Our summer field season is quickly winding its way to a close. Tom and I spent the last month and a half furiously attaching radio transmitters to 7-day-old nestlings. Radio transmitters are small devices (0.5 gram in our case) that emit a regular short-range pulse that can be detected using directional antennae.
Each device transmits at a slightly different frequency so we can keep track of who is who. We attach them on day 7 because they are big enough to handle it, but not so old that they are likely to prematurely leave the nest if disturbed.
We attach transmitters to figure out:
- basic survival rates during the post-fledging period
- whether fledglings use habitat types other than their breeding habitat.
Fledglings of many species have recently been found to move out of their breeding habitat into nearby and often early-successional forests that presumably provide better ground cover and/or more abundant food resources.
To attach the device we use the same leg-loop harness that we used for the geolocators. However, we used a thinner and more elastic material to accommodate growth and allow the devices to naturally fall off later in the year. Once we get the device attached, we wait patiently for those nestlings to grow big enough to leave the nest; usually on day 12 or 13 after hatching.
We've used up our stock of transmitters for the year, tagging about 70 birds. At one site, we found high levels of nest predation during the last part of the nestling stage. Some of the nestlings were killed and then buried 4 inches to 5 inches under the ground, apparently something that free-ranging pet cats are known to do. Cats are a huge threat to small birds and other wildlife so keep those cats indoors!
Most of our nestlings survived long enough to make it out of the nest. Once young birds leave the nest they are called fledglings. Kirtland's Warbler fledglings are almost completely helpless when they leave the nest, and are typically not able to do much more than clumsily hop around on the ground. Luckily, their plumage is well camouflaged for their environment and they are really good at sitting perfectly still and being quiet.
After a day or two spent hopping around on the ground, Kirtland's Warblers manage to get up into the low branches of the Jack Pines where they will spend much of their lives. Flight abilities are weak at this point. They can make it from tree to tree, although with little control. It's best described as a somewhat controlled crash landing. The parents are very defensive at this point and will often pretend to have broken wings, fluttering around in the undergrowth and making lots of noise to draw attention away from their precious young.
If that doesn't work they will aggressively attack any human or non-human intruders. Tom and I had more than a few adults land on our backs or the antenna as they engage in attack maneuvers.
Predation rates during the first few days out of the nest have been fairly high. But it appears that if birds can make it past this period, they have a pretty good chance of making it to independence. Causes of death are somewhat unclear. We haven't seen any evidence of starvation. But this isn't too surprising because it has been a great year for insects and blueberries, both of which Kirtland's love to eat. It's quite difficult to tell what type of animal killed a fledgling even with the carcass in hand, but we certainly suspect that small mammals and some avian predators such as hawks have been involved.
We have seen a few of our fledglings move into much older stands of Jack Pine, but for the most part, birds have remained within 600 meters to 900 meters of the nest in the same habitat they were born. Over the next year, we will more carefully analyze these movements and try to determine whether young (6-7 year) or older (9-12 year) Jack Pine stands provide equally good habitat for fledgling survival.
The batteries on the transmitters only last about 35 days and that matches up well within when the fledglings become independent from their parents. As of today, we only have 3 fledglings with active transmitters. Once those batteries die, we are officially done for the season.
Stay tuned throughout the winter as we will be traveling to three islands in the Bahamas (Cat Island, Abaco, and San Salvador) and Cuba to help reveal a fuller picture of the Kirtland's Warbler wintering range.
Also, don't forget to check back next spring as we begin to re-capture adults returning with geolocators. These devices will provide us with fairly accurate data on wintering location and both spring and fall migration routes.