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Technical Thematic Report No. 12. - Landbird trends in Canada, 1968-2006

Mixedwood Plains Ecozone+(BCR 13)

Contributors: Mike Cadman, Gilles Falardeau, and Lyle Friesen

The Mixedwood Plains is one of the smallest of the ecozones+ and has the highest human density. The vegetation is diverse, with a mix of coniferous and deciduous forests, including Carolinian forests. Alvars, tallgrass prairies, and wetlands also occur. Many areas have been converted to agriculture and urban development. The Mixedwood Plains Ecozone+ is well covered by the BBS and results presented are considered representative of the ecozone+ as a whole. Trends differ by habitat assemblages, with forest birds faring best overall, while grassland birds and other open habitat birds show significant declines (Table 16). Grassland birds show the greatest decline of all groups; abundance has dropped by over 60% since the 1970s. This region is home to many nationally and provincially listed bird species.

Table 16. Trends in abundance of landbirds for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey. The following table shows the BBS Abundance Index by decade from the 1970s to the 2000s.Footnote1
Species AssemblageTrend
(%/yr)
P1970s
(BBS)
1980s
(BBS)
1990s
(BBS)
2000s
(BBS)
Change
Forest Birds1.1% 50.656.964.267.333%
Shrub/Successional0.1% 117.2123.5122.5125.27%
Grassland-3.1%*155.4120.386.459.9-61%
Open / Agricultural-1.8%*133.8124.990.474.8-44%
Urban / Suburban-0.7%*425.9394.3364.4352.2-17%

Table 16 - Footnotes

Footnote 1

In this table: P is the statistical significance; * indicates P<0.05; n indicates 0.05<P<0.1; no value indicates not significant; “Change” is the percent change in the average index of abundance between the first decade for which there are results (1970s) and the 2000s (2000-2006).

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Forest bird assemblage

The overall trend for the forest bird assemblage is positive (Figure 14) although individual species show a variety of increasing, decreasing, and stable trends. The forest bird assemblage includes a wide variety of species that differ in habitat requirements, foraging habits, and migration pattern, thus these differences among individual species are not unexpected. Many species are likely responding to increased forest cover through successional changes in abandoned marginal farmland (Crins et al., 2007). The overall positive trend for the assemblage is reflected by several typical forest species, especially those not particularly sensitive to human disturbance (Black-capped Chickadee, Warbling Vireo, and White-breasted Nuthatch, Table 17). Nevertheless, some worrying declines are apparent despite increased forest cover. For example, the Eastern Wood-pewee, a species that winters in South America and undertakes a long annual migration, has lost 55% of its population since the 1970s (Table 17). The decline in this species is one of many among the aerial insectivores. In contrast, the Red-eyed Vireo, which often inhabits the same forest, is increasing. Within mature forests, where forest cover has not changed, other forest birds, such as the Brown Creeper, Least Flycatcher, and Cerulean Warbler, have shown signs of decline in the past couple of decades (Environment Canada, 2006; Canadian Wildlife Service, Environment Canada, unpublished data; Environment Canada, unpublished data). Although a BBS trend is not available for Cerulean Warbler in Canada because of small sample size, the North American BBS trend indicates a highly significant long-term decline (-4.1% per year), the highest of any warbler in North America (Sauer et al., 2008). This decline is also reflected in results from the Atlas of the Breeding Birds in Ontario (Cadman et al., 2007), and was one reason for the species’ assessment as a Species of Special Concern in 2003 (COSEWIC, 2003) and as Endangered in 2010. The Wood Thrush has remained stable in this ecozone+ despite showing severe population declines in many parts of its breeding range since the mid-1960s and continuing degradation of its forest habitat in both North America and Central America. An increase in forest cover in the Mixedwood Plains has likely helped the population.

Figure 14. Annual indices of population change in birds of forest habitat for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey.

graph

Long Description for Figure 14

This line graph shows annual indices of population change in birds of forest habitat for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey from 1968 to 2006. The abundance index shows an overall increasing trend, from approximately 50 to 70 over the period.

