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ConservationPosted 2/6/14
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Range Changes for
Golden-winged and Blue-winged Warblers

John Confer, NYSOA Conservation Committee
Published in the October 2013 iissue of NY Birders

photo by Benjamin Van Doren

I have studied Golden-winged (GWWA) and Blue-winged (BWWA) warblers for over 40 years. Many others have conducted important research on GWWA, but no one else has published as extensively about the ecological interactions of these species. The following account of conservation efforts for the GWWA makes considerable reference to my own work.

Golden-winged Warbler, photo by Benjamin Van Doren
Golden-winged Warbler
photo by Benjamin Van Doren

During the last half-century, shrubland birds of the eastern United States declined at 4.1%/yr, an exponential a decline of ~50% every 19 years. During this time, 76% of the shrubland species showed a statistically significant decline while only 21% have significantly increased. Woodland species have done much better with slightly over 50% showing a trend for increasing abundance. (See Breeding Bird Survey, Sauer et al. 2012). Both these trends are widely attributed to reforestation with a gain of forest habitat and a loss of shrubland nesting habitat (e.g., Tefft 2006).

Among the shrubland birds, the plight of the Golden-winged Warbler (Vermivora chrysoptera) (GWWA) is particularly severe with a decline of 6.3%/yr1 resulting in extirpation in OH, IN, IL, most of New England, southern portions of Ontario, the southern third of MI, MN, and WI with major reduction in NJ (from ~200 nesting pair to ~20 in the last decade), major reduction in most of New York and in the lower elevations of the Appalachians (Buehler at al. 2006, Confer et al. 2011). The unusual severity of the GWWA decline, much greater than most shrubland species, can not be explained by habitat loss. In fact, areas where GWWA were recently eliminated still have appropriate habitat: in Virginia only 35 of 863 shrub patches supported GWWA (Wilson et al. 2007), and in New England none of 328 census points in shrubland habitat had GWWA, although Blue-winged Warblers (Vermivora pinus) (BWWA), which followed GWWA into New England, were common (Confer and Pascoe 2003). Gill (1980) first noted that extirpation of GWWA correlated with the expansion of BWWA into the Golden-wing range. GWWA extirpation has occurred in almost every instance of BWWA expansion throughout about half the existing and former range of the GWWA (Confer et al. 2011).

Extensive documentation of this correlation was followed by studies of nesting success. For MN (Will 1986) and in upstate NY (Confer et al. 2003), GWWA nesting success was reduced as proximity to BWWA increased. For the Hudson Highlands, very low nesting success for GWWA occurred in uplands that had many BWWA (Confer et al 2010). GWWA nesting success was low in a habitat restoration project in Hudson Highlands in the presence of BWWA (Confer unpub.). In addition to this well documented competitive effect on number fledged, the transmission of GWWA genes is reduced by hybridization with neighboring BWWA. Almost universally, replacement of GWWA by BWWA even where suitable habitat remains occurs and is due to hybridization and competition. Prolonged coexistence of these closely related species is well documented for only two locations: the Hudson Highlands in southern New York, particularly in Sterling Forest State Park (SFSP) (Eaton 1914, Confer et al. 2010) and in the Appalachian Plateaus and Ridge and Valley Physiographic provinces of Pennsylvania (Larkin and Bakermans in press).

A decade of studies shows that the coexistence of these two species in Sterling Forest (SFSP) is due to a unique situation in swamp forests. In swamp forests, the proportion of GWWA to BWWA is significantly higher than in uplands: 88% in swamp forests vs 54% in uplands. This lowers competition and hybridization. In addition, predation is lower in swamp forests than in uplands; 36% predation in swamp forests vs 59% in uplands. Consequently, swamps provide nearly 50% more fledglings per nest. In fact, nests in swamp forests produce enough young to sustain the swamp population and extra young that sustain the population in upland sinks.

