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Article Excerpt
Migratory timing is a consequence of endogenous and exogenous factors (Berthold 1996). For example, genetic control mechanisms influence factors including onset of migratory restlessness, temporal pattern of migratory activity, migratory direction, and annual patterns of body mass cycles (Berthold 1996). Events that occur during passage also influence migratory timing, such as competing with other migrants for limited resources (Moore and Wang 1991), satisfying nutritional demands under time constraints (Loria and Moore 1990), and avoiding predation while balancing conflicting demands between predator avoidance and food acquisition (Moore 1994). Additionally, migrants must adjust to unfamiliar surroundings (Aborn and Moore 1997, Petit 2000), cope with unfavorable weather (Richardson 1978) and choose the appropriate direction for the next migratory flight (Moore 1990). Further, exigencies in wintering areas may influence migratory timing. Age and gender-related dominance asymmetries, via influencing winter habitat occupancy, have been related to northerly departure time and timing of arrival in breeding areas (Marra et al. 1998, Norris et al. 2004, Norris and Marra 2007). A successful migrant integrates these factors, tailoring its migratory strategy in an effort to optimize efficiency and speed of movement (Alerstam and Lindstrom 1990, Mabey 2002) because time and condition upon arrival at the migratory destination may have fitness consequences (Rowe et al. 1994, Moore et al. 2005, Smith and Moore 2005a).
Intrinsic factors also have a role in defining migration patterns, passage rates, and arrival timing. Migratory strategies within-species may diverge leading to intra-specific differential migration and/or differential passage (e.g., Bensch et al. 1999). Differential migration arises when migratory patterns vary intraspecifically, leading to geographic or habitat segregation of gender or age classes during the non-breeding season (Terrill and Able 1988, Cristol et al. 1999). Differential passage is defined as the temporal segregation of gender or age classes during migration. The classic example of differential passage is early migration and arrival of territorial individuals (generally males in North American landbirds) in breeding areas (Francis and Cooke 1986), which is considered advantageous for gaining access to the highest quality territories (Bensch and Hasselquist 1991, Aebischer et al. 1996, Lozano et al. 1996, Smith 2003). Differential passage may be a consequence of differential migration patterns, for example, if all individuals of a species initiate migration within the same time frame but must travel different distances. Alternatively, it may be an integral part of intra-specific differences in optimal migration strategies that balance the expenditure of time and energy to maximize fitness and shape departure timing and/or rate of migration (Sandberg and Moore 1996, Mabey 2002, Smith 2003). Given well-documented intra-specific differences in migration patterns and our general lack of understanding of the ultimate and proximate causes driving those differences, there is need to examine variability in migratory timing not just across species but also within species across age and gender.
Our work on stopover and breeding ecology of landbird migrants (Smith and Moore 2003, 2005a, b) in Michigan's eastern Upper Peninsula provided an opportunity to examine the patterns of American Redstarts (Setophaga ruticilla) passing through and arriving at a northerly stopover and breeding site, and to assess arrival timing in response to annual variation in environmental conditions. Our objectives were to: (1) compare arrival pattern by age and gender in redstarts that arrived at our site to breed, (2) compare passage pattern by age and gender in redstarts stopping to rebuild fat stores prior to continuing migration, and (3) relate extrinsic factors such as weather and food abundance to patterns of arrival and passage.
METHODS
Study Site.--The study was conducted during the spring migratory and breeding periods at a 5-ha site on the shoreline of northern Lake Huron in Michigan's eastern Upper Peninsula (46[degrees] 2' N, 84[degrees] 35' W). Forest vegetation at this site was a mixture of conifers including 61% northern white cedar (Thuja occidentalis), 10% balsam fir (Abies balsamea), 5% white spruce (Picea glauca), and deciduous species including 7% quaking aspen (Populus tremuloides) and 2% paper birch (Betula papyrifera) (Smith 2003). Vegetation composition was representative of the lowland coniferous forests common in shoreline habitats throughout the entire eastern Upper Peninsula of Michigan (Smith et al. 1998).
Bird Sampling.--We trapped American Redstarts daily using mist-nets (12 x 2.6 m, 30 mm mesh) from 7 May through 6 June in 1997, 4 May through 28 June in 1998, 30 April through 23 July in 1999, 27 April through 29 July in 2000, and 27 April through 26 July in 2001. We operated between 25 and 30 permanently positioned mist-nets with the exception of 1997 in which we had 10 nets. We augmented...
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