Productivity of Wood Storks in Florida

The primary goal of this study is to gather productivity data for storks nesting in Florida in order to examine the variation and trends in fledging success within and among colonies and years.

CURRENT FWC PROJECT

The current Florida Fish and Wildlife Conservation Commission's (FWC) research project is entitled "Productivity of Wood Storks Mycteria americana in North and Central Florida." The primary participants are James A. Rodgers and Steve Schwikert working out of the Gainesville Wildlife Research Laboratory. Rodgers and Schwikert are assisted by numerous volunteers and cooperators working around the state.

One of the objectives of the Wood Stork Recovery Plan developed by the U.S. Fish and Wildlife Service, identified productivity levels exceeding a minimum standard to ensure continued viability of the stork population in the United States. Specifically, knowledge of the number of fledged young per nest, or birds mature enough to leave the next, must be determined for a representative number of colonies during the same year for a minimum of three years. The reclassification from endangered to threatened could be accomplished when there are 6,000 nesting pairs and annual productivity is greater than 1.5 fledglings per nest calculated over a three-year average.

Thus, the primary goal of this study is to gather productivity data for storks nesting in Florida in order to examine the variation and trends in fledging success within and among colonies and years. The project duration will include the breeding seasons from 2003 to 2009. The data on the reproductive success of the north and central Florida stork colonies will be compared with the storks in the southeastern United States by examining the effects of colony size and geographical location on breeding success within and among colonies and years. These data ultimately will be used to determine if the stork population in the United States meets recovery criteria for downlisting the species.

Study methods

Between 15 to 20 active wood stork colonies in north and central Florida are being monitored by FWC personnel for 3-5 continuous nesting seasons. These sites include both marine estuarine sites along the coast and interior freshwater sites. Concurrent studies are occurring in both Georgia and South Carolina.

Field research entails marking and monitoring nests from March to August. Either all nests (colonies less than #100 nests) or a sample of the nests (for example, 25-50 percent of nests at larger colonies) are monitored on a biweekly schedule during the breeding season. The study is designed to monitor stork colonies with differences in nest numbers and densities, at interior freshwater and coastal marine-estuarine sites, and with latitudinal and longitudinal dispersal throughout Florida. Care is taken to reduce researcher's effects on the breeding storks and other species of colonial water birds by minimizing nest monitoring during pair-formation and early egg-laying periods. Colonies also are visited during the cooler morning and late afternoon and no visits occurred during inclement weather. The time spent at each nest is further minimized by use of two people to observe and record data and map nest distribution. After the nestlings are three to four-weeks old, they are counted from a distance with binoculars to avoid pre-fledging of nestlings.

Wood stork nestlings

Results of the 2003 Breeding Season

The average fledging rate (number of large, fully feathered birds attaining 7-8 weeks of age) of wood storks at 14 colonies in north and central Florida was 1.49 fledglings for each nest during the 2003 breeding season. For only successful nests (nests that fledged at least one stork), the average fledging rate was 2.15 fledglings per nest. About 70.8 percent of monitored nests fledged at least one bird.

Significant differences existed among colonies in the average fledging rate, which ranged from 0.21 to 2.21 fledglings a nest. A cluster of colonies in Pasco and Hillsborough counties (Cypress Creek north of Tampa and in New Port Richey) in the west-central region and the Jacksonville Zoo colony, located north of Jacksonville in the northeast region, exhibited the greatest fledging rates. Colonies that exhibited low fledging success appeared to be widely distributed in both north and central Florida. However, the two most northern and western colonies of Ochlockonee North and Chaires, found north of Tallahassee in Leon County, both exhibited below average fledging success.

An examination of the distribution of the number of fledglings per nest provided more insight into the fledging success within each colony. Cypress Creek and Jacksonville Zoo exhibited high fledging rates due to low numbers of complete nest failures (which means no fledglings) and high numbers of three-fledgling nests. In contrast, Chaires exhibited a low fledging rate due to low numbers of two-fledgling and three-fledgling nests and high numbers of complete nest failures.

Nest failures appeared to be evenly distributed during the breeding season among most colonies. However, there were three colonies that exhibited a sizable number of nests failures associated with severe weather (wind speeds exceeding 20 mph and rainfall exceeding one inch per hour) during a short interval. Chaires (prior to May 13), Dee Dot south of Jacksonville (prior to May 23), and Croom in Hernando County (prior to June 17) all experienced a large number of nests that either were abandoned or collapsed as evidenced by unattended nests, fallen nest structures, or dead nestlings under the nest trees. The low fledging rate at Devils Creek in Pasco County was the result of an apparent abandonment of all but two of the nests by parent birds.

