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HISTORY:
It is a little known fact that the Loggerhead Shrike historically
was absent from Ontario. Its range expanded into Ontario and Eastern
Canada with the clearing of forests for agriculture. The first
provincial record was near Hamilton in 1860. During the first half
of the 1900s, shrikes nested fairly commonly in southern Ontario.
DECLINE: A population decline began in the mid-1900s and is
now continent-wide. Doug Clarke (1972) was one of the first to
comment on the disappearance of the Loggerhead Shrike in Ontario.
During the first breeding bird atlas 1981–1985, the province’s
breeding population was 50 to 100 pairs (Cadman 1987). By the second
atlas 2001– 2005, most shrikes were confined to the Carden Alvar and
Napanee area (Chabot 2007). Surveys by Wildlife Preservation Canada
in 2017 found 26 breeding pairs and 60 fledged juveniles, plus 8
unmated birds. This more than doubled the 11 pairs in 2015 (lowest
since 1991) and was a marked increase over the 18 pairs in 2016.
Captive-bred released shrikes made up 17% of the wild population in
2017 (Wheeler 2017). The Loggerhead Shrike would be extirpated as a
breeding bird in Ontario without the annual releases (128 juveniles
in 2017) of captive-bred birds to augment the population.
CAUSES: The reasons proposed for the decline include habitat
loss, road kills, pesticides, West Nile Disease, predation, and
mortality during migration and on the winter range. Pesticides have
not been shown to lower the reproductive success in shrikes (Yosef
1996) and Ontario pairs fledge healthy juveniles. Researchers in
Ontario and elsewhere often remark that shrike numbers seem lower
than the apparent carrying capacity of the habitat. Carden, Napanee,
Manitoulin Island, Grey-Bruce, and Smith Falls all appear to have
breeding habitat that is unoccupied by Loggerhead Shrikes.
CONCLUSION: If habitat isn’t limiting the breeding population
of Loggerhead Shrikes, what is? Cadman (1987) and (Yosef 1996)
reported that road kills were a significant factor in the shrike’s
decline. The decline correlates strongly with the phenomenal
increase in motor vehicles and roads in North America. The shrike’s
habit of frequenting roadsides and swooping low to the ground for
prey make it vulnerable to vehicle strikes. A study in Virginia
found that road kills caused 29% of fall and winter mortality,
second only to predation (Blumton 1989). During migration shrikes
cross hundreds of busy roads twice annually. Road kills are almost
certainly the overriding limiting factor now controlling shrike
numbers in Ontario. Ontario’s Loggerhead Shrikes are now an isolated
outpost population quite distant from the nearest and declining
populations in the United States. Collisions with vehicles (not
breeding habitat) probably are the principal factor limiting shrike
numbers in Ontario. Shrikes would be gone as a breeding bird in the
province without the annual releases of captive-bred birds by
Wildlife Preservation Canada and its partners. Loggerhead Shrikes
will soon be returning to Ontario. Get out and enjoy viewing them
and other grassland specialty birds on the Carden Alvar. |
Loggerhead Shrikes in hawthorn with impaled grasshopper.
Illustration by Christine Kerrigan |
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Blumton, A. K. 1989. Factors Affecting Loggerhead Shrike Mortality
in Virginia. Master’s Thesis. Virginia State University, Blacksburg,
VA.
Cadman, M. 1987. Loggerhead Shrike. In, M.D. Cadman et al., eds.,
Atlas of the Breeding Birds of Ontario. University of Waterloo
Press.
Chabot, A. 2007. Loggerhead Shrike. In, M.D. Cadman et al., eds.,
Atlas of the Breeding Birds of Ontario, 2001–2005. Co-published
by Bird Studies Canada, Environment Canada, Ontario Field
Ornithologists, Ontario Ministry of Natural Resources, Ontario
Nature.
Clarke, C.H.D. 1972. Of Shrikes and Hawthorns. Ontario Naturalist
11(2).
Wheeler, H. 2017. Eastern Loggerhead Shrike Recovery. Issue 27.
Wildlife Preservation Canada.
Yosef, R. 1996. Loggerhead
Shrike. In, Birds of North America. Online. The Cornell Lab
of Ornithology. Ithaca, New York. |