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52 STATE OF THE ESTUARY HERONS & EGRETS CONTEXT The sight of a large, prehis- INDICATOR This indicator is comprised toric looking bird—dark grey-blue or stark white—standing or lifting off from a marsh is magnificent and surprising. Great egrets and great blue herons are used to indicate popu- lation responses to different habitat condi- tions. Great egrets prefer- entially forage in small ponds in emergent wetlands and in areas with shallow, fluctuating water depths. In contrast, great blue herons forage along the edges of larger Photo: Rick Lewis bodies of water and creeks and are less sensitive to water depth. Nesting abundance and success of these two, large wading birds are affected by changes in land-use, hydrology (especially water circulation and depth), geomorpholo- gy, environmental contamination, vegetation characteristics, and the availability of suitable prey. Heron and egret nest abundance is a valuable metric for assessing biotic condition in estuarine and wetland ecosystems. of two indices: heron and egret nest density and nest survival. Analysis uses data from ongoing regional heron and egret studies by Audubon Canyon Ranch. Their repeated annual nest counts at all known colony sites provide extensive measurements of nest abundance and an index of regional breed- ing population sizes. To facilitate comparisons among regions, nest abundance is converted to nest density, based on the number of nests per 100 km 2 within foraging range of suitable wetland feeding areas. Re- sults are provided for each year (1991-2014), for all known nesting colonies in the Central Bay, the North Bay, and Suisun Bay, and the combined area of all three. Nest survival is a measure of nest success. Great egret and great blue heron nests are considered to successfully survive if at least one young survives to minimum fledging age of seven or eight weeks, respectively. The number of young fledged per successful nest 2015 is used, separately, as a food web indicator (see chapter on Processes). Nests are sam- pled in approximate proportion to colony size. In colonies with fewer than 15 active nests, all nests initiated before the colony reaches peak nest abundance are treated as focal nests. At larger colonies, random sam- ples of at least 10-15 focal nests are selected. The nest survival indicator, calculated as the annual, arithmetic mean of apparent nest success, between species, is based on the proportion of focal nests that remain active through the nesting cycle, from nest initiation or early in the incubation period, at 40-50 colony sites within 10 km of the historic tidal wetland boundary. The nest survival index is sensitive to nest predation and colony disturbance by native and introduced nest predators (especial- ly by human commensal species such as raccoons and ravens), land development and human activity near heronries, and severe weather. Such ecological processes can vary over space and time in response to habitat changes, dynamics of predator populations, and changes in human land use, and are therefore likely to differentially affect nesting colonies of herons and egrets. The benchmark for nest density is the average nest density observed from 1991-2000, across the Central Bay, the North Bay, and Suisun Bay. The benchmark for nest survival is the average nest survival from 1994-2000 across the same three regions.