The wood thrush is a medium-sized North American songbird belonging to the thrush family (Turdidae). Its scientific name is Hylocichla mustelina. The wood thrush breeds in eastern North America and migrates to Central America for the winter. This species prefers mature deciduous and mixed forests with a dense understory.
The wood thrush has experienced significant population declines in recent decades primarily due to habitat loss on its breeding grounds. It is estimated that the global population has declined by over 60% since the 1960s. As a result, the wood thrush has been identified as a species of conservation concern by several organizations.
In Canada, the wood thrush is listed as Threatened under the Species at Risk Act (SARA). This means that the Canadian government has determined that the species faces imminent threats to its survival or recovery. Under SARA, Threatened species receive legal protections including prohibitions against killing, harming, harassing, or capturing individuals. The goal is to prevent further declines in populations and distribution.
This article will examine in detail whether the wood thrush should be considered at risk in the province of Ontario based on an analysis of population trends, habitat loss, and current protections.
Wood Thrush Population Trends
The wood thrush has experienced concerning population declines across its range in eastern North America. According to the 2016 State of North America’s Birds report, the wood thrush population declined by over 60% between 1970 and 2014. Breeding Bird Survey (BBS) data indicates the species declined at a rate of 2.1% per year in that period.
These losses have been particularly pronounced in the core of the wood thrush’s breeding range which includes Ontario. Analysis of BBS data from Ontario found that wood thrush populations declined significantly between 1968 and 2015 at an average rate of 2.04% per year. The estimated population reduction in Ontario over that period is 67%.
Some specific regions of southern Ontario like Pelee Island have seen local wood thrush populations virtually disappear since the 1980s. Point counts conducted in Ontario forests have also indicated sizable declines in wood thrush numbers. For example, surveys of 25 forest tracts in central Ontario found that wood thrush populations declined on average by 35% from 2002 to 2012.
The consistent picture is that wood thrush numbers have dropped precipitously in Ontario over the past five decades. The species can no longer be considered common or secure in the province.
Causes of Decline
The wood thrush decline in Ontario mirrors the broader loss across their breeding range and has similar drivers related to habitat. The primary cause is forest loss and fragmentation.
Wood thrushes need large tracts of mature deciduous or mixed forests with dense understory vegetation. These conditions have been reduced due to clearing for agriculture and development as well as forest management practices. The remaining forest cover is increasingly fragmented into smaller patches imbedded in non-forested landscapes.
Fragmentation leads to higher rates of nest predation and parasitism from brown-headed cowbirds. The forest-interior microclimate that wood thrushes rely on is also altered on fragments, leading to lower reproductive success. One Ontario study found that wood thrush nest success was 50% lower in fragmented versus intact forests.
Wood Thrush Habitat in Ontario
The extent of suitable breeding habitat across Ontario is a key factor in whether the wood thrush should be considered at risk. Unfortunately, habitat loss and fragmentation pressures have been substantial:
– Ontario has lost over 70% of its original wetland cover and 35% of its forest cover to human land uses. About 30% of remaining forests are less than 10 hectares in size.
– Forest cover has declined across southern Ontario since European settlement. For example, forest cover in the Carolinian region declined from over 80% historically to around 11% today.
– Natural forest disturbances like fire have been suppressed, reducing early successional habitat preferred by wood thrush.
– Remaining mature forests are fragmented by roads, energy infrastructure, and urban areas. Roads alone crisscross 7% of Ontario’s forested land base.
Protected Areas
While habitat loss has been extensive, some high quality wood thrush breeding areas do persist, particularly in protected areas:
– About 10% of Ontario is covered by provincial parks and conservation reserves that protect representative ecosystems.
– National and provincial parks like Point Pelee, Rondeau, Algonquin, and Killarney protect large tracts of mature forest habitat. Over 10% of the Carolinian zone is protected for example.
– Some private lands are also conservation areas and natural areas managed for biodiversity.
However, wood thrushes are still vulnerable on protected lands due to small reserve size, predation, competition from cowbirds, and adjacent development pressures. Protected areas alone are likely insufficient to recover wood thrush in Ontario.
Legal Protections for Wood Thrush
As a federally Threatened species, the wood thrush receives protections under Canada’s Species at Risk Act prohibiting killing, harming, or harassing of individuals. However, the positive effects of this designation are limited for a few reasons:
- SARA prohibitions only apply on federal lands, not the 90% of Ontario land that is provincial, municipal, or private.
- Critical habitat for wood thrush recovery has not been defined or protected anywhere in Canada.
