Kirtland's Warbler Study
June 27, 2014 by Nathan Cooper
All things Kirtland's Warbler related are going great up here in northern Michigan. We have now successfully attached 60 geolocators. For those just joining us, a quick reminder about how they work.
Light-level geolocators are small (0.5 gram, less than a paperclip) devices that collect data about ambient light levels. From this, we can determine sunrise and sunset, and then during most of the year, we can infer latitude and longitude within about 100 km. This allows us to track birds like the Kirtland's that are still too small to carry heavier satellite and GPS based devices.
We deployed geolocators across nearly a 100-mile latitudinal gradient, which represents all of the Kirtland's habitat in the lower peninsula of Michigan. Now we just have to wait and see how many we can re-capture next year. Once we re-capture the males and download the light-level data we can begin to understand whether the population all goes to the same general area or spreads out across much of the Bahamas or other places in the Caribbean.
We will also get some important information regarding migration routes and timing for both spring and fall migration. This will be the first such data collected on Kirtland's Warblers, outside of a small pilot study a couple of years ago.
Now that the geolocator part of the study is essentially finished up for this season, we have diverted nearly all of our effort to nest searching. One of my first field jobs as an undergraduate was in Washington State, out on the Olympic Peninsula. It was a beautiful location with lots of cool birds, but I really struggled at nest searching and found it incredibly frustrating. I ended up spending much of my time in the following years either working with Eastern Kingbirds, which have very easy to find nests, or with wintering birds, which aren't breeding at all. As a result, I was a bit intimidated about finding Kirtland's Warbler nests.
However, over the last few weeks of nest searching I have found that I really enjoy it. Kirtland's Warblers nest right on the ground in the thick vegetation often found under the endless rows of small Jack Pines. The nests are quite well hidden and therefore you can't just go around looking for them. Instead, you have to carefully observe the male and female for nesting behavior.
The process first begins during site selection. The males and females will often forage closely together during this period, occasionally dropping down to the ground to check out different possible nest sites. The female will rub around certain areas, trying to find the perfect spot. Once she's found a good spot, she'll begin to gather nesting material. Large pieces of grass, pine needles, and various other bits of vegetation are all collected in 10 minute or so bouts and then taken back to the nest.
The male doesn't help build the nest at all in this species and instead spends his time patrolling the territory and keeping out intruders. During nest-building females are almost completely silent and pretty secretive. Following them can be a real challenge.
If you get too close they fly away or become anxious and won't visit the nest. And if you're too far, you lose them in the dense vegetation. We found a few nests during the 5-6 days that it takes females to build a nest, but mostly we've found them during the incubation phase or nestling phase. During incubation, the female spends about 40-50 minutes of every hour on the nest keeping the eggs warm. She only comes off to feed about once per hour, but luckily the male is willing to help out during this phase.
He forages around the territory, stopping to fight with any intruding males, and singing the whole time. Then as he approaches the nest, his song gets quieter and a bit muffled (since he has so much food in his bill). He typically starts looking around anxiously, presumably making sure no nest predators like Blue Jays are watching.
They often seem to notice us during this time as well, and we've found that lying on the ground and hiding behind trees is often necessary. If not too disturbed, the male then quickly dives down to the nest and feeds the female. For trickier males, it has taken up to two or even three hours of observation for us to find nests during this period.
After 13-14 days of incubation the eggs hatch and the nestlings need to be fed and kept warm. The female often sits on the nestlings, while the male forages and brings food back. As the nestlings get older and approach fledging date on day 9 to 11, the female spends more and more time away from the nest looking for food to feed those hungry nestlings.
All in all it's an exciting time here in Michigan. The nestlings are just beginning to leave the nest and we have just started to put small radio transmitters on them in order to track fledgling survival. More about this (including some video of the attachment process) in a couple of weeks. Next week Tom will write about nesting from a broader perspective and introduce the perils of nesting near populations of the Brown-headed Cowbird.