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Seasonal Trends in Population Density, Distribution, and Movement of American Dippers Within a Watershed of Southwestern British Columbia, Canada

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Seasonal Trends in Population Density, Distribution, and Movement of American Dippers Within a Watershed of Southwestern British Columbia, Canada The Condor 106:815±825 q The Cooper Ornithological Society 2004 SEASONAL TRENDS IN POPULATION DENSITY, DISTRIBUTION, AND MOVEMENT OF AMERICAN DIPPERS WITHIN A WATERSHED OF SOUTHWESTERN BRITISH COLUMBIA, CANADA CHRISTY A. MORRISSEY1,LEAH I. BENDELL-YOUNG1 AND JOHN E. ELLIOTT2,3 1Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada 2Canadian Wildlife Service, Environment Canada, 5421 Robertson Road RR# 1, Delta, BC V4K 3N2, Canada Abstract. American Dippers concentrate on low-elevation streams during fall and winter in many parts of their range; however the breeding origin relative to the wintering location is poorly understood. Our objectives were to identify seasonal changes in the density and distribution of American Dippers, to classify the migratory behavior of the local population, and to determine the origin of winter migrants within a coastal watershed of British Colum- bia, Canada. During 1999±2002, we color banded 522 dippers and radio-tagged 14 in the Chilliwack River watershed. Using mark-resighting techniques, we identi®ed peak densities on the main river during early November (9.8 6 1.4 [SE] birds per stream km), which was nearly 5 times higher than in early July (2.1 6 0.3 birds per stream km). The watershed's total population size, estimated from November surveys, was 429 6 64 [SE] dippers in 1999, 682 6 79 in 2000, 697 6 123 in 2001, and 550 6 72 in 2002. The majority (79± 90%) of the dipper population seasonally migrated, primarily moving from the main river in fall and winter to the higher-elevation tributaries in spring. The remaining dippers (10± 16%) remained resident on the main river year-round. Migrants showed a high degree of winter site ®delity with 67% returning to the same site on the Chilliwack River for 2 or more years. Given the population's de®ned structure and predictable seasonal movements, this study has implications for applying American Dipper populations as indicators of water and habitat quality in North American watersheds. Key words: altitudinal migration, American Dipper, Cinclus mexicanus, indicator spe- cies, mark-recapture, radio-telemetry, watershed. Tendencias Estacionales en Densidad Poblacional, DistribucioÂn y Movimiento de Cinclus mexicanus en una Cuenca del Sudoeste de la Columbia BritaÂnica, Canada Resumen. Cinclus mexicanus se concentra en rõÂos ubicados a baja elevacioÂn en muchas partes de su rango durante el otonÄo y el invierno; sin embargo, se conoce poco sobre la relacioÂn entre el sitio originario de reproduccioÂn y la localizacioÂn durante el invierno. Nues- tros objetivos fueron identi®car los cambios estacionales en la densidad y la distribucioÂn de C. mexicanus, clasi®car el comportamiento migratorio de la poblacioÂn local y determinar el origen de los migrantes invernales en una cuenca costera de la Columbia BritaÂnica, CanadaÂ. Entre 1999 y 2002 anillamos con bandas de color 522 individuos de C. mexicanus y se- guimos con radio-telemetrõÂa 14 individuos en la cuenca del RõÂo Chilliwack. Usando teÂcnicas de marcado y re-avistamiento, identi®camos densidades pico en el rõÂo principal durante principios de noviembre (9.8 6 1.4 [EE] aves por km de rõÂo), lo que fue casi 5 veces maÂs alto que a principios de julio (2.1 6 0.3 aves por km de rõÂo). El tamanÄo total de la poblacioÂn de la cuenca, estimado a partir de los censos de noviembre, fue 429 6 64 [EE] aves en 1999, 682 6 79 en 2000, 697 6 123 en 2001 y 550 6 72 en 2002. La mayorõÂa (79±90%) de la poblacioÂn de C. mexicanus migro estacionalmente, movieÂndose principalmente desde el rõÂo principal en el otonÄo y el invierno a los tributarios de mayor elevacioÂn en la primavera. Los individuos restantes (10±16%) permanecieron como residentes en el rõÂo principal du- rante todo el anÄo. Los migrantes mostraron un alto grado de ®delidad a los sitios de invierno, con un 67% de los individuos retornando al mismo sitio en el RõÂo Chilliwack por 2 anÄos o maÂs. Dada la estructura de®nida de la poblacioÂn y los movimientos estacionales predecibles, este estudio senÄala que las poblaciones de C. mexicanus pueden ser usadas como indicadores de la calidad del agua y del haÂbitat en las cuencas de AmeÂrica del Norte. Manuscript received 2 September 2003; accepted 18 July 2004. 