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. T he mean number of young surviving to 14 days of age in each colony ran ged from 0.47-1.94 per nesting female (includes recycling efforts that involve laying more than 1 clutch). There were marked intercolony dif ferences in clutch size and incidence of recycling. Predation (primari ly avian) was a major factor that adversely affected nest success in 3 colonies and was relatively unimportant in 2 colonies. Residues of DD E, total polychlorinated biphenyls, 2,3,7,8-tetrachlorodibenzo-p-dioxi n, and other compounds in eggs were generally low and apparently had l ittle influence an reproductive success at any of the colonies. Mean e ggshell thinning ranged from 7-11% in comparison to a pre-1947 norm fo r eggs measured in museum collections. Cytochrome P450 enzyme (EROD, P ROD, 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 referen ce 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 decl ine, nest predation rates increased, and nest success decreased at all 3 colonies.