The rusty blackbird (Euphagus carolinus) is the most strongly declining North American landbird. Although little is known about the reasons for its decline, habitat destruction, control of other blackbirds, and contamination with substances like methylmercury are possible causes.
The rusty blackbird breeds in the boreal forest of Alaska and Canada and spends the winter in the wet bottomland hardwood forests in the southeastern United States. The species seems to be more insectivorous than other blackbirds, but little else is known about its breeding and winter ecology.
In spring 2005 the International Rusty Blackbird Technical Group was founded with the aim to investigate these blackbirds’ habitat use. The first data were collected from the winter range of the species. I have started to study these blackbirds’ habitat use and roosting and flocking behaviors in their core winter range in the Lower Mississippi alluvial valley, with other study sites in North and South Carolina.
We are doing intensive trapping at different sites in the valley to collect feather and blood samples. With these samples, we can obtain information about the birds’ breeding grounds through stable isotope analyses; measure contaminant load on the breeding and wintering grounds through methylmercury analyses; assess parasite loads; and conduct genetic analyses to identify the wintering grounds of different populations of blackbirds.
Typical winter habitat of the rusty blackbird.
© Gerhard Hofmann
In addition, we attached radio transmitters to some of the birds. Bottomland hardwood forests repeatedly flood in winter and are often inaccessible. Birds are also difficult to find and easily overlooked. The use of radio transmitters allows us to learn more about the habits of the rusty blackbird.
A Day in the Field
In Mississippi’s Leroy Percy State Park, we are waiting at one of our feeding locations for a rusty blackbird to get caught in a mistnet, a fine-meshed net that birds cannot easily see. It is cold and calm—ideal conditions for catching rusties. At seven o’clock, the first birds have gathered in the trees around the location: about 20 rusties, ten common grackles, and a few red-winged blackbirds.
One, two, then three rusties fly to the ground, and then some grackles follow, but all land in front of the net to feed. Then, the first bird is in the net! We watch with binoculars, hoping it is a rusty but are disappointed to identify a red-winged blackbird. We fear this first attempt to catch a rusty will end unsuccessfully because a rusty never goes into a net in which other birds are already enmeshed. So we remove the red-winged blackbird and wait for the birds’ next visit.
Two hours later we are ready to close the net for the day. The rusties seem to know that there is a net and stay away. But suddenly, a bird is in the net and our gut feeling tells us that it is a rusty. We are right!
After extracting it from the net, we first put bands on the bird's legs. Then we attach a small, 2.5- gram (less than 1/10 of an ounce) radio transmitter on the young male, using elastic threads that loop over the bird’s legs and allow the transmitter to rest on the bird’s hip. Before releasing the bird we check the frequency, and the “peep – peep – peep” tells us the signal is strong.
You might guess that a bird would fly as far away as possible after this traumatic ordeal but the rusty flies only to the next tree. There he starts preening, trying to remove the leg bands and transmitter, but to no avail, and after a while the bird gives up and moves away. The signal gets weaker and I follow the bird, excited about where the journey will end. I pick up a very strong signal after walking about 50 feet, so I know the bird hasn’t gone very far, and although I can’t see it, I know it’s very near.
But when two hours pass without any change in the signal strength, I start to worry. Did the bird manage to remove the transmitter and disappear? Then I see some birds on the ground turning leaves to look for insects and several of them are rusty blackbirds.
Now, the signal strength starts to vary. It seems my rusty has waited for the other rusties and joined them to feed. I cannot see my bird but the signal is sometimes stronger to to the right, then to the left, in a pattern typical of a bird feeding on the ground, flying up into a tree, and then returning to the ground.
The group consists of about ten birds, mainly females. Some noisy grackles join in and their songs and calls soon attract other grackles. Eventually, 100 grackles and ten rusties are feeding on the ground. The group slowly moves deeper in the forest but the signal of my bird is still coming from the same location. But then it takes off and follows the others.
I have difficulty receiving the signal because some power lines strongly interfere with it but I walk in the direction from which I got the last signal, and every now and then I pick up a signal. I walk as fast as possible but many vines and poison ivy block my way, and the ditches I have to climb in and out of get deeper and deeper. Five minutes later, the signal gets stronger so I must be nearer to the bird, but unfortunately, the signal is coming from the other side of a deep creek.