3 Corresponding author. E-mail: [email protected] [815] 816 CHRISTY A. MORRISSEY ET AL. INTRODUCTION species' range (King et al. 1973, Kingery 1996, Monitoring programs involving birds as indica- Campbell et al. 1997). Speculations as to the or- tors of habitat quality are now widely used on igin of these wintering birds include a possible migration from more northern latitudes where both a local and national scale as an effective seasonal freezing is common (Campbell et al. way to measure spatial and temporal trends in 1997) or an altitudinal migration from remote degradation of key ecosystems (Furness 1993). mountain streams (Kingery 1996). Lack of re- In general, knowledge about the ecological im- search in this area has precluded our understand- pacts on monitored populations may serve as an ing of the population dynamics and movement early warning of habitat degradation. However, patterns of the American Dipper. However, the a solid understanding of the basic ecology of an use of marked individuals may facilitate relating indicator species is necessary to effectively in- wintering populations to speci®c breeding ori- terpret population-level effects and monitor key gins. ecosystems. This research addresses several questions Numerous European studies have successfully about the migratory movements of American used the Eurasian Dipper (Cinclus cinclus), an Dippers and temporal trends in dipper popula- aquatic songbird, as an environmental monitor tion density, distribution, and size. Our objec- of riparian habitat including changes in water tives were to document the seasonal changes in quality (Ormerod et al. 1991, Tyler and Ormerod density and distribution of dippers within a wa- 1992, Logie et al. 1996, Buckton et al. 1998, tershed of southwestern British Columbia; to Sorace et al. 2002). Eurasian Dipper populations identify the composition and size of the local show considerable variability in migratory ten- population; and to determine the origin and dency, with some more northern populations movement patterns of winter migrants through demonstrating clear seasonal movement while extensive banding and radio-telemetry. others are more sedentary (Tyler and Ormerod 1994). In North America, recent studies have METHODS also supported the use of the American Dipper STUDY AREA (Cinclus mexicanus) as a biological indicator of The study area is located within the Chilliwack stream condition (Feck 2002, Strom et al. 2002, River watershed (49809N, 121849W), a tributary Morrissey et al. 2004). However, relatively little of the large Fraser River system, located ap- is known about the migration ecology of the proximately 100 km east of Vancouver in the North American species. If the American Dipper Cascade Range of southwestern British Colum- is to be a successful indicator of changes in en- bia, Canada (Fig. 1). The Chilliwack River is vironmental quality, the population's structure 43.5 km in length. It is fed by a large glacial and movement patterns need to be understood. lake at its upper end and converges into the Ved- Most accounts of partial migration in the Eur- der River downstream. The watershed ranges asian and American Dipper have been primarily from 20 to 2500 m elevation and drains an area descriptive or speculative in nature (Bent 1948, of over 1200 km2. The watershed tributaries are Bakus 1959a, 1959b, BalaÂt 1962, Jost 1969, ®rst through third order streams, and the river is Whitney and Whitney 1972, Tyler and Ormerod dominantly a fourth or ®fth order stream. This 1994). Furthermore, the extent of migration and watershed served as a suitable study site because the location of the wintering and breeding sites of its high winter densities of American Dippers, of individual American Dippers have never been as observed in several watersheds throughout examined. In Canada, few records exist on the the region, as well as good road accessibility. American Dipper's abundance and no compre- The Chilliwack River and its tributaries are char- hensive ecological studies have been done on acterized by highly suitable dipper habitat com- this species in British Columbia, despite the fact posed of fast-moving water, a variety of rif¯es that the majority of the Canadian population ex- and pools, large boulders and cliffs, and a mix- ists here (Campbell et al. 1997). Observations in ture of cobble and sand substrate. southwestern British Columbia have indicated high densities of American Dippers occur in fall CAPTURE AND MARKING and winter on several rivers and streams, which From 1999 to 2002, we captured and marked is consistent with reports from elsewhere in the 522 American Dippers in our study area in ac- DENSITY, DISTRIBUTION, AND MOVEMENT OF AMERICAN DIPPERS 817 FIGURE 1. Map showing the Chilliwack River watershed in southwestern British Columbia, Canada. Numbers (1±8) on the watershed map indicate the locations of winter banding and bimonthly survey sites along the main stem of the Chilliwack River. Shaded areas indicate open water lakes and streams. cordance with federal scienti®c, banding, and ed with a numbered USFWS aluminum band animal care permits. We established eight sites and three colored celluloid bands. Adult dippers along the river at approximately 4-km intervals are sexually dimorphic in size and have different for trapping and surveying (Fig. 1). The majority breeding behavior, allowing us to sex them by of dippers were caught at those eight sites from wing chord measurement, by the presence of a January to March, as they wintered on the Chil- brood patch during the breeding season, or by liwack River.
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