Longevity and size of Wood Stork (Mycteria americana) colonies in Florida as guides for an effective monitoring strategy in the southeastern United States

摘要

Wood Storks (Mycteria americana) breed in colonies widely dispersed across approximately 3,350 km(2) within the United States, and effective monitoring of this population presents immediate tradeoffs between coverage, accuracy, and cost. Here, we summarize surveys in Florida 1991-2005 as a first step towards improving existing survey strategies. In order to determine whether counts from aircraft are a suitable technique for quantifying nests, we compared aerial and ground counts at the same eleven colonies in 2004. Across all colonies, aerial counts averaged 8.1% more nests, probably as result of either better visibility or mistakenly including Great Egret (Ardea alba) nests in the count. During the period 1991-2005, statewide totals in Florida ranged from 2,211-6,449 nests, with an apparently increasing trend through time. Annual modal colony size fluctuated from 65-144 nests, with significantly smaller modal size in 2001-2004, suggesting that colony size has decreased over time. Current survey practices are to visit all previously active colonies and all new ones that are reported or that are encountered during flights between known colonies. Surveys are not systematic, and the number or importance of novel, undetected colonies is unknown. In south Florida, where past and potential colony sites have been systematically surveyed annually, turnover (proportion of colony sites different in two surveys) increased rapidly with interval between surveys, and within ten y, >80% of colony sites differed. Annual turnover rates were not uniform across years, and young colonies appeared to have higher turnover than older ones (up to four y). Novel systematic aerial transects across suitable habitat in central Florida revealed approximately one novel colony/525 km(2). Thus, abandonment of old colonies and formation of new ones is a typical and fairly rapid process in this species. Throughout the state, larger colonies were more persistent, and were surveyed more often than small colonies. The bias of the current nonsystematic survey strategy is towards visiting older colonies that are likely to disappear within 15 y, and against finding newer, growing colonies. This is likely to bias estimates of total population downwards compared to true values. We strongly recommend that surveys be geographically systematic, even if this reduces coverage. We suggest these systematic surveys be located in large blocks (hundreds or even thousands of km(2)) in areas with suitable habitat and historically high colony densities.