At this point I have a choice. I can stay here and hope that the bird doesn’t move further away or I can abandon the signal and search for a place tocross the creek. I decide on the second option and keeping the antenna pointed in the direction of the signal,move along the creek. The signal fades but I still have some contact with the bird and half a mile down the creek I find a large tree lying across the creek. This is my chance. I try to tie all my equipment—binoculars, receiver, compass—to my body and walk slowly across the fallen tree using the antenna as an extended arm to keep my balance.
On the other side, I am still getting a signal and no longer care about wading through ditches filled with water—the signal is getting weaker again, so I have to hurry. At the spot where I have abandoned the signal to find a way across the creek, I get a clear signal coming from northwest, then it is suddenly gone. I record another GPS location and move on in a northwestern direction.
No signal is audible but fortunately the birds usually move in the same direction for quite a while. The transmitter has a range of about one mile but trees reduce the range considerably and one has to be much nearer to pick up a signal from a bird on the ground. I have to go around a large water hole and one of my rubber boots is already filled with water.
Then I reach a more open reforesting area with densely packed seven-to-ten feet tall trees. Was there a weak signal coming from the other side? Yes, I hear it again, but I am dreading going through this area. The young trees are closely packed together and thorny scrub is ubiquitous, and I feel like I’ve already run a ten-kilometer race. How much easier it is for a bird just to fly over this thicket!
Finally, I make it to the other side and end up in another part of the wet forest. The birds—still about ten rusties—seem to have mercy on me and spend the next three hours in this area. Some are in the trees, preening their feathers, others walk on the ground, turning leaves and pecking at insects hidden underneath them.
The birds actually prefer locations with water. They wade in two-inch deep water, pecking at insects on the surface or under the water. They also dig in the mud at water’sedge. Occasionally, they feed on broken acorns.
From time to time a flock of grackles joins the rusties but they never stay long. They are very nervous and fly off again only to come back later. The rusties ignore the grackles and continue feeding, giving me a chance to catch my breath and enjoy observing the birds. I am searching for ‘my’ birdand soon find it feeding in the shallow water, turning leaves and behaving like the others.
Around four o’clock the group becomes more active again. The signal gets slowly weaker;while feeding, the birds seem to be moving away from me. Then I get one strong signal but the next is very weak because the bird has taken off again. I find a bridge to cross another ditch, then follow a path along a field adjacent to the forest. My bird is in the forest and moves on while the rest of he group continues feeding and moving steadily in a northwest direction.
I approach Deer Creek, which is 30 feet across, to wide to ford should the birds cross the river. The birds hang around in the forest near the creek for quite a while, then the direction of the signal changes abruptly as they fly over the open field into some trees along the creek. Now I see through my binoculars that some red-winged blackbirds are already there.
It is five o’clock and this congregation of birds suggests that this area may be a night roost. After all, there have been reports that rusties may roost with red-winged blackbirds and grackles. In fact, control programs conducted in roosts 30 years ago aimed to reduce red-winged blackbird numbers, believed to be an agricultural pest, but may have also substantially reduced rusty blackbird numbers.
I walk through the field to get closer to the birds. The field is another reforesting area with small, three-foot high trees, some bushes, and lots of grassy five-foot high vegetation. Hiding between some trees, I see that more birds have arrived—about 200 rusties and 100 red-winged blackbirds. They sing in the treetops, preen their plumage, or fly in small flocks to the ground to feed. My rusty changes position several times, flying back and forth over the open field between the forest and the single line of trees. Half an hour later most of the birds are concentrated in the trees on the forest site. The red-winged blackbirds disappear in small groups and do not come back. They seem to be roosting already.
The sun disappears below the horizon but most of the rusties are still in the trees. I wonder if they will they sleep in the canopy but then birds in flocks of ten to 20 fly to the ground and hide under overhanging grass. I am totally excited! I never expected that a forest bird like the rusty would roost on the ground. More and more rusties follow the first ones. It is nearly dark when all birds are on the ground. I cautiously leave the roost. Next morning, I will follow the bird further.
Rusty blackbird roost.
© Gerhard Hofmann
The next days show that rusties almost always spend the day in the wet forest, often with common grackles. At night, they roost in the open reforesting areas, either with red-winged blackbirds or alone. In Leroy Percy State Park there are at least two separate groups of rusties, and each seems to use different parts of the forest and different roosts.
Whether young and old rusties segregate, and whether the young birds move more around and spend more time at the periphery of the forest than adults are questions that have to be answered next year. It is the end of March and our birds are on the way back to the breeding grounds. When my bird arrives there, its transmitter will have fallen off and only the leg bands will point to our research activity.