When breeding in its vast northern range, the Horned Grebe selects small ponds and lake inlets containing a mixture of emergent vegetation and open water, which are used to good advantage: its cryptic, floating nest is usually concealed in the vegetation, and its intraspecific visual signals, including stereotyped breeding displays such as the unique Weed Rush, are presented in the open water. Monogamous and intensely territorial, the species usually nests solitarily or in loose aggregations, occasionally in small colonies. The young are precocial but must be fed and warmed by a parent for a few days after hatching.
Known to most North American field observers in its contrasting dark-and-white Basic plumage, the Horned Grebe acquires warm colors and performs intricate courtship displays in its breeding habitat. Its holarctic distribution and approachability during nesting season have resulted in many intensive life-history studies at a variety of sites in both the New and Old worlds, making it among the better known breeding species of the Northern Hemisphere. Breeding studies in North America include those of Ferguson ( Ferguson 1977b , Ferguson 1981 ), Faaborg ( Faaborg 1976 ), Riske ( Riske 1976 ), Sugden ( Sugden 1977 ), Ferguson and Sealy ( Ferguson and Sealy 1983 ), Heglund et al. ( Heglund et al. 1994 ), and Fournier and Hines ( Fournier and Hines 1999 ); European breeding studies of note include those of Fjeldså ( Fjeldså 1973c , Fjeldså 1973d , and Fjeldså 1973a ), and Ulfvens ( Ulfvens 1988b , Ulfvens 1988a , Ulfvens 1989a , and Ulfvens 1989b ). Food preferences in North America were investigated early on by McAtee and Beal ( McAtee and Beal 1912 ), Wetmore ( Wetmore 1924b ), and Munro ( Munro 1941b ). The foremost behavioral study in North America is that of Storer ( Storer 1969 ).
Notwithstanding these studies, however, knowledge of population numbers in North America, unlike knowledge of population numbers in Europe, is poor. Most populations breed in low densities at high latitudes, making even rough estimates of population numbers difficult to obtain. Horned Grebes are also widely distributed within their North American winter range, rendering population assessment at that season equally difficult. This gap in knowledge is frustrating because for many decades the North American breeding range of the species has been slowly contracting northwestward. Reasons for this contraction are not immediately obvious, but it may signal a decline in numbers, also suggested by Breeding Bird Survey (BBS) and Christmas Bird Count (CBC) results. Thus, even though the Horned Grebe is still one of the most abundant breeding grebes in North America, its contracting breeding range is cause for concern.