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The American Bittern is intimately tied to wetlands throughout the year, including marshes with extensive tall emergent vegetation (e.g., cattails, tules), sloughs, flooded fields (especially where there is a dense cover of tall grass), lake edges, riparian willow thickets, swamps, riverbanks, sewage ponds, and (occasionally) agricultural fields. Cattail marshes are the preferred habitat throughout its range, particularly during the breeding season, and other habitats are used primarily during migration (especially in the fall). Breeding birds typically inhabit relatively large wetlands (>4 ha), and wetland size may limit abundance in a given region.

Source: Campbell et al. (1990a); Lowther et al. (2009)

Breeds from northern Canada (central B.C., northern Alberta, southern Northwest Territories, northern Saskatchewan, northern Manitoba, central Quebec, and Newfoundland) south to central California, New Mexico, Arkansas, West Virginia, and North Carolina. Winters along the Pacific coast north to southwestern B.C. as well as across the southern United States north to southern Nevada, Arizona, New Mexico, and New Jersey. Also winters throughout Mexico, much of Central America (south to Costa Rica), and Cuba.

Breeding
Rare to uncommon (and always highly local) throughout the southern and central interior, from the eastern slopes of the Coast and Cascade Mountains east to the Rockies, occurring north and west to the Bulkley and Skeena Valleys and north in the central interior to Williston Lake (Mackenzie); it is typically rare and irregular at the northern extremes of its distribution. It is also rare to uncommon in the Peace River lowlands of northeastern B.C.

On the south coast, this species is uncommon in the Lower Mainland (east to Chilliwack, with the largest numbers in the brackish marshes of the Richmond-Delta area) and rare in the Nanaimo area of southeastern Vancouver Island. It occasionally (but regularly) occurs south of Nanaimo on southeastern Vancouver Island (Duncan, Victoria) during the breeding season and may be a sporadic breeder in these areas. Casual to very rare north of Nanaimo on eastern Vancouver Island during the breeding season, but breeding in this area has not been confirmed.

Non-breeding
Uncommon in winter in the Lower Mainland, and rare in the Nanaimo area of southeastern Vancouver Island. Very rare in winter farther south on Vancouver Island, although it is regular in the Victoria area (e.g., Swan Lake). There are no winter records for the interior of B.C.

Migration and Vagrancy
This species is difficult to detect during migration, and most observations are of individuals already on breeding territories (particularly during the spring). Nonetheless, it is a rare to uncommon spring and fall migrant throughout the southern and central interior, and very rare to rare on the south coast away from resident populations in the Lower Mainland and near Nanaimo. Northeastern populations (Peace River lowlands) appear to arrive and depart directly from the breeding grounds in this area.

Spring migrants first arrive in the southern interior (e.g., Okanagan Valley, Creston Valley) in early to mid-April, with peak passage occurring during late April and early May. Spring migrants do not reach central B.C. until considerably later, however, with the first individuals arriving in mid- to late May. Arrival times in the Peace River lowlands of northeastern B.C. are slightly later than those in central B.C., with the first individuals arriving in late May. Populations on the south coast are essentially resident, although there may be a tiny (and largely unnoticed) movement of birds north during April and May (exceptionally as early as March).

Individuals begin to depart the breeding grounds throughout the interior in August, but much of the early movement is attributable to post-breeding dispersal rather than true migration. Southward-bound migration occurs through this region primarily during September. Occasional individuals may linger in the central interior into October, and in the southern interior into November; the latest occurrences in the southern interior are in mid-November. Fall migration is more pronounced (and noticeable) on the south coast than spring migration, with small numbers moving through the Georgia Depression between August and November.

This species is a casual summer vagrant in northern B.C. north of its breeding range in central B.C. and the Peace River lowlands, with records occurring north to the Stikine and Nass Rivers in northwestern B.C. and to the Fort Nelson lowlands in extreme northeastern B.C. It is also casual on the Queen Charlotte Islands off the northern coast of the province, with single winter and spring records from the Masset area.

Source: Campbell et al. (1990a)

The American Bittern has suffered significant population declines (-2.4% per year) over the past 3-4 decades in the United States, with some local populations such as those in the north-central states declining by as much as –4.0% per year over this period. Although no conclusive causes for these declines have been determined, ongoing destruction and contamination of marsh habitats is clearly a considerable contributing factor. Curiously, although there have been severe declines in the U.S., populations in Canada have remained essentially stable over the same period. Thus, although this is not a particularly abundant species in British Columbia, it does not appear to be undergoing population declines here as it is in areas farther south. Nonetheless, this species is obviously sensitive to the extent and condition of wetland habitats and, as a result, it is placed on the provincial “blue list” as a species of special concern by the B.C. Conservation Data Centre (CDC). It is not recognized federally as a species of conservation concern by COSEWIC (Committee on the Status of Endangered Wildlife in Canada).

Source: Lowther et al. (2009)

This species is monotypic, with no recognized subspecies. Western populations, including those in B.C., may average marginally larger, redder, and more brightly coloured and have been called B.l.peeti Brodkorb; however, these differences are extremely weak and this subspecies is rarely recognized.

Source: Lowther et al. (2009)