Unfortunately, GWWA nesting in swamp forests in the Hudson Highlands is threatened by Phragmites australis, an invasive, non-native plant. The tall, dense growth of Phragmites eliminates Carex rostrata, virtually the only substrate used for nesting by golden-wings in swamp forests. The number of golden- wing territories in swamp forests at proposed restoration sites has declined from 20 in 2001-'04 to four in 2009-10 and may be zero this year as Phragmites replaced Carex (Confer pers. obs.). Other swamp forests still support sufficient Carex and provide nesting sites for GWWA; but the continued expansion of Phragmites threatens this population, arguably the most significant golden-wing population in the entire New Jersey– New York–New England region.

This year, the Palisades Interstate Park Commission (PIPC) initiated a project to remove the non-native, invasive Phragmites and restore the native vegetation in several swamp forests. Trained volunteers are surveying unexplored wetlands for vegetative conditions and for nesting GWWA to identify suitable and threatened habitat. Further, in swamp forests that once supported a high nesting success for GWWA, the PIPC personnel will direct the cutting and manual removal of Phragmites followed by spraying of recovering shoots. This wetland restoration avoids the mistake of creating upland habitat that may attract GWWA but is likely to also attract BWWA with major negative effects for GWWA. These efforts by PIPC personnel and the volunteers may document a unique method to restore highly significant habitat that supports the highest nesting success and genetic purity of GWWA in a multi-state area.. Thanks are due to all who are involved.

References

Buehler, D. A., A. M. Roth, R. Vallender, T. C. Will, J. L. Confer, R. A. Canterbury, S. Swarthout, K. V. Rosenberg, and L. P. Bulluck. 2007. Status and conservation priorities of Golden-winged Warbler (Vermivora chrysoptera) in North America. Auk 124(4):1-7.

Confer, J. L., K. W. Barnes, and E. C. Alvey. 2010. Golden and Blue-winged Warblers: distribution, nesting success, and genetic differences in two habitats. Wilson Journal of Ornithology 122:273-278.

Confer, J. L. P. E. Allen, and J. L. Larkin. 2003. Effects of vegetation, interspecific competition and brood parasitism on Golden-winged Warbler nesting success. The Auk 121: 138-144.

Confer, J.L. P. J. Hartman, and A. M. Roth. 2011. Golden-winged Warbler (Vermivora chrysoptera). The birds of North America. Number 020.

Confer, J. L., J. Gebhards, and J. Yrizarry. 1998. Golden-winged and Blue-winged warblers at Sterling Forest: a unique circumstance. Kingbird 39:50-55.

Confer J. L. and S. M. Pascoe. 2003. Avian communities on utility rights-of-ways and other managed shrublands in the northeastern United States. Forest Ecology and Management 185:193-205.

Eaton, E. H. 1914. Birds of New York. University of the State of New York, Albany. New York, USA.

Gil, Frank. 1980. Historical aspects of hybridization between Blue-winged and Golden-winged Warblers. Auk 97:1-18.

Larkin, J.L. and M. Bakermans. The Golden-winged Warbler. In The 2nd Pennsylvania Breeding Bird Atlas (A.M. Wilson, R.Mulvihill and D. Brauning, editors). The Penn State University Press, University Park, PA in press.

Sauer, J. R., J. E. Hines, J. E. Fallon, K. L. Pardieck, D. J. Ziolkowski, Jr., and W. A. Link. 2012. The North American Breeding Bird Survey, Results and Analysis 1966 - 2011. Version 12.13.2011 USGS Patuxent Wildlife Research Center, Laurel, MD

Tefft, B.C. 2006. Managing Shrublands and Old fields: In Managing Grasslands, Shrublands and Young Forest Habitats for Wildlife. Published Northeast Upland Habitat Technical Committee; Editors Oehler, Covell, Capel and Long. U. S. Fish and Wildlife Service. 2011. National Wetlands Inventory website. U.S. Department of the Interior, Fish and Wildlife Service, Washington, D.C. http://www. fws.gov/wetlands/.

Will, T. C. 1986. The behavioral ecology of species replacement: Blue-winged and Golden-winged warblers in Michigan. Thesis. University of Michigan, Ann Arbor, USA.

Wilson, M. D., B. D. Watts, M. G. Smith, J. P. Bredlau, and L. W. Seal. 2007. Status Assessment of Golden- winged Warblers and Bewick's Wrens in Virginia. Center for Conservation Biology Technical Report

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