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Since the first documented successful nesting of Wood Storks (Mycteria americana) in South Carolina in 1981, a total of 16,642 nesting attempts have been recorded. During the past 26 years, annual nesting has increased from eleven to a high of 2,057 nests in 2004. Storks have used 27 different colony sites, however, five sites have supported 76.3% of all nesting attempts. Mean colony size was 118 nests (range 1-547). Colony turnover rate was 0.19 overall based on annual calculations. However, 95.6% of colonies with more than 100 nests in one year were active the following year (N = 68). Only three of 27 sites are in public ownership and 21 sites are in wetlands altered or maintained by man. Production of young has been high each year (x bar = 2.08 young per successful nest) and abandonment of active colonies has been rare. This may result from the availability of varied habitats used by foraging storks. Nesting storks use palustrine habitats associated with rivers, inter-tidal wetlands, isolated wetlands and marsh impoundments. The variety of habitats used, combined with the topography of our coastal plain, provide adequate foraging habitat under a wide range of rainfall conditions.
Wood Storks (Mycteria americana) were first recorded nesting in Georgia in 1965 at Blackbeard Island National Wildlife Refuge (Harris 1995). In 1992, biologists initiated annual aerial surveys of known wading bird rookery sites to document Wood Stork distribution and estimate the number of nesting pairs in Georgia. Additionally, personnel from the U.S Fish and Wildlife Service and Savannah River Ecology Laboratory have contributed annual ground-count data for several colonies. Since surveys began, stork nesting has been documented at least once in 43 locations across the Coastal Plain of the state, with an average of 14 active sites per year. The documented nesting effort for storks has increased from four pairs in one wading bird colony in 1965 to a high count of 1,817 pairs in 19 colonies in 2005. A regression of log-transformed nest totals for the 14-year time-series (1992-2005) showed no significant overall trend in nesting pair numbers (P = 0.84) for that period. However, it is possible that an ongoing rebound from a drought-related crash could continue, resulting in an increasing trend. Thirty-three (77%) of stork nesting locations were on private land, with only ten (23%) nesting locations in public ownership. Seven colony sites have supported an average of 68% of the stork nesting effort since 1992; in 2005 these sites supported 77% of the nesting effort. Although not a systematic census of all potential stork nesting habitat, these standardized surveys have produced a useful estimate of Wood Stork population size, distribution, and stability in Georgia. We believe that Georgia’s Wood Stork population has shown an increase in size and distribution, and that numbers can continue to increase or stabilize as long as hydrologic conditions favorable to the species persist.
Wood Storks (Mycteria americana) breed in colonies widely dispersed across approximately 3,350 km2 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 10 years, >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 4 years). Novel systematic aerial transects across suitable habitat in central Florida revealed approximately one novel colony/525 km2. 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 years, 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 km2) in areas with suitable habitat and historically high colony densities.
Determination of breeding success rates of endangered species such as the Wood Stork (Mycteria americana) and reasons for their variation are vital information needed for monitoring recovery efforts. To provide information in the northern portion of their breeding range, breeding success rates were monitored for nine stork colonies in Georgia and South Carolina in 2004 and 2005. Overall success rates were very high in 2004 (N = 421, x̄ = 2.3 fledged young per nesting attempt), with slightly greater success in inland colonies as compared to coastal colonies. Overall success rates declined in 2005 (N = 359, x̄ = 1.6 fledged young per nesting attempt), with slightly greater success in coastal colonies as compared to inland colonies. Reduced success in 2005 may be due to elevated mid-breeding season rainfall and its negative effects on prey availability. Examination of longer-term breeding success at multiple Georgia stork colonies relative to annual rainfall showed no consistent trends between success and mid-season rainfall. A moderate positive association was found between pre-breeding season rainfall and success for coastal, but not inland, colonies.