 

Table 17. Trends in abundance of selected species of forest birds that are characteristic of the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey. The following table shows the BBS Abundance Index by decade from the 1970s to the 2000s.Footnote1
Forest BirdsPopulation
Trend (%/yr)
 P1970s
(BBS)
1980s
(BBS)
1990s
(BBS)
2000s
(BBS)
Change
Eastern Wood-Pewee-2.8%*4.384.352.721.96-55%
Veery-1.5% 5.714.874.313.91-31%
Rose-breasted Grosbeak-0.6% 4.725.313.763.70-22%
Great Crested Flycatcher-0.1% 4.725.114.904.48-5%
Scarlet Tanager0.6% 0.610.550.640.6811%
Wood Thrush0.8% 2.202.272.252.7324%
Purple Finch1.7% 0.530.700.740.8052%
Downy Woodpecker2.1%*0.861.021.281.3658%
White-breasted Nuthatch2.8%*0.390.530.770.77100%
Warbling Vireo3.3%*3.545.926.907.63115%
Red-eyed Vireo3.4%*5.185.479.5712.98150%
Black-capped Chickadee6.5%*1.494.596.807.85>200%

Species are listed in order from those showing most severe declines to those showing the most positive increases

Table 17 - Footnotes

Footnote 1

In this table: P is the statistical significance; * indicates P<0.05; n indicates 0.05<P<0.1; no value indicates not significant; “Change” is the percent change in the average index of abundance between the first decade for which there are results (1970s) and the 2000s (2000-2006).

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Shrub/early successional bird assemblage

The overall index for the shrub/early successional assemblage is stable (Figure 15). There has tended to be a positive change among birds making use of young forests (such as Chestnut-sided Warbler, Mourning Warbler, and Indigo Bunting), with a tendency towards negative change in birds making use of old-field habitat (such as Field Sparrow, Brown Thrasher, and Gray Catbird) (Table 18).

Figure 15. Annual indices of population change in birds of shrub/early sucessional habitat for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey.

graph

Long Description for Figure 15

This line graph shows annual indices of population change in birds of shrub/early sucessional habitat for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey from 1968 to 2006. The overall abundance index is stable around 120 for the period.

 

Table 18. Trends in abundance of selected species of shrub/early succession birds that are characteristic of the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey. The following table shows the BBS Abundance Index by decade from the 1970s to the 2000s.Footnote1
Birds of Shrub/SuccessionPopulation
Trend (%/yr)
 P1970s
(BBS)
1980s
(BBS)
1990s
(BBS)
2000s
(BBS)
Change
Field Sparrow-2.4% 2.962.771.841.38-53%
Brown Thrasher-2.2%*4.443.132.232.38-46%
Gray Catbird-1.2% 4.864.062.893.59-26%
House Wren0.0% 6.385.424.865.80-9%
Song Sparrow0.1% 38.1543.3241.4940.496%
American Goldfinch0.6%*19.4320.1422.9926.2835%
Indigo Bunting1.0% 2.322.462.612.9527%
Mourning Warbler2.1% 0.480.910.840.5923%
Chestnut-sided Warbler3.4%*0.991.201.942.01102%

Species are listed in order from those showing most severe declines to those showing the most positive increases

Table 18 - Footnotes

Footnote 1

In this table: P is the statistical significance; * indicates P<0.05; n indicates 0.05<P<0.1; no value indicates not significant; “Change” is the percent change in the average index of abundance between the first decade for which there are results (1970s) and the 2000s (2000-2006).

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The Golden-winged Warbler, whose Canadian breeding range is mainly in southern Ontario, was assessed as Threatened in Canada in 2006 (COSEWIC, 2006a). This species underwent a dramatic range expansion in Ontario during the 1930s (McCracken, 1994) which continued until the 1990s. The population is now rapidly declining throughout its North American range, including Ontario. Reasons for the decline are thought to be a combination of habitat loss (in part due to succession of old fields to forest), parasitism by Brown-headed Cowbirds, and hybridization with the Blue-winged Warbler (Vallender, 2007).

