Study looks at Minnesota's forest-breeding birds

The Ups and Downs of Forest Birds

Two decades of survey data paint a generally positive portrait of Minnesota's forest breeding-bird populations—with a few troubling trends.

by Dr. Gerald Niemi

There is a long history of concerns for forest bird populations in Minnesota and throughout North America. Rachel Carson in her classic 1962 book Silent Spring sounded the alarm of disappearing neighborhood bird songs due to widespread use of the pesticide DDT. Clemson University researcher Sidney Gauthreaux used radar to find that neotropical bird migration intensity and volume across the Gulf of Mexico had declined by 50 percent or more from the 1960s to the 1980s. U.S. Fish and Wildlife Service researcher Chandler Robbins and his colleagues also found severely reduced breeding populations of songbirds from the 1950s to the 1980s in the eastern United States due to habitat loss and fragmentation. Princeton University researcher John Terborgh summarized these concerns in his 1989 book, Where have all the birds gone?. Terborgh raised legitimate fears for future songbird populations for many reasons including widespread loss and fragmentation of forests in North, Central, and South America.

Forest bird habitat has been a concern in Minnesota too. Over the past 150 years, Minnesota has lost nearly half of its forest land, mostly in central and southern parts of the state. Today, Minnesota has about 16 million forested acres, down from more than 30 million acres in the mid-1800s.

In the early 1990s, concerns among Minnesota conservationists grew over the loss and fragmentation of forests and about the lack of information on the status of forest birds. These concerns led to a systematic forest-breeding-bird study in northern Minnesota. In 1991 the Legislative Citizen Commission on Minnesota Resources and U.S. Forest Service provided funding to the Natural Resources Research Institute at the University of Minnesota–Duluth to conduct a yearly survey of forest breeding birds in Superior and Chippewa national forests. Detailed analyses of the forest bird population trends observed over the past 17 years illustrate a good news–bad news story: The vast majority of forest bird species appear to be stable or increasing, while some are declining.

Listening for Bird Songs. Each year at least four researchers are recruited for the survey based on their ability to identify and pass a test of more than 80 bird songs. In June and early July, researchers visit more than 900 sites from 5 a.m. to 9:30 a.m., when birds are most actively singing to defend their territories. Each researcher listens for 10 minutes per site and identifies all the bird species seen or heard; over 90 percent of identifications are made by their songs.

Over the past 17 field seasons, the NRRI research team has detected about 240,000 individual birds of some 170 species in the two national forests. A typical 10-minute point count in June will result in 11 species and 17 individuals.

In Chippewa National Forest, researchers collected sufficient data to evaluate the conservation status of 63 bird species. Populations of 40 species were considered stable, 17 species increased significantly, and six declined. Species in decline included the yellow-throated vireo, golden-winged warbler (a species with over 40 percent of its breeding population in Minnesota), yellow-rumped warbler, Connecticut warbler, scarlet tanager, and song sparrow. Most of these species are still found commonly throughout their range, and their population declines in the NRRI survey do not appear to be continent-wide. Scarlet tanager and Connecticut warblers may be exceptions, as they have shown regional declines in population.

In Superior National Forest, researchers also collected ample data to evaluate 63 bird species. Populations of 31 species were considered stable and 24 species increased significantly. Eight declined: ruffed grouse, broad-winged hawk, Wilson's snipe, yellow-bellied flycatcher, Swainson's thrush, magnolia warbler, Connecticut warbler, and evening grosbeak. Some of these declines must be viewed with caution. The survey design is not very well suited to broad-winged hawks and Wilson's snipe, which are very secretive and difficult to detect. Ruffed grouse are better detected when drumming in spring. Continental populations of the magnolia warbler appear to be stable, despite the decline in our survey.

A decline in the evening grosbeak, a common species in the 1970s and 1980s, is consistent with widespread continental declines. Well known for its gregariousness at winter bird feeders, it is a solitary nesting species in the summer. Reasons for its decline are unclear, but factors could include disease transmitted at bird feeders and reduced food supplies due to logging of hardwoods on its primary breeding grounds in Canada. Declining counts of evening grosbeaks could also be a sign that this species isn't found as far south anymore, with reduced migrations due to warmer temperatures in winter in Canada.

Complex Trends. For each forest bird species in our survey, the 17 years of trend data tells a complex story about how it lives out its life history among the perils of weather, availability of food, competitors, diseases, parasites, and the hazards of migration or the challenges of surviving winter.

Climate change could partly explain the positive trend among northern Minnesota's permanent residents whose species are increasing the most. They might be benefitting from warmer winters and higher overwintering survival rates. Earlier emergence of insects in spring, as climate warms, could also benefit early-nesting species of permanent residents. And winter bird feeding, which subsidizes permanent residents over a harsh cold season, could be a factor as well. For example, the breeding population of the red-breasted nuthatch has increased 6 percent per year—or more than doubled over the past 17 years! Other permanent residents seeing population increases include gray jays, hairy woodpeckers, pileated woodpeckers, and American goldfinches.

On the other hand, populations of long-distance migrants (neotropical migrants to Central and South America) illustrate a complex pattern of change that is not easily explained. For instance, the ovenbird is one of the most common forest birds in Minnesota during the summer. Its distinctive "teacher, teacher, teacher" song can be heard from virtually all upland forests—maple, aspen, and pine. Ovenbirds had a steady increase in population for about eight years from 1995 to 2003, then the population trend flattened from 2003 to 2008. Recently it has shown a slight decline from 2008 to 2011. Many other common bird species of Minnesota's forests—red-eyed vireo, American redstart, chestnut-sided warbler, and white-throated sparrow—have trends similar to the ovenbird.

The Connecticut warbler is also a long-distance neotropical migrant, but a much rarer species than the ovenbird. Its relatively loud "wee cherr cherr, wee cherr cherr, wee cherr cherr, wee" song is disappearing from spruce and tamarack bogs in northern Minnesota. This species has been declining more than 7 percent per year during the past 17 years. There is evidence of a widespread decline of the Connecticut warbler throughout the United States. Other species that nest in lowland coniferous habitat—wet, mossy, mosquito-infested stands of black spruce, tamarack, and white cedar—are also declining, such as the yellow-bellied flycatcher, Swainson's thrush, and yellow-rumped warbler. These species may be sensitive to logging and fragmentation of large forested bogs. They also could be vulnerable to climate change because a warming climate is detrimental to boreal tree species at the southern extent of their range.

Range Shifts? Climate change is affecting forest bird populations in numerous ways, such as changes in the distribution of tree species. Many forest bird species that are currently found in the northernmost regions of Minnesota will likely find more suitable habitat, temperature, and precipitation conditions for their survival in the climate to the north in Canada.

If those range shifts occur, monitoring programs such as the annual NRRI surveys in Chippewa and Superior forests will be essential to evaluating the health and, wherever possible, aid in decisions to protect habitat for Minnesota's forest birds.

Author Gerald J. Niemi, Ph.D., is a Professor of Biology at the University of Minnesota-Duluth, and a Senior Research Associate at the Natural Resources Research Institute.

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