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Effect of cyclic and declining food supply on great grey owls in boreal Sweden

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Abstract:

In this study of 35 years of data, we examine the short-term (cyclic) and long-term relationship between breeding success of great grey owls (Strix nebulosa Forster, 1772) and their food supply (bank voles (Clethrionomys glareolus (Schreber, 1780)), grey-sided voles (Clethrionomys rufocanus (Sundevall, 1846)), and field voles (Microtus agrestis (L., 1761))) in northern Sweden. Annual number of owl nests showed a 3 year cyclicity, which as predicted, corresponded to the length of the vole cycle in the region. Mean annual brood size also fluctuated and was positively dependent on the vole supply during the same spring. In this region, there has also been a decline in vole numbers in recent decades, from high-amplitude cycles in the 1970s to subsequent low-amplitude cycles. Correspondingly, and as predicted, mean annual brood size of the owls also declined, although only during the third years of the vole cycle when vole supply in spring and brood size of the owls is at its highest level in high-amplitude cycles. We predict that in the long run the vole decline, associated with increasingly milder winters, and the reduction of the brood size of the owls, especially in years of high owl breeding success, will have serious implications for the population of great grey owls in Scandinavia.

L’examen de données sur 35 ans, nous sert à examiner les relations à court (cycliques) et à long termes entre le succès de la reproduction des chouettes lapones (Strix nebulosa Forster, 1772) et leur source de nourriture (les campagnols roussâtres (Clethrionomys glareolus (Schreber, 1780)), les campagnols gris-roux (Clethrionomys rufocanus (Sundevall, 1846)) et les campagnols des champs (Microtus agrestis (L., 1761))) dans le nord de la Suède. Il existe un cycle de 3 ans dans le nombre annuel de nids de chouettes, ce qui correspond, comme prédit, à la période du cycle des campagnols de la région. Il y a aussi une fluctuation de la taille des couvées qui est en corrélation positive avec l’approvisionnement en campagnols durant le printemps correspondant. Dans cette région, il y aussi eu un déclin des nombres de campagnols au cours des dernières décennies, depuis les cycles de forte amplitude des années 1970 jusqu’aux cycles subséquents de faible amplitude. Comme prédit, il y a aussi eu un déclin correspondant de la taille annuelle moyenne des couvées des chouettes, mais seulement durant la troisième année des cycles des campagnols, lorsque la présence de campagnols au printemps et la taille des couvées atteignent leur maximum dans les cycles de forte amplitude. Nous prédisons qu’à long terme le déclin des campagnols, associé aux hivers de plus en plus doux, et la diminution de la taille des couvées des chouettes, particulièrement les années de fort succès reproductif des chouettes, vont avoir d’importantes conséquences sur les populations de chouettes lapones en Scandinavie.

Document Type: Research Article

Publication date: December 1, 2008

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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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