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Published: June 15th 2008
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Telemetry
Using the antenna and receiver to find a tagged bird. With the second phase of the Brain Hormone Project behind me, and another couple weeks until we begin the third and final phase, I’m finally able to delve into my project - at least, the project I developed under Dustin’s supervision and with his help.
To sum it up quickly, Dustin’s past research has shown that Superb Starling females show a higher rate of infidelity, based on extra-pair copulations (matings outside of a pair-bond, verified by DNA analysis of young), on territories of lower quality (based on vegetation cover and insect abundance). In other words, the frequency of extra-pair copulations is inversely proportional to territory quality - the better the territory, the less likely the female is to mate outside of her pair-bond. Conversely, the lower the quality of the territory, the more likely it is that the female is raising offspring sired by a male other than the one with whom she built her nest and raises the chicks.
My job is to find out why.
The plan is to test two hypotheses: one, that territory quality is correlated with mate quality, so that males on lower quality territories are of lower quality themselves, influencing females to
Outside the field office
Going through the processing routine. seek out better genes; or two, assuming that females are inherently promiscuous, that lower quality territories necessitate longer forays for food, so that males cannot guard their females as well from mating with neighboring males. I’ll test the first hypothesis by looking at parasite loads of males on territories varying in quality. The second hypothesis requires knowing where pair-bonded males and females are at any given time, for a set amount of time, so that distance between male and female can be averaged for a given site and compared to other sites. This means I get to play with radio telemetry - by applying transmitters to the pairs, I can find and follow them, observing their activity and noting distances between male and female.
The first step is to catch a bird. And not just any bird - but a breeding male or female from a known nest. Fortunately, that’s pretty easy this time of year since the birds are breeding in full-swing, and we can target birds by setting up mistnets directly in front of the acacia trees harboring their nests. Once we catch a bird, which can actually take a surprisingly long time considering the mistnet is
just inches from the nest’s entrance, we immediately began a course of hormone sampling. This requires drawing blood at 3, 10, 30, 60, and 90 minutes, which is then spun in a centrifuge for five minutes to separate the red blood cells from the plasma, which we draw out and put in a labeled tube that immediately goes on ice. These samples will eventually be analyzed for stress hormones.
Following the hour and a half of hormone sampling, we measure the standard biometrics, such as mass, wing length, tarsus length, and bill length. We also replace any of the plastic colored bands that may have fallen off since the bird was last captured. Next, I take some pyrethrin powder, a natural insect repellent not harmful to birds, and dust the bird along the feather tracts throughout the body - after dusting, these feather tracts are ruffled (hence the term "dust-ruffling") over a colored piece of paper to dislodge any ectoparasites such as lice, which will subsequently be counted. We also take a drop of blood to make a smear on a slide to later be analyzed for endoparasites.
The last step is to equip the bird with the
transmitter, which is activated by removing a small magnet taped to it. The transmitter is a small, greyish, rectangular device with a long wire trailing off the back end and some elastic string looping through the top and bottom to create a sort of backpack-type harness. These loops are fitted over the bird’s legs so that the backpack sits on the bird’s lower back and the wire projects past the tailfeathers. Preliminary aviary trials have shown that the transmitters, when worn this way, cause the bird no harm and do not hinder it in any way. Once we decide that the transmitter is fitted well, and we’ve tested the frequency with the receiver, we let the bird go ("it is very happy!" Wilson says after every release as the bird flies away in a scolding chatter).
Now that we have a bird fitted with a transmitter, we’re able to find that particular bird any time we want by using a receiver attached to an antenna that will pick up the frequency emitted by the transmitter (assuming the bird doesn’t wander so far from the territory that the frequency will be too weak to be picked up by the antenna).
Applied Transmitter
How the transmitter rests on the lower back of the bird By angling the antenna in certain directions, and adjusting the dials on the receiver while listening to the intensity of the beep emitted by the receiver, I’m able to tell roughly in which direction and how far away the bird is. Sometimes a little triangulation is necessary, but we can usually find our study subject in ten minutes or less.
At this point, the aim is to follow the birds for an hour and make note at set time intervals how far apart the male and female are from one another. This is not as easy as it sounds, however - if we approach the birds to mark their locations at a given time, then we alter their behavior. Simply estimating the distance is too subjective. So, basically, we’re still in the process of fine-tuning the protocol to get accurate and unbiased data. When we finalize the protocol, however, we’ll ultimately end up following three pairs on three separate occasions at each of the nine sites. If there is a difference in distance between the male and female across the sites in relation to territory quality, it will suggest that mate-guarding is more difficult (and therefore less effective) on
Sexy Tail
Lateral view of bird with transmitter lower-quality sites and thus the female can seek other males with which to mate, thus explaining the inverse correlation between habitat quality and extra-pair copulations.
Get it?
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Great explanation
Great explanation and pictures even if I don't entirely "get it"! Reminds me of Bustard telemetry...remember? And here you are!