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Grassland bird assemblage

Grassland birds in the Mixedwood Plains are showing dramatic declines (Figure 16), particularly since the beginning of the 1980s. This is similar to results in other Canadian ecozones+ and throughout North America (Sauer et al., 2008). Several species have lost 50% or more of their population over the last four decades (Table 19), likely due to the combined effects of conversion of marginal farmland to forest and more intensive use of remaining agricultural lands where most of these birds nest and winter. The number of wind farms has increased dramatically over the last few years and this trend is expected to accelerate in coming years (CanWEA, 2010). Many current and proposed wind farms in Ontario are in prime grassland areas because these areas often have high wind potential and because they present the fewest logistical constraints to construction. Concerns have been raised that the presence of wind mills might result in lower nesting densities of Bobolinks, Eastern Meadowlark, and other grassland birds because of avoidance or abandonment of areas too close to the structures (Arnett et al., 2007). The relatively stable population of Northern Harrier in this assemblage may reflect its use of large marshes and fens as well as grassland.

Figure 16. Annual indices of population change in birds of grassland habitat for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey.

graph

Long Description for Figure 16

This line graph shows annual indices of population change in birds of grassland habitat for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey from 1968 to 2006. The abundance index shows dramatic declines, in particular since the beginning of the 1980s, where levels were around 150. In 2006 the index had declined to around 50.

 

Table 19. Trends in abundance of selected species of grassland birds that are characteristic of the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey. The following table shows the BBS Abundance Index by decade from the 1970s to the 2000s.Footnote1
Grassland BirdsPopulation
Trend (%/yr)
 P1970s
(BBS)
1980s
(BBS)
1990s
(BBS)
2000s
(BBS)
Change
Bobolink-3.6%*59.3348.2129.5618.64-69%
Eastern Meadowlark-3.5%*28.6318.8514.4310.13-65%
Vesper Sparrow-3.4%*5.344.333.012.05-62%
Savannah Sparrow-2.6%*54.4239.9731.8823.68-56%
Horned Lark-2.3%*7.148.245.613.74-48%
Grasshopper Sparrow-1.6% 0.910.630.900.65-29%
Northern Harrier0.6% 0.380.480.670.36-4%

Species are listed in order from those showing most severe declines to those showing the most positive increases

Table 19 - Footnotes

Footnote 1

In this table: P is the statistical significance; * indicates P<0.05; n indicates 0.05<P<0.1; no value indicates not significant; “Change” is the percent change in the average index of abundance between the first decade for which there are results (1970s) and the 2000s (2000-2006).

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Other open habitat bird assemblage

Birds in this assemblage use a variety of open lands found in agricultural and other landscapes. The fact that so many different species (for example, raptors and passerines, as well as short-distance and neotropical migrants) are declining in this group (Figure 17, Table 20), suggests that problems on the breeding ground may be a common cause. Loss of old-field habitat due to succession and the intensification of agricultural practices which involve the removal of hedgerows are likely important causes. The Eastern Bluebird is an exception within this assemblage. This species was assessed as rare in the 1970s by COSEWIC, but as its population increased, it was de-listed in 1996 (COSEWIC, 2007a). Warmer winters over the last 20 years have likely helped the population. In southern Ontario, the population recovery was greatly assisted in the 1980s and 1990s by projects of concerned citizens that erected and maintained thousands of nest boxes. Tree Swallows are also a beneficiary of nest box programs and have a stable/increasing population in this ecozone+, in contrast to the declines seen in other swallow species. Nevertheless, their populations are experiencing declines further north and in Canada as a whole, especially over the last two decades. This assemblage contains several species of aerial-foraging insectivores, which are declining across Canada (Blancher et al., 2009; Nebel et al., 2010).

Figure 17. Annual indices of population change in birds of other open habitats for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey.

graph

Long Description for Figure 17

This line graph shows annual indices of population change in birds of other open habitats for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey from 1968 to 2006. The abundance index shows steady declines over the period from approximately 160 to 70.

