Horned Grebe, Fernhill Wetlands, Forest Grove, Oregon on 24 November 2010 by Greg Gillson.
Eared Grebes breed throughout the west from southern Canada to Minnesota, south to Texas, and west to California. In the Pacific NW they breed east of the Cascades and Sierra-Nevada Mountains.
Horned Grebes are more northerly breeders. Though a few isolated pairs may breed south to eastern Oregon, most Horned Grebes breed from southern Canada (British Columbia to the Great Lakes) northward to the Arctic and west into Alaska.
In winter, both species move south to open water. Horned Grebes winter primarily on the ocean and bays from Alaska to California and from Texas to Florida and north along the Atlantic States, also on fresh water in the Southeast.
Eared Grebes tend to winter primarily on open fresh water or estuaries from California to Louisiana, south through Mexico (the National Geographic 5th edition field guide map is more accurate than the big Sibley guide, in this regard).
In the Pacific NW, both grebes winter uncommonly in the valleys west of the Cascades and along the coast. Horned Grebe is more expected, especially along the immediate coast.
In their breeding attire, Eared Grebes are blackish with a wispy yellow ear patch. Horned Grebes are reddish with dark wings and head and yellow "horns" of feathers above and back from the eye.
In "winter" (non-breeding) plumage both species are generally dark gray above and paler below. Horned Grebes tend to be more black-and-white, Eared Grebes more dusky overall, especially on the neck. But first year Horned Grebes can be as dusky-necked as Eared Grebes. Thus, the identification of some individuals can cause confusion--even for experienced birders.
Please compare the photos of the typical non-breeding Horned Grebe above with Eared Grebe below. Both photos are from Forest Grove, Oregon in November, but in different years.
Eared Grebe, Fernhill Wetlands, Forest Grove, Oregon on 27 November 2008 by Greg Gillson.
Field guides might point to the varying contrast between the dark and light shades of the neck. However, you will find identification easier if you concentrate, rather, on the shape of the birds.
Specifically, note on the Horned Grebe the thicker neck and larger head with flatter crown. The bill is pointed, but symmetrical above and below.
In contrast, the Eared Grebe is thinner necked with a smaller head. Note that the crown is highly peaked above the eye, not flat. The bill is straight on the upper mandible and with the lower mandible more angled. This gives the appearance of a thinner, sharper, upturned bill.
Using the shape of the head, bill, and neck, you will be able to separate these birds more accurately, throughout the year.