Development of Nests
Contingency of Nests
Historical nesting of Loggerhead Sea Turtles on the Georgia Coast (after GaDNR).
Sea turtle nesting in Georgia has been tracked since 1964 on Little Cumberland Island, 1965 on Blackbeard Island, and 1973 on Wassaw Island and is now being tracked on virtually all 12 Georgia Barrier Islands. The statistical trend of declining Loggerhead nests became statistically significant in 2004 at a 1.2%/year decline.
Sea turtle nesting on St. Catherines Island has been tracked sporadically since 1984 and continuously since 1989. Since inception of the St. Catherines Island Sea Turtle Program in 1992, 1896 nests have been documented producing 112,025 hatchlings that reached the sea. Loggerheads deposit an average of 112.3 nests per year on St. Catherines Island, with a range of 51 to 184 nests per year. The number of nests per year is highly variable due to turtles nesting during sporadic years (rebuilding resources by taking a year or more off).
Nesting on St Catherines Island is compiled as part of our Summary of Productivity.
The developmental history (contingency) of sea turtle nests has received almost no attention in the literature or on the Internet. It has been observed that the length of incubation is dependent upon nest temperatures, those deposited in the coolest part of the year taking the longest to hatch and those deposited across the hotter part of the year taking the shortest time to hatch. The gender of sea turtles, including Loggerheads, is determined by the temperature of development during the second trimester of development. The temperature at which gender switches is called the pivotal temperature. The pivotal temperature for Loggerheads varies according to location, but in Georgia is 29.9o C. Nests with a lower pivotal temperature produce males and those above the pivotal temperature produce females, as the pivotal temperature is approached the gender of hatchlings becomes variable. This implies that degree-days are one of the determining factors in incubation time.
The Georgia Department of Natural Resources recommends keeping a nest record for each nest on a separate card. The St. Catherines Sea Turtle Program keeps the same information on Monitoring Sheets and formerly kept it on a database, saving space in the field notebook.
Soon, the Georgia Southern University Sea Turtle Program @ St. Catherines Island will develop a database formatted to support the GaDNR Card System and we will be able to visually present individual nest histories downloaded from the database, and, we hope, uploaded onto web pages, either as individual nests, or as aggregate records.
About 1994 we began tracking the number of nests deposited and emerged by day of the year. This led to the ability to graph nests/day against time and integrate the data with rainfall data and summer storms. A crude sliding bar was devised in 1996 (as a separate object in AppleWorks) with a 60-day spacing allowing an estimate of number of emergences that might be expected during the hatching season. This concept seems worth formally pursuing in 2007 in order to calibrate the bar-slider with a script to adjust its length according to degree-days.
Temporal distribution of nests (orange) and emergences (blue) for 1996 St. Catherines sea turtle nests. Hurricane Bertha disrupted nesting in mid-July.
Last updated: 12/15/2014