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About Our Wood Stork Tracking Program

The need for information about movements and survival of storks

Wood Storks have undergone very sharp declines in population since the 1960’s, and though their numbers appear to have stabilized, the goal is now to understand the species well enough to cause populations to increase. We know a lot about the nutritional requirements and nesting success of this species, but there is still very little information about survival, habitat use, and movement patterns throughout their range.

One persistent question is whether the species is producing enough young to keep the population levels stable or increasing – this is basic demography. To answer this, we need to know how many young are being produced (good data and monitoring programs exist for the most part) and how long individuals survive as young and adults (little or no data here). So there is an obvious need to measure survival rates of this species.

Unlike a lot of endangered species, this is a wide-ranging animal, and there are also plenty of questions about its movements and habitat needs. Where do they go after they leave the nesting colony in the summer? What habitats do they use? Do their needs change throughout the year? Are they protected in these areas? Do they mix with populations from Mexico or use habitats in the Caribbean?

Designing the study

The only way to get these kinds of information is to follow the fates and habits of individually marked individuals. For most animals, habitat characteristics are crucial in determining how well they breed and survive, so the survival and habitat use information may complement each other. We hope to use information about individually tracked storks to 1) build a population response model, and 2) determine what kinds of habitat are used, and important for survival, especially during the first year of life. As the first few months of a bird’s life are often the period of time when birds experience their highest mortality (imagine a toddler out roaming the streets of New York on its own!), we decided to focus our efforts on juvenile storks (ones that hatched within the year). We know from previous studies that the birds are likely to move extensively during the non-breeding season – maybe several hundred kilometers.

How do you follow a long-distance gypsy?

This movement behavior presents a major obstacle for researchers – how do you follow them? Previous work has shown that banding and tagging of storks is a lot of work for very little information – and it relies on observers being in the right place at the right time, and requires that huge numbers of birds be marked. Radio transmitters have been used for decades by wildlife biologists, but they only can be detected at a distance of a couple of kilometers. If the birds move around in a space the size of three southeastern states, it would be like finding needles in a large haystack. Finally, there are transmitters that transmit a signal which can be detected and located by orbiting satellites – these can be detected remotely anywhere in the world.

Our study is funded by the U.S. Army Corps of Engineers (Jacksonville District) and the U.S. Fish and Wildlife Service as part of an effort to better understand and manage wetlands in the Everglades and the southeastern United States.

How do satellite transmitters work?

Satellites that orbit the Earth pick up the information sent out by the transmitters. These satellites, owned by a French company named Argos, can locate and receive data from around the world. This information is then beamed back down to a collection station in Maryland, USA that houses powerful computers. These computers process the data, sending us a daily email detailing the locations of our storks. A more thorough description of how these satellites work and explanation of the Doppler Effect can be viewed at this site.

Although there are many types of satellite transmitters used in research today (see section on other tracking projects), we had to find some that were very light – less than the 3% of bird’s body weight recommended by the U.S. Fish and Wildlife Service. Our satellite transmitters are made byMicrowave Telemetry, Inc. and weigh only 35 grams. In an effort to maximize their life span and yet reduce the need for a heavy battery, we use solar powered transmitters. Non-solar-powered transmitters of roughly the same size have a typical life span of 1-2 years, while our solar-powered transmitters are thought to last at least 3 years. Similar transmitters used on White Storks in Europe are still working after 6 years. We are hoping that this will allow us to follow some of our birds into reproductive maturity (approximately four years of age).

Our transmitters were worn on the bird like a backpack. The ‘straps’ of the backpack are looped under each wing and were made of Teflon ribbon. The two straps also crossed on the chest of the bird to give a more secure fit. The backpack will eventually fall off because it is held together at a single point by cotton stitching, which rots over time. A thin piece of rubber padding was attached to the bottom of each transmitter to avoid abrasion on the back.

Here is Becky Hylton holding one of the young storks, showing the yellow bill typical of first-year birds, the fuzzy down on the back of the head identifying it as a large nestling, and the solar-powered satellite transmitter with antenna on its back.

Study findings and progress

During 2002, we placed 33 transmitters on juvenile storks that had hatched in April – June at a single colony in the Everglades. Of these, five died while in the colony, and 28 fledged successfully. Of the birds that left the colony, seven have died as of early September 2002. This means that the postfledging survival is about 75%, much higher than we anticipated. At this stage we believe this is a particularly good year for storks fledging from south Florida, since we know of other years when virtually no birds even survived to leave the colony.

To see where the young storks headed after they fledged from the colony, look at some of the maps of where these birds are.

Who’s involved in this study?

Dr. Peter Frederick is a professor in the Department of Wildlife Ecology and Conservation at the University of Florida. He has been working in the Everglades for 15 years studying wading birds and the ecology of wetlands in various locations around the world.

Becky Hylton works with Dr. Frederick as a Master of Science student. She has been working with wading birds in the Everglades for 3 years, and heads up the field aspect of this project.