Jaci's project looks at movement of individual Swainson's Thrushes as a function of age and fat classes. She is interested to see which individuals move South and which individuals don't. The assumption is that birds must have sufficient fat to cross the Gulf, so fatter birds are expected to fly South. In terms of age, birds hatched this year may be less liable to fly South because the water is such a great barrier. For a bird who has never migrated before, this might be enough of a deterrent to cross-Gulf migration, so hatch-year birds may instead follow the coast to Mexico.
Jaci ordered about 70 transmitter this season for various research projects. Besides thrushes, we are also radiotagging Gray Catbirds and Red-eyed Vireos. Each transmitter has a specific radio frequency (it's radio signal is distinctive in length of "beeps" and time between beeps) and weight. The transmitters are stored attached to magnets, which turn off the transmitters to prevent the batteries from running down and prevent us from detecting the transmitters when they aren't attached to birds.
Attaching Transmitters To BirdsIn order to follow the movements of individual birds, Jaci attaches radio transmitters. The process looks something like this:
1. Before attaching a transmitter, it is a good idea to make sure it works! Jaci uses a radio tuned in to the transmitter's frequency and removes the magnet to test it.
2. After catching a Swainson's Thrush and determining its age/fat class, Jaci prepares to attach a transmitter by cutting feathers along the spine. The top of the transmitter should line up with the birds elbow, so she uses this bone as reference when cutting feathers. As a safety measure, birds must be at least 24 grams to carry a transmitter (we are permitted to attach transmitters that are 3-5% of the bird's weight).
3. Jaci cuts a section of feathers on the bird's back. When a bird's feathers are cut, they don't grow back like they would if they were pulled. Also, the transmitter adheres better to the leftover feather bases than just skin, and the feather tracts are visible, making it easier to center the transmitter. For obvious reasons, it is important to cut only body feathers and not flight feathers!
4. Eyelash glue is put along the bird's spine. This glue provides a flexible attachment from bird to transmitter.
5. The transmitter pack (transmitter + battery + fabric base), with superglue around the edges, is placed over the eyelash glue. Scissors are used to push the edges down around the transmitter, essentially supergluing the fabric to the bird.
6. The transmitter pack only needs a few minutes to adhere. Jaci also checks that there are no sticky parts of the transmitter pack that could get in the way of the wings. If necessary, she cuts more body feathers.
7. After obtaining a bird+transmitter mass and making sure it doesn't exceed 5% of the bird's original mass, the thrush is released. The transmitters and glue weigh 1.2 grams on average- and most of this weight is from the battery. The battery will only last two weeks at the most, so the next step is to make sure we get all the data we can in that time.
Tracking Birds: A Passive ApproachJaci's data collection relies primarily on three radio towers she put up near the site last year. This passive data collect requires a fair amount of work up in advance, i.e Jaci had to get permission from the Army to borrow the towers, permission from Bon Secour Wildlife Refuge and Fort Morgan Historical Site to put up the towers, and then come down early last year to set them up, but once the towers are in place they are relatively easy to maintain.
Jaci and Tom Radzio go out to check on the tower closest to our house. If a bird is above the canopy, the tower can collect information within a 25 kilometer radius.
There are six directional antennae on the top of the tower, pointing N, S, NE, SE, NW and SW. Whichever of these antennae detects the strongest signal from the bird is the direction the bird is traveling. Each antennae is also color-coded for easy reference.
The tower is powered by a lawnmower battery, which is in turn charged by a solar panel at the base of the tower. The colored cords at the top of the picture correspond to the different antennae. The tower must be programmed with which frequencies to "listen" for, but there is only so much memory in the system. To listen to a larger number of frequencies, you have to sacrifice how often the tower listens for those frequencies. So when we know a bird has gone South for good, or if it has been over two weeks since we released the bird (and therefore it is unlikely that the battery is still working even if the bird is still around), those frequencies are removed from the queue.
Tom Radzio checks the tower's clock. It is important that all of the towers are synchronized in their data collection. Data also is periodically downloaded to a computer for archiving and future analysis. The data collected from the tower takes the form of a graph with time along the bottom axis, and different colored lines to indicate the signal strength for a given frequency (yep, the same colors as those antennae!).
Tracking Birds: An Active ApproachSome people don't want to have a tower collect the data for them. Arlow is one of those people.
Arlow follows our birds over land. When the weather is good, he and Jaci can also fly to check in on our birds. He has an antenna on his car (locals and police give the car a lot of strange looks), and tunes into the frequencies of the birds we have tagged in the evening. Especially if our tower data indicates that one of our birds didn't fly straight South, Arlow tries to figure out where they've gone. He does most of his work at night because that's when birds tend to migrate. (With eating during the day and flying at night, there is a big question of when these birds sleep, or even whether they sleep at all! One idea is that they let half of their brain go to sleep when flying . . .). So Arlow comes to our house in the afternoon, collects the transmitter frequencies of birds we tagged in the morning, and goes out at dusk to find them. Arlow is also perpetually accompanied by three dogs he has rescued. When he lets them run, he puts a radio collar on the ring leader so he can find them if they get lost or go too far.
That's it in a nutshell. Perhaps the most exciting part of this research is that this season a few US researchers have set up towers like ours on the Yucatan Peninsula in Mexico . . . and they've heard our birds arrive! This is great news on several accounts, not the least being proof that the radio transmitters do not hamper the birds so much that they cannot make the Gulf crossing. Data from this pilot study is likely going to be incorporated into greater research and soon we can figure out the answer to fun questions like, "How long does it take the birds to fly over the Gulf?"
