Photo by R. C. Tweit
Black-crowned Night-Herons are unusual among herons in their propensity to eat birds; especially, the nestlings of other colonial waterbirds in the breeding colony in which they nest (Kushlan and Hancock 2005). These nestlings may constitute an important component of the diet of some individuals and their chicks. They obtain nestlings by walking through the colony.
In Texas, an important influence on waterbird populations is the 20-25 year precipitation cycle (Telfair 2002). Between the early 1960s and late 1980s, there was a significant upward trend in the cycle; since the, the trend has begun to decrease (Tom Spencer, Texas Forest Service, pers. comm.). The trend varies regionally and there are intermittent drought years and irregular intervals with pronounced wet/dry springs-summers such as those associated with El Niño/La Niña years and massive slow-moving atmospheric disturbances (Dr. Robert K. Peters, NWS Observer for Tyler, Texas, pers. comm.). Inland wetland habitats are particularly subject to the effects of these trends. Unfortunately, there is a lack of data about the status of Black-crowned Night-Herons in Texas before the 1970s; so, the possible relationship between its breeding populations and the precipitation cycle is unknown for earlier years.
Black-crowned Night-Herons are cryptically-colored and nocturnal. In addition to large colonies in coastal areas, there are many scattered small ones inland and many of these are not censused. Thus, they are probably more common than reported.
DISTRIBUTION: Black-crowned Night- Herons occur in a wide variety of habitats. They prefer vegetated margins of shallow freshwater or brackish rivers, streams, ponds, oxbow lakes, playa lakes marshes, swamps, and mudflats. Human-made habitats including pastures, agriculture ponds, reservoirs, canals, ditches, and aquaculture ponds are also used. Dense cover for day roosts is important (Kushlan and Hancock 2005).
Black-crowned Night-Herons nest in scattered locations in Texas. Based upon Texas colonial waterbird censuses (1973-1990), most of the breeding population is coastal (80-98%). Breeding also occurs in the eastern north- central region, and in the High Plains of the Panhandle (TBBA). In 1933, three nests with eggs were reported at San Elizario in El Paso County in the Trans-Pecos region (Oberholser 1974). In 1991, they established new nesting colonies in the southern and northern Panhandle, in Crosby and Sherman cos, respectively (Lasley and Sexton 1991); in 1994, 25 nests were found at the new reservoir at McNary in Hudspeth County in the Trans-Pecos (Lasley and Sexton 1994); and, in 2002, a pair with two juveniles was discovered at Rio Grande Village in Brewster County, a noteworthy record since there were no previous breeding records for that area of the Trans-Pecos region (Lockwood et al. 2002).
SEASONAL OCCURRENCE: Black- crowned Night-Herons breed from early February to late July with eggs found from February 20 – June 24; and newly hatched chicks as late as June 20 (Oberholser 1974). They are common residents in coastal areas, locally uncommon to common summer residents west of the forested areas of east Texas except in the South Plains and High Plains of the Panhandle where they are fairly common to locally abundant. In winter they are rare to locally uncommon inland except in the Panhandle and forests of east Texas where they are casual visitors (Lockwood and Freeman 2004).
BREEDING HABITAT: The nesting habitats of Black-crowned Night-Herons are highly variable, e.g., shrubs and trees up to 50 m (164 ft) tall, but also in emergent rank vegetation and on the ground. They often nest in rural, suburban, and urban settings and are particularly attracted to nesting in zoos (Kushlan and Hancock 2005). Most colony sites are on islands, in swamps, or over water, suggesting predator avoidance (Davis 1993). Nesting is usually colonial, in single-species or multi-species colonies among other colonial waterbirds; but, Black-crowned Night-Herons form discrete single-species groups within the larger colony and they place their nests in close proximity (Kushlan and Hancock 2005). Some large multi-species colonies are reestablished annually for as many as 30+ years, but others are not. Reasons for this difference are unknown. Some of the large colonies are considered nuisances when they are located near human habitation (Telfair et al. 2000).
STATUS: According to Texas colonial waterbird censuses from 1973-1990, the breeding population of the Black-crowned Night-Heron fluctuated between 453 and 2,090 pairs. This data reveal an annual decreasing trend of -3.5%. Unfortunately, comprehensive inland annual breeding censuses have not been conducted since 1990. However, coastal data between 1973-2000 were analyzed by McFarlane (2002). He found a highly significant annual decreasing trend of -2.2%. Nevertheless, within these time spans, there were years with high census numbers; so, the decline may be related to regional responses to the precipitation cycle (see Introduction and Distribution discussions above). North American Breeding Bird Survey (BBS) data for Texas (Sauer et al. 2005) give annual trends of 17.2% (1966-1979), 3.0% (1980-2005), and 2.1% (1966-2005); thus, indicating an overall upward trend statewide. However, the BBS data for the Coastal Prairies (in agreement with the analysis of MacFarlane 2002) indicate a decreasing trend: -9.0% (1966-1979), -15.6% (1980-2005), and -13.2% (1966-2005).
Text by Raymond C. Telfair II (2007)
Literature cited.
Davis, W. E., Jr. 1993. Black-crowned Night-Heron (Nycticorax nycticorax). InThe Birds of North America, No. 74 (A. Poole and F. Gill, eds.). The Birds of North America, Philadelphia, PA.
Kushlan, J. A. and J. A. Hancock. 2005. The herons. Oxford University Press Inc., New York.
Lasley, G. W. and C. Sexton. 1991. Texas region (summer season). Am. Birds 45: 1135-1139.
Lasley, G. W. and C. Sexton. 1994. Texas region (summer season). Field Notes 48: 960-964.
Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M. University Press, College Station.
Lockwood, M. W., C. E. Shackelford, B. Freeman, and G. Lasley. 2002. Texas (spring migration). N. Am. Birds 56: 324-329.
McFarlane, R. W. 2002. Texas colonial waterbird trends (Powerpoint presentation). http://www.fws.gov/texascoastalprogram/TCWC.htm.
Oberholser, H. C. 1974. The bird life of Texas. University of Texas Press, Austin.
Sauer, J. R., J. E. Hines, and J. Fallon. 2005. The North American Breeding Bird Survey, results/analysis 1966-2005. Version 6.2.2006. USGS Patuxent Wildl. Res. Cnt., Laurel, Maryland. http://www.mbr-pwr.usgs.gov/bbs/bbs.html.
Telfair, R. C. II. 2002. Inland heronries in Texas. Texas Partners in Flight Newsletter, Texas Parks and Wildl. Dept. 9: 20.
Telfair, R. C. II, B. C. Thompson, and L. Tschirhart. 2000. Nuisance heronries in Texas: characteristics and management. 2nd ed. Wildl. Diversity Prog. , Wildl. Div., Texas Parks and Wildl. Dept. PWD BK W7000-134 (1/00).