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Mount St. Helens
Pacific Northwest Research Station

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Portland, OR 97204

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Frequently Asked Questions

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(FAQ 5) How were birds initially affected by the eruption?

 

The song sparrow nested in shrubs in the blowdown and scorch zones. Four speckled eggs are incubating in the song sparrow nest shown here (photo by Charlie Crisafulli).

 

Bird survival during the 1980 eruption depended on the distance of birds from the volcano and disturbance zone. All birds died throughout the entire 230-square-mile blast area and in areas crushed by the debris avalanche. In contrast, many birds outside the blast area but in the path of mudflows likely fled to safety, and birds in tephra-fall areas were temporarily displaced. The power of flight gives birds tremendous ability to move freely, and scientists observed some birds flying into the blast area within days after the eruption. These early immigrants stayed and nested wherever habitat and food were available. After the eruption, the pattern of bird colonization was strongly influenced by habitat structure and complexity, which differed substantially across the disturbance zones. These differences determined which bird species, and how many bird species, were found in each zone. (Birders can print a complete bird checklist for the Mount St. Helens area.)

 

Conditions and recolonization within the pyroclastic flow zone

The pre-eruption forest was completely destroyed and covered with rocky rubble, leaving a barren habitat. Only bird species that nested and foraged on the ground, such as the American pipit (Anthus rubescens) and horned lark (Eremophila alpestris), were able to live in the pyroclastic flow zone.


Conditions and recolonization within the debris avalanche zone

The debris avalanche scoured parts of the Toutle River valley and buried the rest, replacing the former valley with an unusual topography of rock-and-sand hummocks interspersed with natural hollows where small ponds and seeps formed. The Toutle River carved a canyon through the avalanche deposit, creating terraces and a dynamic flood plain. Initially, these diverse habitats offered habitat only for ground-nesting birds like the common nighthawk (Chordeiles minor) and killdeer (Charadrius vociferus), and a few species of waterfowl. But the ponds and seeps developed into biological hotspots, filled with algae, reeds, and cattails and surrounded with thickets of willows, alder, and herbs. Plants also grew along the margins of the avalanche deposit. A spectacularly diverse assemblage of birds colonized all these habitats as they developed.


Conditions and recolonization within the blowdown and scorch zones

The trees toppled by the blast and standing dead trees (snags) created an abundant supply of large dead wood, used by several bird species. Tree saplings and shrubs buried in late-winter snowbanks survived, as did many dormant plants, creating habitats with some complexity. Many bird species colonized these small patches of surviving vegetation. Common species included those that nest and forage on the ground like the dark-eyed junco (Junco hyemalis) and white-crowned sparrow (Zonotrichia leucophrys), species that use snags for nesting or foraging, such as the mountain bluebird (Sialia currucoides), northern flicker (Colaptes auratus), and Vaux’s swift (Chaetura vauxi), and species that use shrubs, like the song sparrow (Melospiza melodia).


Conditions and recolonization within the tephra-fall zone

In the tephra-fall zone, the eruption’s only effect was the burial of understory herbs, mosses, tree seedlings, and very small shrubs. Birds most likely abandoned these areas temporarily, but scientists found many bird species had returned to the tephra-fall zone within a few weeks. Over the next few years, scientists found the same species of birds in the tephra-fall zone that they found in nearby undisturbed sites, although the total number of individual birds using the ash-covered forest floor was likely reduced for the first few years after the eruption.

 

More bird species colonized the disturbance zones as habitat complexity increased. In the years since the eruption, habitat complexity increased in all disturbance zones as surviving plants grew and spread and other plant species became established. Scientists found that additional bird species colonized the blast area as habitat complexity increased, and that the appearance of new species was closely connected to the developing vegetation.

 

The colonization of additional bird species occurred in stages, with species colonizing an area when the vegetation reached a threshold of development that met the birdsí habitat requirements.

 

Bird colonization of streamside vegetation

The most dramatic threshold since the 1980 eruption occurred about 10 years after the eruption. Willow (Salix spp.) and alder (Alnus spp.) shrubs created verdant riparian vegetation along most streams in the blowdown zone, and several Neotropical songbirds such as the yellow warbler (Dendroica petechia) and willow flycatcher (Empidonax traillii) colonized the woody thickets along the streamsides. These species nest or forage in the woody riparian vegetation.


Bird colonization of cottonwood trees

As cottonwood trees (Populus trichocarpa) grew in moist areas, two additional bird species, warbling vireo (Vireo gilvus) and black-headed grosbeak (Pheucticus melanocephalus) colonized. The vireo and grosbeak used the extensive canopies of 50-foot-tall cottonwood trees for nesting and foraging.


Bird colonization of conifer stands

Conifer saplings that survived the 1980 eruption beneath snowbanks had grown into small, dense stands by the mid-1990s and ushered in the arrival of hermit thrush (Catharus guttatus), which forage on the ground under dense cover, and seed-eaters like pine siskins (Carduelis pinus).


Bird colonization of ponds, wetlands, and lakes

Two large new lakes, more than 130 ponds, and dozens of wetlands formed on the debris avalanche deposit. These water bodies provide habitat for many aquatic bird species, including puddle ducks such as the mallard (Anas platyrhynchos), diving ducks such as the ring-necked (Aythya collaris), and grazers like the Canada goose (Branta canadensis). Spotted sandpipers (Actitis macularia) feed and nest along the lake shores. Great blue herons (Ardea herodias), which stand up to 4 feet tall, hunt the wetlands and shallow pond waters, and hard-to-spot soras (Porzana carolina), small birds in the rail family, live in the wetlands.


The return of raptors

Birds of prey have come back to the volcanic landscape as their food sources increased. For example, osprey (Pandion haliaetus) and bald eagles (Haliaeetus leucocephalus) forage for fish in the lakes, red-tailed hawks (Buteo jamaicensis) soar the skies hunting for rodents on the ground, and American kestrels (Falco sparverius) hover, then pounce on grasshoppers and other prey. The short-eared owl (Asio flammeus), which lives in open country and sometimes hunts by day, has also returned. Several other raptors use the area during the summer or while traveling through on their migration routes.


Scientists found that in the early years after the 1980 eruption, the bird assemblages, or groups of species, in the disturbance zones were quite different, corresponding to the dramatically different habitats offered. As plant communities developed, however, creating habitats with more structure and complexity, more bird species colonized each zone and the assemblages for the zones developed some similarities. Even so, as recently as 2005, significant differences still remained among the bird assemblages for the different disturbance zones.

 

Bird colonization into the future

Only the initial stages of succession have occurred among birds in the Mount St. Helens blast area. In the first quarter century after the 1980 eruption, scientists found that more and more bird species colonized the blast area, with the total number of bird species steadily increasing. Further stages of succession—or the replacement of one species by another—have not occurred yet among birds.


If no large eruptions or other large disturbances such as wildfire occur in the near future, scientists expect that bird species replacements will occur as forests become widespread across the Mount St. Helens landscape, and birds of open habitats are replaced by forest bird species. (The eruption in late 2004 and early 2005 has been confined, for the most part, to the crater.) Scientists expect that as succession occurs, the bird assemblages in the disturbance zones will become more and more similar, eventually converging into an assemblage of bird species similar to those of other Pacific Northwest forests.

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US Forest Service - Pacific Northwest Research Station - Mount St. Helens
Last Modified:  Thursday, 28 March 2013 at 14:15:24 CDT


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