about 32", with a surprising range of 24-39", quite unusual for a single species. The weight average is 9 pounds, with a range of 3.5 to 17.6 pounds! The heavy body helps with diving, and in fact some of the bones are solid, which is unusual for a migratory bird. The molt pattern is also unusual, in that all the flight feather are replaced during a flightless 6 week period, primaries and secondaries at the same time (synchronously), occurring in spring just before migration. The heavy weight of the loon requires a near full complement of flight feathers, which probably explains the unusual molt strategy. The species breeds all across Canada, and in some of our northernmost states. Historically the breeding range extended further south, and included Northern California. They are long-lived, and lay only 2 eggs each year.

You'd think the heavy birds would have trouble in the air, but in fact they are strong fliers and speed along at 75 mph during migration. The adults have few predators, aside from the occasional Sea Otter or Bald Eagle.

The 3rd photo (5943) shows how a black bill can appear silvery in certain light (same can happen with the glossy feathers and bills of ravens and crows). Again note the red iris, as well as hints of color on the head and neck. 

One last photo shows the red eye and the rear-mounted feet. The breeding plumage adults will leave soon, but here we always have some over-summering Common Loons, which are probably subadults or unhealthy birds. They can breed at 2 years of age, but it can take 3-4 years to attain full adult alternate plumage.

And that's what I learned in school today.