Earth & Sky

Our thanks to the following individuals and institutions who assisted in the preparation of this script:

Thanks to:

Philip W. Huber
Wildlife Biologist
US Forest Service
Mio Ranger Station
Mio, Michigan

Mike DeCapita
Biologist
US Fish and Wildlife Service

For more information visit the following Web site(s):

The Kirtland’s Warbler Page (USDA Forest Service)

Author’s Notes:

Interview with Phil Huber:

To understand the Kirtland’s Warbler, you have to first understand jack pine. You see, the warblers absolutely depend on jack pine. They are habitat specialists.

Jack pines are scrubby looking pine tees that depend on fire for regeneration. They have serotinous cones-cones that only open and release their seeds when heated by fire. They also have extremely flammable needles, especially in the spring when water content is low. Crown fires are very common in jack pine. In fact, the natural occurrence of these catastrophic fires would be-if it weren’t for present day fire suppression efforts-about once every 28 years! Those fires would be started by lightning, and by humans occupying North America since the last glacial retreat.

Huber points out that it is still important to put out fires because of the increasing urban interface between management lands and people.

Jack pines produce cones at a very early age for trees – about 3 years (in contrast to, say, the red pine which makes it’s first cones at 25 years of age). And after fire, the jack pine stands regenerate quickly and very densely.

Now that people are suppressing fires, the jack pines are growing old, which is very unusual, and their life cycle has been interrupted.

Enter Kirtland’s Warbler. Kirtland’s depend on the dense young Jack Pine stands you find after fire. The best trees for the birds range from 5 to 20 feet tall and are only up to about 15 to 20 years old. The Warbler’s are ground nesting birds, and use the thick cover of the Jack Pines to build their nests. They use the stands for cover, to find food like blueberries, and sedges. They may use the area for insect foraging opportunities and the low hanging pine branches for cover.

Jack pine grows on sandy, infertile soils. Glaciers deposited the soils about 10,000 years ago. Other trees don’t grow there because the soils are very nutrient poor. “It’s a nasty place,” says Huber. “It’s dry, frosty, and there are weather extremes. The trees tolerate a lot in this unique ecosystem.”

The birds live in the Bahamas for seven to eight months per year and then migrate to Michigan in May. Most of the breeding population nests in the lower peninsula of Michigan during this time. Since fire suppression has been ongoing for the last hundred years, the birds have not had the kind of breeding habitat they need. In fact, in the 1960s biologists could see that people would need to manage areas of young Jack Pine habitat in order to save the Kirtland’s Warbler from extinction.

So, the U.S. Forest Service, along with the Michigan Department of Natural Resources, created 24 natural areas for Kirtland warbler management in Michigan. These 24 natural areas provide enough habitats to sustain about 1000 pairs of breeding warblers. The management strategy for the jack pine includes harvest and replanting of trees. So far, fire is avoided because it’s so uncontrollable in jack pine stands. Fires start easily and are very difficult to control.

In addition, there is an economic benefit to a management strategy based on cutting and replanting. The contractor comes in, removes the timber, pays for that timber, and those funds go back into replanting. Seventy-five percent of the money goes directly back to the sites and 25 percent goes back to the surrounding community. The management schedule involves 2710 acres per year that are harvested and replanted.

How is it done? First managers plant two-year-old trees. At seven years old, the Kirtland warblers begin to use those trees. And Kirtland’s warblers require very large stands of trees. In fact one pair requires at least six to 12 acres. No birds will occupy the stand unless it is at least 80 acres. The larger the size of the stand, the more birds tend to inhabit it. Some birds return to the same stand year after year.

These jack pine stands remain occupied for an average of 10 years. If the trees grow too tall, the warblers move out. These trees are allowed to grow to maturity, to about 40 to 60 years old, at which time there are harvested. Then the cycle begins again and new two-year-old trees are planted.

In early 1980s there were only about 200 pairs of birds, and in 1987 only 167 pairs. A large forest fire Michigan in the middle 1980s provided 12,000 acres of new Jack Pine habitat. This was a big boost for the population of Kirtland’s warblers. During the 1980s 1990s, the population expanded. In 1999, the Census count found 905 pairs. And during the 2000 Census, 891 pairs were counted. So the target of 1000 nesting pairs has almost been reached. To census birds, researchers go into the woods, run transects, count singing males and map their location. This takes about one and 1/2 weeks in the early summer each year.

Says Huber, “ We need to continue to do this into the future-the habitat must be managed. Without management the Kirtland’s warblers will be doomed to extinction.”

And he stresses the point that successful management uses commercial harvesting of timber to benefit an endangered species. If logging were stopped on this land it would doom the Kirtland’s warbler.

Finally, Huber brings up another big factor in the success of Kirtland’s warblers-brown-headed cowbirds. When forests in Michigan were cleared for farmland the brown-headed cowbird found the habitat attractive and moved in. Brown-headed cowbirds are nest parasites. They lay their eggs and other bird’s nests in the other birds are forced to raise their young. The U.S. Fish and Wildlife Service have trapped cowbirds every year since 1972 to reduce nest parasitism. About 3 to 7000 cowbirds are removed for Michigan every year. These cowbirds can have a significant impact on Kirtland’s warblers, and without the trapping program warblers could suffer serious declines.

Interview with Mike DeCapita:

Brown headed cowbirds are nest parasites that evolved with other bird hosts. Many of these other bird hosts evolved defenses against them-but not the Kirtland’s’ warbler.

When the forest of Michgan was cut in the last two centuries, the cowbird spread (it doesn?t do well in forests). Prior to the turn of the last century, Kirtland’s had never been in contact with cowbirds. So, when the cowbirds moved in, they had tremendous impact on the warblers. Studies in the 1960s showed that brown headed cowbirds were parasitizing between 50 and 70% of Kirtland’s nests! This reduced their reproductive potential to less than on young per pair, per year. This is a “prescription for extinction” says Mike DeCapita. “The birds simply couldn’t replace themselves in number.”

THe 1961 census counted about 500 pairs of Kirtland’s. By 1971, the census count had dropped to just 200 pairs. This brought on a great alarm. Both habitat decline (as described above) and the cowbirds, were blamed as partly responsible. So the USFWS started to trap and remove cowbirds starting in 1972. This had an immediate effect on the parasitism rate-which dropped from over 50% to between 0 and 9%.

But the population did not start to increase again until the late 1980s. This was probably due to a sudden increase in good habitat brought on by a large wildfire a few years earlier (and subsequent young jack pine growth). So, cowbird control was important to maintain, but the most important underlying factor for Kirtland’s was the availability of good habitat.

There are millions of cowbirds over the American landscape and they just keep coming back. It’s a program that will have to be kept intact if people hope to keep the Kirtland’s warblers from going extinct-just as the habitat management program will have to stay in place.

Says DeCapita, “We’re lucky we found a program that works. It’s expensive, but it works. And it will have to go on forever. That’s what’s most troubling to me, if we didn’t help, then I think the birds would go. The Kirtland’s depends on us for its existence, and that existence is paid for by tax dollars, which are subject to politics. I’d rather see the money secured in some other, more secure way. Perhaps a benefactor interested in preserving this beautiful species would step forward to start a Kirtland’s warbler trust fund, that would keep the Kirtland’s from going extinct.”