Nesting ecology of Black-crowned Night-Herons at Lake Calumet Wetlands

We examined the nesting ecology of a Blackcrowned Night-Heron (Nycticorax nycticorax) colony located at wetlands associated with Lake Calumet in south Cook County, Illinois, during the 2002 and 2003 nesting seasons. This area of southeastern Chicago has been greatly impacted by heavy industry, solid and chemical waste disposal, urbanization, and altered hydrology. Black-crowned Night-Herons (BCNH) have nested at five known locations at Lake Calumet wetlands during 1984???2003. Emergent cover (giant reed, Phragmites australis) was of primary importance to this colony for nesting during that time. Cottonwoods (Populus deltoides) also were used for nesting from the late 1980s to mid-1990s. During 1993???2003 the herons began arriving at the colony as early as March 10. During the two years of this study the earliest indications of nest building and courtship occurred during the first week in April; the first pairs and precopulatory displays were observed during second week of April in both years. The egg-laying period extended from April 20 to June 12 in 2002, and from April 16 to May 27/28 in 2003. Hatching occurred from mid-May to the first few days of July in 2002, and from mid-May to June 19/20 in 2003. Juvenile dispersal in 2002 occurred from mid-July through late August, and from early July through mid-August in 2003. Reproductive parameters in BCNH nesting at the north end of Indian Ridge Marsh (IRM), the primary nesting location for this colony in both years, were typical for this species. In 2002 the ???recruitment??? rate (number of young/nest surviving to 15 days) of 1.74 young/pair was below the threshold of 2.0???2.1 young/nesting pair thought to be necessary to maintain BCNH populations. However, recruitment increased to 2.22 young/ pair in 2003, which was among the highest previously reported. The most important cause of nest failure was poorly constructed (flat) nests which allowed the eggs to roll out into the water. Although some eggs were lost to gulls and some hatched young were taken by unknown mammalian or avian predators, predation was not an important cause of nest losses at IRM.published or submitted for publicationis peer reviewe