Productivity of great blue herons in King County, Washington

We studied productivity of great blue herons (Ardea herodias) at their nesting colonies in King County, western Washington in the 2000 breeding season. Rapid urbanization has been hypothesized to have limited heron population growth in recent years. Of the 354 active nests in 2000, 260 (74%) nests produced fledglings. Mean productivity for all active nests was 1.77 (SD = 1.37). Mean productivity for the 260 successful nests was 2.42 (SD = 1.01). Calculations of productivity for the active and successful nests are similar to or greater than results of 8 of 10 similar calculations from studies conducted in the Pacific Northwest, and one study that used band return data to determine the minimum productivity required to maintain a stable population. Herons that began incubating earlier in the season (prior to 17 April) had higher productivity than those that began nesting later in the season (P < 0.001).