The Black-crowned Night-Heron (Nycticorax nycticorax) is the most widespread heron in the world, breeding on every continent except Antarctica and Australia, where the genus is represented by the Nankeen (or Rufous) Night-Heron (Nycticorax caledonicus). Although widespread and common in North America, its coloration and behavior, as well as its nocturnal and crepuscular feeding habits -- especially outside the breeding season -- render it less noticeable than many diurnal herons. This heron is an opportunistic forager that feeds on a wide variety of terrestrial organisms, but its diet consists primarily of fish and other freshwater and marine organisms.
This night-heron's diet has magnified its exposure to contaminants, especially DDT, a persistent organochlorine pesticide that appears to have caused reproductive failure in some populations and may have contributed to subsequent local population declines in the 1960s. Since then, extensive sampling has shown that while some populations continue to accumulate contaminants, these appear to have had minimal effect on breeding success and population levels.
Currently, populations in some areas are declining for a variety of reasons, mostly habitat-related, prompting a handful of states to list the species as threatened or endangered. With a few exceptions, however, North American populations are generally considered to be stable or increasing.
Black-crowned Night-Herons may be vulnerable to changes in climate, however. Of major concern are sea-level rise and the potential for increased temperature and changes in precipitation, which would reduce shallow marsh foraging areas, modify the availability and the timing of prey, and shift the distribution of predators. Henny et al. 2002
This species has been often used as an indicator of environmental quality, primarily because it is an upper trophic level bird that nests colonially, has a wide geographic distribution, and tends to accumulate contaminants ( Custer et al. 1991 ). In addition, its piscivorous feeding habits, flexibility in selection of nesting and foraging habitats, tolerance of degraded habitats, and ability to habituate to certain forms of disturbance (e.g., vehicular traffic) make the species particularly well-suited for service as a sentinel of environmental contamination in urban environments ( Levengood et al. 2007 ). As such, the literature is rich with references to the use of the species as an indicator of a wide range of contaminants worldwide (e.g., Fasola et al. 1998 , Custer et al. 1994 , Kim and Koo 2007 , Henny et al. 2002 , Henny et al. 2007 , Rattner et al. 2000 ).
For naturalists who enjoy the shore and marsh, this heron's distinctive call is a quintessential sound of dusk and night. In Life Histories of North American Marsh Birds, A. C. Bent ( Bent 1926 ) eloquently described the night-heron as it leaves to feed: “How often, in the gathering dusk of evening, have we heard its loud, choking squawk and, looking up, have seen its stocky form, dimly outlined against the gray sky and propelled by steady wing beats, as it wings its way high in the air towards its evening feeding place in some distant pond or marsh!”