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Effect of altitudinal migration within a watershed on the reproductive success of American dippers

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Abstract:

Distinct changes occur in the distribution of American dipper (Cinclus mexicanus Swainson, 1827) populations during the breeding season. Small numbers of American dippers remain resident on the wintering site, while the majority of birds make short altitudinal movements upstream. Therefore, American dippers breed over large elevation gradients within a watershed, using both the main river and its associated tributaries. I hypothesized that altitudinal migration of American dippers would affect their timing of breeding and ultimately their productivity. Additionally, since the main river and its tributaries differ in habitat, elevation, and nesting substrates, I hypothesized that these variables would also influence American dipper breeding performance. In the Chilliwack watershed of British Columbia, Canada, I followed 99 pairs of American dippers: 23 in 1999, 40 in 2000, and 36 in 2001, of which approximately 65% were residents and 35% were migrants. Resident pairs on the lower elevation river initiated nests earlier and a greater proportion had second broods, contributing to slightly higher nest success and annual productivity compared with migrants on tributaries. Reduced productivity was primarily associated with later onset of breeding, which increased the likelihood that nests were lost to predation or flooding and reduced the probability of initiating a second clutch. Timing of breeding was affected by migratory status and year, but elevation and habitat did not directly influence breeding performance.

Il se produit des modifications caractéristiques de la répartition des populations de cincles d'Amérique (Cinclus mexicanus Swainson, 1827) durant la saison de reproduction. Un petit nombre de cincles d'Amérique demeurent sur les sites d'hiver, alors que la majorité des oiseaux font de courts déplacements en altitude vers l'amont. Les cincles d'Amérique se reproduisent sur un important gradient d'altitudes dans un bassin versant et ils utilisent à la fois le cours principal et ses tributaires associés. Mon hypothèse veut que la migration en altitude des cincles d'Amérique affecte le moment de leur reproduction et, par conséquent, leur productivité. De plus, puisqu'il y a des différences d'habitat, d'altitude et de substrats de nidification entre le cours principal et les tributaires, une seconde hypothèse veut que ces variables affectent aussi la performance reproductive des cincles d'Amérique. Dans le bassin versant de la Chilliwack, Colombie-Britannique, Canada, 99 couples de cincles d'Amérique ont été suivis, 23 en 1999, 40 en 2000 et 36 en 2001, dont environ 65 % étaient résidants et 35 % migrateurs. Les couples résidants dans la portion aval de la rivière commencent la nidification plus tôt et produisent une plus grande proportion de secondes portées, ce qui leur donne un succès au nid et une productivité annuelle légèrement supérieurs à ceux des couples migrateurs dans les tributaires. La réduction de la productivité est surtout reliée au début tardif de la reproduction qui augmente la probabilité de perdre des nids à cause de la prédation ou de l'inondation et diminue celle produire une seconde portée. Le moment de la reproduction est affecté par le statut migratoire et l'année; l'altitude et l'habitat n'affectent, cependant, pas directement la performance reproductive.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: May 1, 2004

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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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