Reproduction of Black-crowned Night-Herons related to predation and contaminants in Oregon and Washington, USA

We studied reproductive characteristics of Black-crowned Night-Herons (Nycitcorax nycticorax) at 4 colonies in south central Washington and 1 colony in north central Oregon in 1991. Nest success, adjusted using the Mayfield method, was significantly different between colonies and ranged from 12-84% to hatching and 12-73% to 14 days post-hatching. The mean number of young surviving to 14 days of age in each colony ranged from 0.47-1.94 per nesting female (includes recycling efforts that involve laying more than 1 clutch). There were marked intercolony differences in clutch size and incidence of recycling. Predation (primarily avian) was a major factor that adversely affected nest success in 3 colonies and was relatively unimportant in 2 colonies. Residues of DDE, total polychlorinated biphenyls, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and other compounds in eggs were generally low and apparently had little influence an reproductive success at any of the colonies. Mean eggshell thinning ranged from 7-11% in comparison to a pre-1947 norm for eggs measured in museum collections. Cytochrome P450 enzyme (EROD, PROD, and BROD) induction in livers of pipped embryos by colony ranged from low to average in comparison with other colonies throughout the U.S. Average EROD and BROD activities were highest at Sand Dune island and were lowest at Potholes Reservoir which was designated the reference colony In relation to our study of 3 of the 5 colonies in the early 1980s, residues of DDE and several related compounds appeared to decline, nest predation rates increased, and nest success decreased at all 3 colonies.