Loggerhead Shrike
Limiting Factors
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One of the most troubling factors that have led to the
decline of loggerhead shrike populations has been habitat loss and fragmentation on the breeding grounds.
In particular, the intensification of farming operations including the conversion of native grasslands and trees to cultivated croplands has
played a major role in reducing shrike nesting and foraging opportunities
and decreasing productivity. Farms during the early to mid-1900s were relatively small with a considerable amount of grassy pasture where shrikes thrived.
In those portions of the Canadian prairie provinces where Loggerhead Shrike declines
have been the greatest, there was a 39% decline in native grasslands between 1949 and 1986.
In areas where substantial numbers of shrikes remained, there had been only a 12% decline in occurrences of unimproved pasture.
In more recent times, contamination has also become a concern. The
Loggerhead Shrike's position near the top of the food chain puts it at a high risk of accumulating chemical residues.
The
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continental decline in Loggerhead Shrike populations coincided with
the introduction and increased use of organochlorides between the 1940s and 1970s.
The decline also corresponded with initial use of chemicals for grasshopper control.
As a result of the widespread usage of these chemicals egg shells of shrikes
are becoming thinner and there has been evidence of a significant negative relationship between DDE (a metabolite of DDT) concentrations and shell thickness of recent eggs.
It should be noted, however, that no studies have produced evidence of a direct effect of chemicals on Loggerhead Shrike populations, and it is likely that most effects on shrikes were indirectly caused by a reduction in prey.
Fortunately, the use of organochlorides is no longer widespread.
Climate change and inclement weather may be a source of significant mortality for young shrikes at the northern fringe of the species' range in Alberta.
Predation is another limiting factor for Loggerhead Shrikes. It is
a natural phenomenon but is also influenced by the activities of man. For instance, Loggerhead Shrikes are most commonly found near
human-created corridors like roadsides and fencerows, and mortality for shrikes in these areas can be higher than in more natural habitats.
Predation may also be an important influence on shrike populations on the wintering grounds, where habitat availability is declining, and birds are forced into marginal areas or to wander.