 

Table 20. Trends in abundance of selected species of birds of other open habitats that are characteristic of the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey. The following table shows the BBS Abundance Index by decade from the 1970s to the 2000s.Footnote1
Birds of Other Open HabitatsPopulation
Trend (%/yr)
 P1970s
(BBS)
1980s
(BBS)
1990s
(BBS)
2000s
(BBS)
Change
Bank Swallow-4.6%*33.4231.1611.807.08-79%
Brown-headed Cowbird-3.7%*24.8416.9511.147.92-68%
American Kestrel-3.0%*1.111.280.790.45-59%
Barn Swallow-1.9%n38.6335.9825.3320.90-46%
Northern Rough-winged Swallow-1.8% 1.421.200.970.74-48%
Baltimore Oriole-1.5% 9.097.726.055.95-35%
Red-tailed Hawk-0.8% 0.920.790.620.66-29%
Eastern Kingbird-0.5% 8.359.878.696.53-22%
Tree Swallow1.0% 12.8317.9419.5815.3720%
Eastern Bluebird7.6%*0.130.130.921.09<200%

Species are listed in order from those showing most severe declines to those showing the most positive increases

Table 20 - Footnotes

Footnote 1

In this table: P is the statistical significance; * indicates P<0.05; n indicates 0.05<P<0.1; no value indicates not significant; “Change” is the percent change in the average index of abundance between the first decade for which there are results (1970s) and the 2000s (2000-2006).

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Urban and suburban bird assemblage

Birds typical of urban and suburban landscapes are a diverse group. Unlike many other species, they are united by their ability to tolerate human disturbance. Despite this, the assemblage is showing a slight decline (Figure 18). Many species in this group are not exclusively urban and also occur in more natural habitat while others, such as House Sparrow and House Finch, are associated almost exclusively with human habitation. Population trends are varied, ranging from the very sharp decline shown by Chimney Swift, to substantial increases in Blue Jay, Mourning Dove, and House Finch (Table 21).

Figure 18. Annual indices of population change in birds of urban/suburban habitat for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey.

graph

Long Description for Figure 18

This line graph shows annual indices of population change in birds of urban/suburban habitat for the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey from 1968 to 2006. The abundance index shows a slight decline varying from approximately 475 to 340 over the period.

 

Though the House Finch is a species native to western Canada, its eastern Canada population is more recent. The House Finch was introduced into the cities of the eastern United States from the western United States in the 1940s and from there spread to eastern Canada. The House Finch first arrived in the Mixedwood Plains in the mid-1970s, increased quickly to a peak in the mid-1990s, but has since declined steadily. The recent decline is likely a result of disease, as House Finches are particularly susceptible to conjunctivitis which is easily spread at bird feeders (Dhondt et al., 1998).

Introduced Eurasian birds are a feature of this assemblage; both House Sparrow and European Starling have shown significant declines in recent decades while Rock Pigeon numbers are stable (Table 21). The decline in House Sparrow mirrors declines in Europe (Pan-European Common Bird Monitoring Scheme, 2007), which is thought to be due in part to a reduction in food supply because of less grain spillage as a result of reduced horse use, loss of nesting habitat, and an increase in pollution and predators (e.g. Baillie et al.,2007).

Table 21. Trends in abundance of selected species of urban
/suburban birds that are characteristic of the Mixedwood Plains Ecozone+, based on data from the Breeding Bird Survey. The following table shows the BBS Abundance Index by decade from the 1970s to the 2000s.Footnote1
Urban / Suburban BirdsPopulation
Trend (%/yr)
 P1970s
(BBS)
1980s
(BBS)
1990s
(BBS)
2000s
(BBS)
Change
Chimney Swift-5.5%*2.601.530.970.59-77%
House Sparrow (I)-2.9%*69.6566.5340.2530.57-56%
European Starling (I)-1.4%*163.20135.79120.79105.39-35%
Common Grackle-0.7% 75.9559.9758.8664.62-15%
American Robin0.8%*52.0658.9363.8965.1225%
Chipping Sparrow0.9%*13.0715.6516.5316.1423%
Rock Pigeon (I)0.9% 15.7920.8819.9420.5130%
Blue Jay2.8%*4.206.348.289.39123%
Mourning Dove3.0%*14.3920.2825.5633.66134%
House Finch7.7%n0.000.355.862.72<200%

Species are listed in order from those showing most severe declines to those showing the most positive increases

Table 21 - Footnotes

Footnote 1

In this table: P is the statistical significance; (I) indicates an introduced species; * indicates P<0.05; n indicates 0.05<P<0.1; no value indicates not significant; “Change” is the percent change in the average index of abundance between the first decade for which there are results (1970s) and the 2000s (2000-2006).

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