Results of the 2004 Breeding Season

The average fledging rate of wood storks at 19 colonies in north and central Florida was 1.53 fledglings per nest during the 2004 breeding season. For only successful nests, the average fledging rate was 2.07 fledglings at each nest. About 71.3 percent of monitored nests fledged at least one bird.

Significant differences in the average fledging rate existed among colonies (range=0.25 to 2.37 fledglings per nest) during 2004. Colonies exhibiting higher or lower fledging rates again appeared to be evenly distributed across north and central Florida.

An examination of the distribution of the number of fledglings per nest provided additional insight into the fledging success within each colony. Jacksonville Zoo and Chaires exhibited high fledging rates due to low numbers of complete nest failures and above higher numbers of two-fledgling and three-fledgling nests. In contrast, Bird Island near Vero Beach, Lake Russell near Kissimmee, Little Gator Creek near Dade City, Matanzas Marsh near St. Augustine, and Pelican Island near Sebastian exhibited low fledging rates due to low numbers of two-fledgling and three-fledgling nests and high numbers of complete nest failures.

Jacksonville Zoo again possessed the greatest fledging rate in 2004. While several colonies rebounded and exhibited higher fledging rates (Chaires, Devils Creek), other colonies (Little Gator, Lake Russell, Matanzas Marsh) exhibited lower productivity in 2004 compared to 2003. Several stork colonies exhibited noticeably fewer nests in 2004 compared to 2003. Pumpkin Hill north of Jacksonville had no nesting activity in 2004 compared to 2003, which had 120 nests. Colonies that possessed fewer nests in 2004 included Cypress Creek (175 nests in 2003; 59 in 2004), New Port Richey (225 nests in 2003; 178 in 2004), Lone Palm in Lakeland (175 nests in 2003; 82 in 2004), and Lake Rosalie near Lake Wales (125 nets in 2003; 46 in 2004).

Results of the 2005 Breeding Season

The average fledging rate of wood storks at 22 colonies in north and central Florida during 2005 was 0.49 fledgling per nest. For only successful nests (a successful nest is one that produces at least one stork), the average fledging rate was 1.88 fledglings for each nest. Only about 42.3 percent of monitored nests fledged at least one bird. Significant differences in the mean fledging rate existed among colonies (range=0 to 2.40 fledglings a nest) during 2005. Colonies exhibiting higher fledging rates were concentrated in the northeast regions of the state (Jacksonville Zoo, Dee Dot, Pumpkin Hill) and northwest (Chaires, Ochlockonee North, Ochlockonee South). Whereas, colonies possessing lower fledging rates appeared to be evenly distributed across north and central Florida, three of four colonies monitored in the southeast region were at or below this index.

Several unusual events occurred during the 2005 nesting season. Storks started nesting at Little Gator Creek in Pasco County but abandoned the 12-15 nest starts and the entire breeding season prior to mid-March. Storks abandoned 10-12 nest starts at Ochlockonee South, located north of Tallahassee about mid-March, began renesting in early April, and exhibited a partial abandonment prior to mid-June. About 80 of 154 monitored nests (51.9 percent) at Croom were abandoned prior to mid-May. Storks abandoned 41 of 42 nests (97.6 percent) at Matanzas prior in late-June. All 29 nests at Pelican Island were abandoned. Finally, no storks nested at Devils Creek. An examination of the distribution of the number of fledglings per nest provided further insight into the fledging success within each colony. Jacksonville Zoo, Chaires, Ochlockonee North, and Dee Dot exhibited high fledging rates due to below average number of complete nest failures (or no fledglings) and above average number of two-fledgling and/or three-fledgling nests. In contrast, Bird Island, Lone Palm, Deseret Ranch near Holopaw, Matanzas Marsh, and Pelican Island exhibited low fledging rates due to below average number of two-fledgling and three-fledgling nests and above average number of complete nest failures. The one bright spot in the 2005 nesting season was the renesting of storks at Pumpkin Hill after no nesting in 2004.

The 2005 nesting season followed an above average year of rainfall and hurricane activity. Storm damage to nesting trees may have precluded storks nesting at Devils Creek. Inclement weather resulted in the eventual collapse during the nesting season of a cypress tree and contributed in large part to the overall low productivity rate at Lake Rosalie. The elevated water levels at these interior freshwater sites in 2005 would not directly disrupt nesting within the colony as occurred at Little Gator Creek and Ochlockonee South, and partial abandonment as occurred at Matanzas and Croom. Rainfall also did not directly affect the nesting failure at the marine estuarine site of Pelican Island. However, large amounts of rainfall and the elevated water levels associated with the foraging sites around a colony are suspected to cause the over-dispersal of aquatic freshwater prey and result in non-breeding or abandonment of nesting by storks.