The combined rate for 21 Wood Stork (Mycteria americana) colonies during 2003-2005 was 1.19 ± 0.09 fledglings/nest (N = 4,855 nests). The mean fledging rate of 0.61 fledgling/nest for colonies during 2005 was significantly less than the rate of 1.47 fledglings/nest in 2003 and 1.43 fledglings/nest in 2004. A comparison of the annual fledging rates for a subset of 13 colonies monitored during all three years indicates the overall probability of fledging >1 nestling was significantly greater in both 2003 (69.7%) and 2004 (69.8%) than in 2005 (38.5%). Though the probability of fledging one stork was similar among years (range 9.7-12.0%), the probabilities of 0, 2, or >3 fledglings were significantly different in 2005 than in either 2003 and 2004. Colony size and longitude had no significant effect on mean colony productivity. However, latitude did have a significant positive effect on colony productivity. Thus, colonies farther north in Florida tended to exhibit greater productivity. The colonies in the northeastern and northwestern regions of the state generally had greater fledging rates than did colonies farther south in Florida, especially the southeast region. This study also identified objectives for management activities and future research. There is a need to develop an unbiased estimator of productivity that takes into consideration the lack of nesting during some years, which would allow managers to more accurately estimate stork productivity at the regional level. Because some colonies out perform others, an index-type system of colonies should be developed to designate colonies that perform above a modeled fledging rate, thus offering long-term insight into stork productivity. A re-examination of the productivity indices listed in the Wood Stork Recovery Plan is needed to ensure the current status of the U.S. breeding population.
Wood Storks (Mycteria americana) breeding in the southeastern United States exhibit lengthy inter- and intra-regional movements in response to resource availability. One type of movement, post-breeding dispersal, has resulted in storks from this population temporarily moving into the Gulf Coast states of Alabama and eastern Mississippi. Concurrently, other Wood Storks, presumably of Mexican/Central American origin, are frequently observed in large numbers in the Mississippi River Valley and nearby western states during the late-summer months and the proximity of dispersing storks from the two regions might result in population mixing. In a preliminary examination of the origins of Wood Storks observed in Gulf Coast states and the likelihood of population mixing, we deployed ten satellite transmitters on storks in those areas in June-July of 2003 and monitored their movements. All storks captured in eastern Mississippi moved into southern Florida. Storks captured in western Mississippi went to eastern Mexico or western Guatemala. One stork from Louisiana went to Mexico and the other went to southern Florida, suggesting that population mixing may be occurring on their non-breeding season habitats.
We modeled population dynamics and extinction probabilities for the endangered Wood Stork (Mycteria americana) using count data from synoptic aerial surveys, annual measures of productivity from throughout the southeastern U.S., and survival data from satellite-tagged juveniles. Using a simple, count-based diffusion approximation approach we were able to quantify an increasing population trend since 1976. High inter-year variability resulted in wide confidence intervals and we could not eliminate the possibility of long-term population decline in spite of recently measured population increases. We also used a stage-based population matrix model to incorporate observed differences in survival rates among age classes. Fledging success, and survival of fledglings, one, and two-year-old birds were estimated using data from the satellite telemetry study. Because the survival rates of three-year-olds and adults are presently unknown, we analyzed population dynamics over a range of these values. Long-term population growth was most sensitive to changes in adult survivorship. This deterministic matrix model indicated that adult survival rates >0.94 were necessary to maintain a growing population, an estimate considerably higher than that observed in the European White Stork (Ciconia ciconia). This study underscores the need for reliable estimates of juvenile and adult survival in Wood Storks, and for a conservation focus on the factors that affect adult survival. It also provides a tool for understanding and projecting potential trajectories of the Wood Stork population in the Southeastern United States.
In 1984, the U.S. Fish and Wildlife Service listed the U.S. breeding population of Wood Storks (Mycteria americana) in their known range of Florida, Georgia, South Carolina, and Alabama as “endangered” under the Endangered Species Act, due to a population decline of 75 percent from the 1930s to the 1980s. In 1997, an updated Recovery Plan for the Wood Stork established two metrics to for determining the biological status: numbers of nesting pairs and regional productivity measured over time. Five-year averages of nesting pairs from annual aerial surveys show an increasing trend (1981-85, 1991-95, 2001-2005). Since listing in 1984, the average number of active colonies has almost doubled, however colony size has decreased. Current productivity estimates are similar to those recorded in the 1970s and 1980s. Monitoring continues to indicate that nesting numbers and productivity rates are variable within and among years and within and among colonies. Variability is likely related to environmental factors affecting the quality of local and regional nesting and foraging habitats. Loss of natural foraging habitat continues to be a major threat, while the effects of the increasing number of constructed wetlands and their use as foraging areas is unknown. Habitat Management Guidelines are being updated and will address: managing colony sites, controlling predation and human disturbance, and core foraging habitat issues. One of the most important initiatives is the Comprehensive Everglades Restoration Plan (CERP), which has identifies the restoration of healthy wading bird populations as an expected benefit. It is anticipated that CERP will be a large contributing factor toward the recovery of this species.