Status, trends, and implications for the conservation of Heron and Egret nesting colonies in the San Francisco Bay area

摘要

Nesting distribution, abundance, and reproductive success of colonially nesting herons and egrets in the central coastal region of California surrounding the San Francisco Estuary were analyzed from 1991 to 2005. Nesting activity among nine major wetland subregions was compared with regard to nesting distribution, nest survivorship, productivity of Successful nests, habitat characteristics, and intraseasonal timing. An average of 73 active colony sites y(-1) supported approximately 62 Great Blue Heron (Ardea herodias) colonies, 25 Great Egret (Ardea alba) colonies, 13 Black-crowned Night-Heron (Nyclicorax nycticorax) colonies and twelve Snowy Egret (Egretta thula) colonies each year. Regional nest abundances were stable or increasing in all species. Annual changes in nest abundance were consistent with shifts in colony site preference as wetland suitability and disturbance patterns fluctuate across years and over space. Decreases in colony size were significantly related to higher rates of nest mortality, suggesting that conspecific nest failure may stimulate shifts in breeding distribution. A sharp decline in regional nest abundances Suggested the effects of reduced recruitment associated with increased juvenile mortality in winter, rather than a decline in productivity. Persistence of colony site use increased substantially at sites that reached at least 20 active nests. Subregional increases in nesting abundance coincided with the restoration of tidal marshes. Significant declines in regional reproductive success of Great Blue Herons, Black-crowned Night-Herons, and Snowy Egrets resulted primarily from declining nest survivorship, which coincided with regional increases in abundances of Common Ravens (Corvus corax). We found small but significant Subregional differences in nest survivorship, productivity, and overall reproductive success. Local and subregional productivity of Successful Great Blue Heron and Great Egret nests fluctuated within larger-scale variation across the region, whereas nest survivorship was associated with processes at local or subregional scales.