Results of the 2006 Breeding Season

In a slightly modified area of coverage, the average fledging rate of wood storks within north and central Florida during 2006 was 2.25 fledglings/nest (n=889 nests at nine colonies). For only successful nests (fledged at least one stork), the average rate was 2.59 fledglings/nest (n=776 nests). About 87.9 percent of monitored nests fledged at least one bird and 83.4 percent of nests fledged 2+ birds. Significant differences in the mean fledging rate (range=0 to 2.59 fledglings/nest) existed among colonies. Differing rates among colonies were due to different frequencies of complete nest failures (no fledglings) and nests with two fledglings and/or three-fledglings. The three highest fledging rates were at colonies along the southeastern coastal region of the district, whereas the three lowest fledging rates occurred in the northeast part of the state. Water levels were generally lower during the 2006 nesting season due in part to the low rainfall during 2005-2006. Most stork colonies exhibited their greatest fledging rates in 2006 compared to previous years. However, lack of flooded nest trees resulted in the lack of breeding storks at the Pumpkin Hill colony. The failure of most nests at Matanzas Marsh was associated with low water levels and depredation, or plundering, of nests by raccoons.

Results of the 2007 Breeding Season

The average fledging rate of wood storks within north and central Florida during 2007 was 0.71 fledgling/nest (n=186 nests at six colonies). Only about 50.0 percent of monitored nests fledged at least one bird and only 31.1 percent of nests fledged more than two birds. Significant differences in the mean fledging rate existed among colonies (range=0.27 to 1.45 fledglings/nest) during 2007. There also were fewer nests recorded at these six active colonies in 2007 compared to all previous years. Six other colonies active 2006 contained no stork nests in 2007. The reason for lack of nesting at three of these colonies probably was due to little or no water present under the nest trees (primarily cypress trees) prior to the nesting season. The reason for no nesting at the other sites is unclear but these colonies have exhibited a continued decrease in nest numbers during recent years. A comparison of the combined fledging rate for all colonies within the north and central Florida region indicates the rate in 2007 was the second lowest fledging rate recorded during the past four nesting seasons of this study.

Results of the 2008 Breeding Season

The average fledging rate of wood storks (Mycteria americana) within the St. Johns River Water Management District (SJRWMD) of north and central Florida during 2008 was 1.51 fledglings/nest (n=419 nests at six colonies). About 72.3 percent of nests fledged at least one bird and 60.8 percent of nests fledged more than two birds. Significant differences in the mean fledging rate existed among colonies (range=1.02 to 2.13 fledglings/nest) during 2008. There were more nests per colony and higher fledging rates at these six active colonies in 2008 compared to 2007. Three other colonies active in previous years were not active in 2008. The reason for no nesting at these sites is unclear but may be due to no water present under the nest trees prior to the nesting season or little flooded foraging habitat. These colonies have exhibited a continued decrease in nest numbers during recent years. A comparison of the combined fledging rate for all colonies within the SJRWMD region of Florida indicates the fledging rate in 2008 was in the middle of the range of fledging rates recorded during the past five nesting seasons.

Graph of wood stork colonies

Annual number of wood stork nests recorded at colonies in the SJRWMD region of North and Central Florida. Colony abbreviations are as follows: PH=Pumpkin Hill, JZ=Jacksonville Zoo, DD=Dee Dot, MM=Matanzas Marsh, LD=Lake Disston, HO=Hontoon Island, HI=Horseshoe Island, DR=Deseret Ranch, KR=Kemper Ranch US192S, PI=Pelican Island, NF=North Fork, and BI=Bird Island.

Acknowledgments

This project is a cooperative effort among numerous agencies and individuals including Gabrielle A. Griffin (Florida Institute of Technology), Donna Bear-Hull (Jacksonville Zoological Gardens), Bill Brooks (U.S. Fish and Wildlife Service), Paul Elliot (Southwest Florida Water Management District), Gian Basili and Becky Trudeau (St. Johns River Water Management District), J. B. Miller (Florida Department of Environmental Protection), Kristin Ebersol (Florida Department of Environmental Protection), and Amy Kalmbacher (Florida Department of Environmental Protection). We also wish to thank those individuals, organizations, and agencies that allowed us access to the colonies on their properties or under their jurisdiction: Dee Dot (Keith Kelly and Dee Dot Timberlands, Inc.), Croom (Vincent Morris and Florida Forest Service), New Port Richey (Al Lolli and Ken Tracy), Lake Rosalie (Bob Armington and the Armington family), Lake Russell (Sandy Woiak and The Nature Conservancy), Cypress Creek (Jill Lehman and Hillsborough County Parks and Recreation), and Lone Palm (Joe Hodge, Lone Palm Golf Club). Numerous other individuals assisted in the collection of data, especially Bill Brooks, Bert Charest, J.B. Miller and LeAnn White. Funding was provided in part by grants from the U.S. Fish and Wildlife Service in the Jacksonville Field Office and the St. Johns River Water Management District in Palatka.



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