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Table of Contents:
Keywords: Birds, disturbance, early-successional habitats, management, populations |
Conservationists have been concerned about evidence of population declines over the past thirty years in Neotropical migrant birds that breed in the forests of the eastern United States (Aldrich and Robbins 1970, Briggs and Criswell 1978). Only in the past decade or so, however, have researchers become widely concerned about birds that breed in early-successional shrubland habitats, such as regenerating forest habitat created by wildfire or catastrophic weather events. In fact, these shrubland species are exhibiting more consistent declines than species that breed in mature forest (Askins 1993), a trend that is also reflected in the declining availability of early-successional shrubland habitat in New England. For example, six of the ten most rapidly-declining bird species in Massachusetts, according to the National Breeding Bird Survey, are early-successional species: Brown Thrasher (Toxostoma rufum), Eastern Meadowlark (Sturnella magna), Eastern Towhee (Piplio erythropthalmus), Northern Bobwhite (Colinus virginianus), American Kestrel (Falco sparverius) and Field Sparrow (Spizella pusilla); Sauer et al. 2003). The goal of this article is to review the population trends of shrubland birds in relation to past trends in habitat conditions and to describe our current understanding of the habitat requirements of these species and the issues surrounding their conservation.
Part of the reason why concern for early-successional shrubland birds is less prevalent among conservationists than is warranted, given evidence of recent habitat declines, may have to do with the deeply ingrained sense that mature forests are more natural than early-successional habitats resulting from disturbance. From Wordsworth to Muir to Smokey the Bear, big trees have retained a sort of sanctity, perhaps as a reaction to the excesses wreaked upon the forests of our old world ancestors. Closer examination, however, reveals a long history of disturbance in North America, beginning with effects of Pleistocene glaciations and browsing by large mammals such as the progenitors of our modern elephants (Askins et al. 2000). These events corresponded to the period during which the evolution of many species of songbirds that we know today was taking place (Mengel 1968), and, thus, could not be more natural in relation to the ecology of these species.
After the extinction of the Pleistocene mammals, Native Americans assumed the role of the Pleistocene mega fauna as agents of habitat creation for early successional habitats by frequent and large-scale burning to encourage game (Askins 2000). These activities likely resulted in a mosaic of open grasslands, savannahs, regenerating fields and mature forest that would have provided abundant opportunities for early-successional species to flourish (DeGraaf and Miller 1996, Askins 2000). Although the precise amount of early-successional habitat that was available is not known, it is clear that natural and anthropogenic disturbance maintained substantial amount of these habitats in the Northeast. In fact, there is evidence that the “primeval forest” encountered by early colonists was an artifact of the previous decimation of Native Americans by contact with the first colonists (DeGraaf and Miller 1996).
The widespread view that early-successional shrubland birds are generalists and will persist in the absence of management in areas peripheral to development or management, is another reason why the plight of these species may have received less attention from the conservation community than might be merited based on population and habitat trends. Contrary to this view, some of shrubland birds exhibit “area-sensitivity,” the tendency to prefer larger patches of habitat to small isolated patches, a phenomenon originally described in mature forest bird species. For example, Prairie Warblers (Dendroica discolor), a priority conservation species exhibiting declines in much of New England, preferentially occupy larger openings (Annand and Thompson 1997). It is true that some species of early-successional shrubland birds are regularly encountered on forest edges or small roadside openings; however it appears that, in some cases, these edge type habitats might represent marginal habitats in which these birds are unable to breed successfully. Prairie Warblers and Yellow-breasted Chats (Icteria virens), the latter of which is a shrubland species largely extirpated from New England, occupy field edges as well as natural and anthropogenically-created shrublands in Missouri; however, far fewer individuals in edge habitats were able to attract mates compared to other habitats (Fink and Thompson 2002).
Finally, many shrubland bird species have a narrow range of tolerance to the habitat conditions. In uplands regenerating after clearcutting, Mourning Warblers (Oporornis philadelphia) are able to occupy a site for only a few years after cutting before the structure of the habitat becomes unsuitable for nesting (DeGraaf 1991), whereas “mature forest species” like Red-eyed Vireos (Vireo olivaceus) and Ovenbird (Seiurus aurocapillus) are able to occupy a site more than ten years after disturbance until the site is disturbed again. These findings emphasize the fact that shrubland species are, indeed, habitat specialists and their welfare is not necessarily assured in the absence of active conservation and management efforts.
Until we are able to bring back the mastodons, we will have to rely on human intervention to create habitat for many disturbance-dependant bird species. It is unlikely that we will have the financial resources to create habitat for these specialized species solely for the purpose of their conservation, therefore, in most cases, we will have to rely on human activities that create these types of habitats as a byproduct. In the Northeast, early-successional habitats are produced by activities that include maintenance of powerline rights-of-way and silviculture.
Powerline Rights of Way Powerline rights-of-way (ROWs) are potentially valuable to early-successional bird species because they are maintained in a constant state of early succession to prevent contact between transmission wires and the vegetation (Askins 1994). The potential of ROWs as habitat for shrubland bird species may be affected by ROW width. Some species of shrubland birds, such as the Prairie Warbler, avoid smaller habitat patches (Annand and Thompson 1997). The reason for this is not clear; however, it may have to do with the tendency of these species to avoid edges (Woodward et al. 2001), which comprise a greater proportion of a habitat in a smaller patch. The reason why birds avoid edges may be related to higher predation near edges (see below). Indeed, preliminary results from an ongoing study on the effect of ROW characteristics indicate that Prairie Warblers in ROWs occur in greater densities in wider ROWs (King and DeGraaf, In Prep).