- Enforcement of SARA for a small songbird is challenging.
- The Act does not protect habitat on non-federal lands or address degradation threats like fragmentation.
Under Ontario’s Endangered Species Act, the wood thrush is listed as Special Concern, a weaker designation. The species has no habitat protection here and could potentially be downlisted or delisted by the province in the future.
Overall, current legal protections for wood thrush in Ontario are limited in scope. They have likely prevented some direct mortality but do not address the habitat issues driving population declines. Stronger provincial habitat protections and land use planning would benefit the species.
Conclusion
The wood thrush has declined dramatically in Ontario over the past 50 years due to extensive habitat loss and fragmentation pressures. Remaining populations are now highly localized and concentrated in protected areas. While the wood thrush receives some protections as a federally Threatened species, current legal measures do not adequately address ongoing threats to breeding habitat in Ontario.
Given the significant historical population declines and continuing downward trends, the wood thrush should be considered a species at serious risk in Ontario unless stronger efforts are taken to protect and restore habitat across the province. Maintaining Canada’s remaining breeding populations will require a concerted effort to increase suitable forest habitat through ecological restoration, land use policy, and stewardship initiatives.
References
- Environment and Climate Change Canada. 2020. Management Plan for the Wood Thrush (Hylocichla mustelina) in Canada. Species at Risk Act Management Plan Series. Environment and Climate Change Canada, Ottawa.
- COSEWIC. 2012. COSEWIC assessment and status report on the Wood Thrush Hylocichla mustelina in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. ix + 46 pp.
- Rosenberg, K.V., T. Will, D.A. Buehler, J.L. Swarthout, W.E. Thogmartin. 2016. Dynamic Distributions and Population Declines of Golden-winged Warblers. Chapter Three in Streby, Henry M., David E. Andersen, and David Buehler (editors). Golden-winged Warbler Ecology, Conservation, and Habitat Management. Studies in Avian Biology (no. 49), CRC Press.
- Environment Canada. 2016. North American Breeding Bird Survey – Canadian Trends Website, Data-version 2015. Environment Canada, Gatineau, Quebec K1A 0H3.
- Burke, D.M. and E. Nol. 2000. Landscape and fragment size effects on reproductive success of forest-breeding birds in Ontario. Ecological Applications 10:1749–1761.
- Friesen, L.E., T.J. Underwood, R.W. Pierson, and W.J. Cadman. 2002. Wood Thrush Population Trends in Ontario, 1968-1998. Ontario Birds 20:2-12.
- Cadman, M.D., D.A. Sutherland, G.G. Beck, D. Lepage, and A.R. Couturier, eds. 2007. Atlas of the Breeding Birds of Ontario, 2001 – 2005. Bird Studies Canada,Environment Canada,Ontario Field Ornithologists, Ontario Ministry of Natural Resources, and Ontario Nature, Toronto, xxii + 706 pp.
- Tozer, D.C. 2016. Birds of Algonquin Park. Friends of Algonquin Park, Whitney, Ontario. 248pp.
- COSSARO (Committee on the Status of Species at Risk in Ontario). 2010. COSSARO Candidate Species at Risk Evaluation Form for Wood Thrush (Hylocichla mustelina).
- Austen, M.J.W., M.D. Cadman and D.A. Sutherland. 1994. Wood Thrush (Hylocichla mustelina) in The Birds of North America, No. 146 (A. Poole and F. Gill, Eds.). The Academy of Natural Sciences, Philadelphia, Pennsylvania; The American Ornithologists’ Union, Washington, D.C.
- Robichaud, C.B., M.A. Villard, and C.S. Machtans. 2002. Effects of forest regeneration on songbird movements in a managed forest landscape of Alberta, Canada. Landscape Ecology 17:247–262.
- Hobson, K.A. and E. Bayne. 2000. Breeding bird communities in boreal forest of western Canada: consequences of “unmixing” the mixed woods. Condor 102:759-769.
- Elliott, K.A. 2006. Declining numbers of wood thrush in a fragmented Ontario landscape. Ph.D. thesis, Queens Univ., Kingston, ON. Vi + 150 pp.
- Ontario Ministry of Natural Resources and Forestry. 2015. State of Ontario’s Natural Resources – Forests 2015. Queen’s Printer for Ontario.
- Austen, M.J.W. and C.M. Francis. 2004. Systematic salvage harvests have little effect on breeding bird communities in northern Ontario. Forest. Chron. 80: 66–70.