Clark's Grebe
Aechmophorus clarkii
Clark's grebe closely resembles the western grebe and occurs in the same colonies together with it. Storer and Nuechterlein in 1992, following earlier morphological studies by Storer and others, define the species as being distinguished from the western grebe by an overall paler plumage on its back, as well as a larger portion of its face covered in white, as it extends above the eyes, rather than just below them. A distinguishing feature is its bill, which is bright yellow in the US, whereas the Western Grebe's bill is greenish-yellow in the US, which had been noted by others. Voice: The calls of Clark's grebe are similar to its western counterpart, however, during courtship the birds make an 'advertising call' to attract mates—this sounds like a single, extended kreeeed as opposed to the two-note kreed-kreet of the western grebe. The calls of the grebe tend to vary very little between sexes.
ID text from Wikipedia, CC BY-SA 4.0
Western Grebe
Aechmophorus occidentalis
The western grebe is the largest North American grebe. It is 55–75 cm (22–30 in) long, weighs 795–2,000 g (1.753–4.409 lb) and measures 79–102 cm (31–40 in) across the wings. It is black-and-white, with a long, slender, swan-like neck and red eyes. It is easily confused with Clark's grebe, which shares similar features, body size, behavior and habitat, and hybrids are known. Clark's grebes generally populate more of the southern part of North America Other differences are whiter flanks and paler gray backs when comparing Clark's grebe to the western grebe.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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