Forest bird population crashes
DULUTH, Minn. — The number of birds counted each summer on Minnesota’s Chippewa and Superior National Forests nearly doubled from 1994 to 2009 but crashed by 30 percent this year after back-to-back cold, late springs.
And the legacy of this year’s poor nesting season could last into next year, with fewer chicks hatched this summer and even fewer birds returning to be counted in 2015.
That’s the conclusion of the annual Summary of Breeding Bird Trends released recently by researchers at the Natural Resources Research Institute of the University of Minnesota Duluth.
The number of total birds counted in the survey, now in its 20th season, has moved up and down in fits and spurts but has trended up over the long haul.
Every June, at the peak of nesting and singing, researchers go to predetermined locations in the forest, near specific habitat, and tally every bird they hear. They counted just more than 10,000 different birds in 1994. That number peaked at nearly 20,000 birds as recently as 2009, dropped in 2010, rose again for three years, and then crashed to 12,500 birds this June.
That’s down 36 percent from peak numbers and 30 percent in one year.
This year’s decline likely was spurred by fewer surviving chicks in 2013. The same kind of late spring conditions appear to have occurred again this year, with another decline likely next year, said Gerald Niemi, University of Minnesota professor, forest bird expert and co-founder of the study.
“That’s what we’d expect to see, down again,” Niemi said.
Trend was up
Niemi and others involved in the survey say the recent weather-related decline detracts from what had been a markedly upward trend in many forest bird species during the past 20 years.
Even with 2014’s decline included, over the 20 years of the survey, most of the 73 species counted are doing OK. Some 37 percent have increased significantly on one or both of the forests, 40 percent are stable and 23 percent of the species are declining at a significant rate.
Scientists say they often don’t know why some bird numbers are going up and some down, but a few patterns are emerging.
As with other animal types, habitat generalists in the bird family seem to be doing well. American robins for example, which can nest and thrive just about anywhere, are increasing in both national forests.
While there are some exceptions, species that live here year-round, or that migrate short distances, also seem to be doing well. Black-capped chickadees, blue jays and cedar waxwings all are increasing on the national forests.
Other species are tied to obvious trends. The Cape May warbler specializes in eating budworms. With a long, heavy outbreak of budworms chomping away on balsam fir in the Superior National Forest in recent years, the population of Cape May warblers has dramatically increased.
“It’s one of the few species where a specific and individual cause for an increase is quite clear,’’ said Edmund Zlonis, NRRI researcher who now heads the study. “For most of the species, we just don’t know what’s driving the population up or down.”
Zlonis said climate and habitat changes may be playing key roles in forest bird populations — not just in northern Minnesota forests but along migration routes and in the birds’ wintering areas. He said it’s easy to notice the much more erratic swings in population counts in recent years.
“Weather-related ups and downs (in bird populations) like we are seeing recently have likely always occurred,’’ he said. “The questions are … will dramatic variability (in weather) be more common with climate change and how will birds adapt, if at all? The jury is probably still out for both.”
Less logging, more old trees
There’s a good hunch, for example, that habitat change is why pileated woodpeckers are doing so well. The big, bug-eating bird chooses old aspen trees as its favored habitat, both to build nests and hammer away for insects. Aspen that are 80 years old and older are its favorites.
Those old trees are rare in areas where aspen are cut at 40 or 50 years for local mills. But on national forests, where there’s been less focus on aspen clear-cuts in recent years, the average age of aspen is increasing, and there are more of the old trees that pileateds favor.
That increase in the average of trees in the northern forests, not just aspen but other species as well, is in large part due to declining timber harvest. A decade ago as many as 4 million cords of wood were cut each year in Minnesota. That number has dropped to about 2.5 million cords in recent years.
Two factors have combined to leave more trees to grow older. U.S. Forest Service timber policy has favored leaving more old trees in the woods and more standing trees after logging, including smaller and fewer clear-cuts. Even more striking has been the decline in the amount of wood needed for regional mills. There’s simply less demand to cut trees than there was 20 years ago, with several paper mills and most of the region’s waferboard plants now shuttered.
Niemi began the survey when Minnesota’s wood products industry was expanding amidst growing concern that the appetite for wood would outstrip the sustainable supply of trees, reducing critical habitat for many forest birds.
That didn’t happen, Niemi notes, and many of the state’s northern forest birds have actually seen better times of late. In fact, Minnesota has about 5 percent more forested acres now than 35 years ago, a recent state survey found.
“If you asked me if we have a lot of forest birds now compared to 150 years ago, I’d say we’re probably way down. Minnesota once had 31 million acres of forest; now we’ve got less than 16 million,’’ Niemi said. “But in the shorter term, maybe the last 20 or 30 years, maybe things have rebounded a bit and maybe the birds responded. I’d say, this year notwithstanding, that the recent trend has been pretty good.”
Nearly one-fourth of the bird species counted on the national forests are not doing so well. Some may be species that favor more open, recently logged areas. For others, no one knows what’s wrong.
Connecticut warblers, which nest exclusively in lowland conifers like black spruce bogs, are doing especially bad. Their numbers have crashed an average of 8 percent each year of the survey, with a more than 80 percent decline in their abundance.
And it’s not just on the two Minnesota forests. Across its summer range in Canada and the northern U.S., Connecticut warblers are declining rapidly. As with many others on the declining list, Connecticut warblers are long-distance migrators, so-called neo-tropical birds that fly all the way to Central or South America to spend the winter.
“Personally, I’m very concerned about the Connecticut warbler because there is a clear, one-way trend downward,’’ Zlonis said.
Evening grosbeaks, Swainson’s thrush and the yellow-bellied flycatcher also favor lowland conifers and all are declining.
While there’s been some increase in conifer harvest in parts of the bird’s range, Zlonis said there’s no solid research showing that northern habitat issues are the cause of the Connecticut warbler’s demise. Their numbers may be affected by declining habitat in their warm winter homes. And the little birds face many perils over the thousands of miles they must fly to get there.
“There’s been some literature that Connecticut warblers may be especially vulnerable to collisions’’ with towers, buildings and other man-made structures, Zlonis said, to the point that high migration mortality may be affecting the population decline.
“Like so many other areas, the more we learn, the more questions come up,” he said. “It may be climate related. We are at the southern edge of this type of lowland conifer habitat, and maybe we are starting to see some impacts of that on the habitat and indirectly on the birds. Or maybe it’s affecting the birds directly. We can’t say for certain what’s causing it.”
Birds and forest management
The annual survey has been funded each year by the U.S. Forest Service. Each year when the results are tallied, NRRI researchers present their findings to forest managers and wildlife biologists on the Chippewa and Superior forests.
It’s hoped the survey might help plot management of the forest into the future, potentially seeing more effort made to encourage habitat for long-term declining species.
“If nothing else the Forest Service has real numbers, over a pretty long period now, that they can answer questions on basic bird numbers,’’ Niemi said.
One mandate of the Forest Service is to provide habitat for forest wildlife and “forest birds are an important part of the biodiversity of the region,’’ said Kelly Barrett, wildlife biologist for the Chippewa National Forest.
The region’s national forests are an important incubator for continental bird populations, Barrett noted, and help make up for the loss of forest habitat in other parts of the Midwest. That makes the national forests here focal points for bird conservation efforts, she said.
The annual NRRI survey provides the national forests with local, long-term population trends for many forest bird species. Those trends can be used to look at changes in habitat and changes in Forest Service management of the forest. NRRI staff also is helping the forest managers look ahead, modeling scenarios that gauge the impact of changing climate on habitat and bird species
“The program provides the Forest with a close tie to the best science available,” Barrett said