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Habitat use by Loggerhead Shrikes in Ontario and Quebec

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Abstract:

Once common throughout northeastern North America, the migrant race of Loggerhead Shrike (Lanius ludovicianus migrans) has undergone a drastic decline since the middle of the last century. The subspecies was designated as endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 1991. To gain a better understanding of the factors affecting the decline of this species in eastern Canada, habitat selection by Loggerhead Shrikes breeding in Ontario (Smith's Falls, Napanee, and Carden limestone plains) and southern Quebec was studied in 1991 and 1992. Nest trees used by breeding shrikes were compared with similar arbitrarily identified trees in suitable unoccupied habitat to determine if there was a nest-tree preference. Territories where successful nesting attempts were made (i.e., young fledged from at least one of the eggs laid) were located over a 2-year period; data were not collected for unsuccessful nests or nests used for double-brooding. Thirty-seven nests (50%) were located in hawthorn shrubs (Crataegus spp.) and 29 nests (40%) in red-cedar trees (Juniperus virginianus). White cedar (Thuja occidentalis), buckthorn (Rhamnus catharticus), and ash (Fraxinus spp.) were used infrequently (10%). Sixty-two nests (86%) were located in isolated trees or in a copse.

Ten nests (14%) were located in hedgerows. Fifty-one (64%) nest sites were located in an actively grazed pasture. The other nest sites were located in idle pasture, hayfields, or old fields. Nest trees and arbitrarily chosen trees in suitable unoccupied habitat did not differ significantly in average height, width, or canopy concealment. Few differences were detected in the average height of the vegetation or the composition of ground cover within a 10-m radius of nest trees and arbitrarily chosen trees. The average numbers of shrubs per hectare did not differ between breeding sites and suitable unoccupied habitat. Nest trees in the Smith's Falls core breeding area were located significantly closer to roads than arbitrarily chosen trees in suitable unoccupied habitat. Habitat suitability was also assessed according to the density of perches (trees and shrubs), which directly affects the amount of actual utilizable habitat in a territory. Significant differences were found in the amounts of actual habitat and potential habitat. The amount of habitat around active nest sites, historic nest sites, and suitable unoccupied sites was significantly greater around active nest sites. Since few statistically significant differences were found between habitat occupied by shrikes and that which was not used, it is not possible to build a predictive model of suitable breeding habitat for shrikes in this study area.

Autrefois répandue dans tout le nord-est de l'Amérique du Nord, la race migratrice de la Pie-grièche migratrice (Lanius ludovicianus migrans) a subi un déclin sérieux depuis le milieu du siècle. Cette sous-espèce a été mise sur la liste des taxons menacés par le Comité sur le statut des espèces en péril au Canada (COSEPAC) en 1991. Afin de mieux comprendre les facteurs qui peuvent expliquer le déclin de cette espèce dans l'est du Canada, nous avons étudié le choix de l'habitat chez les pies-grièches en période de reproduction en Ontario (Smith's Falls, Napanee et les plaines calcaires de Carden) et dans le sud du Québec en 1991 et 1992. Les arbres occupés par des nids ont été comparés à d'autres arbres semblables identifiés de façon arbitraire dans des habitats adéquats inoccupés pour déterminer si les oiseaux nichent dans des espèces particulières d'arbres. Les territoires où la nidification a été réussie (i.e., où au moins un oeuf a donné un oisillon prêt à l'envol) ont été repérés sur une période de 2 ans; ces résultats ne tiennent pas compte des nids où la nidification a été ratée ou qui ont servi à deux couvées. Trente-sept nids (50 %) ont été repérés dans des buissons d'aubépine (Crataegus spp.) et 29 nids (40 %) dans des genévriers de Virginie (Juniperus virginianus). Le Thuya occidental (Thuja occidentalis), le Nerprun cathartique (Rhamnus catharticus) et des frênes (Fraxinus spp.) ont aussi été utilisés, quoique rarement (10 %). Soixante-deux nids (86 %) étaient localisés dans des arbres isolés ou dans un bosquet.

Dix nids (14 %) ont été repérés dans des haies. Cinquante et un nids (64 %) se trouvaient dans des champs

Document Type: Research Article

Publication date: 2001-05-01

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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