The characteristics of ROWs may affect the reproductive success of nesting early-successional shrubland birds as well as their densities. ROWs that are too narrow may not provide sufficient interior habitat to permit successful reproduction. The greater proximity of nests to edges, where nest predators such as eastern chipmunks (Tamias striatus) concentrate (King et al. 1998), might reduce nesting success in narrow ROWs. Several studies have found that predation is higher near edges in early- successional shrubland habitat (Rudnicky and Hunter 1993, Vander Haegen and DeGraaf 1996); King and Byers (2002) report that nest predation was higher near edges of ROWs in one of two years. Although it appears that some powerline rights-of-ways can support viable populations of shrubland birds, such as the Chestnut-sided Warbler (Dendroica pensylvanica) (King and Byers 2001), preliminary results of our ongoing investigations indicate that nest success is greater in wider rights-of-ways.
SilvicultureSilviculture, or forest cultivation and management, is another potentially important source of suitable habitat for early- successional shrubland birds. The key for using silviculture to benefit these species is that the forestry treatments must remove enough of the canopy to encourage growth of a thick shrub layer. Clearcuts, shelterwood cuts (removing overstory trees to reduce stand density), or patch cuts (creating an open area surrounded by forest) are suitable for producing shrubland habitats for birds. Single-tree selection appears to do little or nothing to encourage early-successional shrubland birds and appears to compromise the quality of the remaining forest habitat for mature forest species such as like Scarlet Tanagers (Piranga olivacea) and Brown Creepers (Certhia americana) as well (King and DeGraaf 2000). Size of a silviculture patch as well as treatment type is important in creating suitable breeding habit for shrubland bird species. Treated patches should be a hectare or more in size; smaller openings appear to be of lower quality, as indicated by earlier nest initiation dates in larger patches in New Hampshire (King and DeGraaf, in review).
Finally, isolation appears to interact with patch area. In an ongoing study of wildlife openings in the White Mountain National Forest in New Hampshire, we are finding that open patches as small as 0.5 hectares support breeding Chestnut-sided Warblers if the smaller patches are adjacent to large source populations like clearcuts, or surrounded by numerous other smaller patches (King and DeGraaf 2001); Chestnut-sided Warblers are less able to attract mates in isolated patches of regenerating forest in this size range.
We conclude, on the basis of our findings and those of others, that the future of early-successional shrubland bird species is by no means assured in the absence of active management. Clearly, efforts to create and maintain early-successional habitat must be conducted as part of a coordinated conservation program that includes sensitivity to the habitat needs of mature-forest dependent species as well as to other values of the forest. Key to these efforts is the recognition on the part of conservationists that preservation of habitat is not a static process, but must recognize the importance of the processes with which the target organisms evolved.
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Annand, E. M., and F. R. Thompson, III. 1997. Forest bird response to regeneration practices in central hardwood forests. Journal of Wildlife Management 61:159-171.
Askins, R. A. 1993. Population trends in grassland, shrubland, and forest birds in eastern North America. Current Ornithology 11:1-34.
Askins, R. A. 1994. Open ROWs in a heavily forest landscape: Impact on shrubland and forest-interior birds. Wildlife Society Bulletin 22:339-347.
Askins, R. A. 2000. Restoring North America’s Birds: Lessons from Landscape Ecology. Yale University Press, New Haven.
Briggs, S. A., and J. H. Criswell. 1978. Gradual silencing of spring in Washington: selective reduction of species of birds found in three woodland areas over the past 30 years. Atlantic Naturalist 32: 19–26.
DeGraaf, R. M. 1991. Breeding bird assemblages in managed northern hardwood forests in New England. Pages 153-171 in Wildlife and habitats in managed landscapes (J. E. Rodiek and E. G. Bolen, Eds.). Island Press, Covelo, California.
DeGraaf, R. M, and R. I. Miller. 1996. The importance of disturbance and land-use history in New England: implications for forested landscapes and wildlife conservation. Pages 3-35 in R. M. DeGraaf and R. I. Miller, editors. Conservation of faunal diversity in forested landscapes. Chapman and Hall, New York, New York, USA.
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King, D. I. and R. M DeGraaf. 2000. Bird species diversity and nesting-success in mature, clearcut, and shelterwood forest in northern New Hampshire, USA. Forest Ecology and Management 12:227-235.
King, D. I., R. M. DeGraaf and C. R. Griffin. 2001. Productivity of early-successional shrubland birds in clearcuts and groupcuts in an eastern deciduous forest. Journal of Wildlife Management 65:345-350.
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King, D. I. and R. M. DeGraaf. In review. Effects of group-selection opening size on the distribution and reproductive success of an early-successional shrubland bird. Forest Ecology and Management.
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Woodward, A. L., A. D. Fink, and F. R. Thompson. 2001. Edge effects and ecological traps: Effects on shrubland birds in Missouri. Journal of Wildlife Management 65:668-675.
1. Eastern meadowlark, adult and young © Isidor Jeklin. Courtesy of the Global Bird Photos Home Page: http://www.globalairphotos.com
2. Mourning warbler, female on nest © Isidor Jeklin Courtesy of the Global Bird Photos Home Page: http://www.globalairphotos.com
3. Powerline right of way © 2003 David I. King
4. Syracuse University forest patch clearcut, Sept. 1974, Chad Oliver. Courtesy of The Rural Technology Initiative http://www.ruraltech.org/
5. Chestnut-sided warbler © 2003 David I. King
6. Chestnut-sided warbler nest © 2003 David I. King
The views and opinions expressed in all articles that appear in "Conservation Perspectives" are those of the authors and do not necessarily reflect those of NESCB.