Some montane songbirds have defied expectations about how animals respond to warming temperatures. Ecologists hypothesized plant and animal populations would try to stay within their natural climate conditions and move north or upslope seeking cooler temperatures as climate change heats up their habitats. But a new study of lowland and montane birds in New Hampshire found instead that the majority of montane species went to lower elevations. While other studies have reported individual species moving downslope or south, this is the first to document the majority of an area’s bird community moving against expectations. Testing the accuracy of this hypothesis is possible and necessary because the big climate change experiment is underway.
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Over a 17 year period (1993 to 2009), researchers measured elevational distributions of 28 bird species on mountainsides in New Hampshire’s White Mountains (part of the northern Appalachian Mountains). They visited 768 locations on 42 separate transects at intervals during breeding season and recorded all birds seen or heard. Species included yellow-bellied flycatcher, white-throated sparrow, winter wren, Swainson's thrush, Nashville warbler, yellow-rumped warbler and dark-eyed junco. Some of the birds occupy lower elevations while others are montane species. The study found that the lower elevation species responded as expected. Ten of 16 species (63%) moved to higher elevations, an average of 325 feet higher over the study period. But montane species defied predictions: 9 of the 11 species (82%) moved downslope an average of 62 feet in elevation.
Although the study wasn’t designed to identify reasons for the unexpected movement to lower elevations, researchers speculate the higher mountain birds are following suitable habitat. Forest composition in the White Mountains has changed, especially in the ecotone between the montane and lowland vegetation. Red spruce and paper birch trees have died, in part due to acid rain, and native balsam firs have moved in to replace them. All the montane bird species that moved downslope are closely associated with balsam fir, a tree that handles a wide range of temperatures and elevations. It grows from sea level to 6,000 feet elevation across the northern U.S. from Minnesota to the Atlantic coast, and far north into Canada. When the mid-elevation spruce and birch trees died, the higher elevation balsam firs replaced them by extending their range downslope.
This study adds more evidence that human activities like climate change and pollution are affecting the distribution of mountain species. It also points out that applying conventional expectations when designing conservation programs can lead to failures. Planning for climate change impacts to these birds usually would involve protecting upslope habitat. If balsam fir availability is more critical than temperature for these birds, then protecting and expanding adjacent fir habitat (both up and down slope) is of greatest importance. Conservation responses to climate change challenges must be meaningful. The global experiment began years ago.
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