Wing Morphology Varies with Age but Not Migratory Habit in American Dippers

—We investigated variation in morphology of American Dippers (Cinclus mexicanus) in the Chilliwack River watershed, British Columbia in relation to gender, age and migratory habit. Male dippers had linear dimensions that were 2–9% longer and, on average, were 16% heavier than females. Adults (AHY) were the same structural size as yearlings (HY). Yearlings, however, had shorter and more rounded wings than adults providing support for the hypothesis that an increased vulnerability to predation may lead to selection for traits that improve take-off performance and maneuverability. Yearlings also had shorter tails suggesting other selective pressures shape tail morphology. Dippers in this population may be sedentary or migrate short distances to breed at higher elevations. We found no evidence that wing or tail morphology varied with migratory habit or that sedentary dippers, that have higher reproductive success, are larger or heavier than migrants. Received 1 November 2007. Accepted 11 May 2008. Migration can impose strong selection pressure on traits that influence speed and efficiency of long-distance flight, and may have a major role in maintaining interand intraspecific variation in wing morphology (Alerstam and Lindström 1990, Alerstam 1991). Aerodynamic theory suggests that costs of long-distance flight are reduced if wings are longer and more pointed, and tails are relatively short (Rayner 1988, 1990; Norberg 1990, 1995). Comparative studies have confirmed that migratory species typically have more pointed wings than sedentary species (Marchetti et al. 1995, Mönkkönnen 1995, Lockwood et al. 1998). Some within species comparisons have reported wing length and/ or wing pointedness increase with migration distance (Pérez-Tris and Tellerı́a 2001, Fielder 2005, O’Hara et al. 2006), and migratory populations have longer and more pointed wings than sedentary populations (Copete et al. 1999, Pérez-Tris and Tellerı́a 2001). Intra-specific differences in wing morphology can evolve rapidly. For example, House Finches (Carpodacus mexicanus) introduced into eastern North America in 1940 soon became migratory (Able and Belthoff 1998) and, by 2002, had more pointed wings than sedentary House Finches in western North America (Egbert and Belthoff 2003). 1 Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada. 2 Corresponding author; e-mail: [email protected] Other studies have found that wing length or pointedness is not correlated with migration distance or migratory behavior (Keast 1980; Mulvihill and Chandler 1990, 1991; Burns 2003). These results may occur because avian flight apparatus is also shaped by other, potentially opposing, selection pressures. For example, predation may select for shorter rounder wings that improve take-off performance (Norberg 1990, Swaddle and Lockwood 2003), and shorter rounder wings and longer tails that enhance maneuverability (Norberg 1990). This may explain why juvenile birds, which are more vulnerable to predators, typically have shorter and more rounded wings than adults (Alatalo et al. 1984). Avian flight apparatus may also be shaped by use of differing foraging strategies, habitat selection, prey size, or sexually selected courtship behaviors (Norberg 1979, Winkler and Leisler 1985, Hedenström and Möller 1992, Marchetti et al. 1995, Stiles et al. 2005). American Dipper (Cinclus mexicanus) populations in mountainous areas are composed of sedentary individuals (residents) that occupy multi-purpose territories year round and migratory individuals (migrants) that overwinter with residents but migrate short distances to higher elevation territories to breed (Morrissey et al. 2004). Long-term monitoring of marked birds has shown that individual dippers rarely switch strategies (Gillis et al. 2008). We investigated whether migratory and sedentary American Dippers differ in mor142 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 121, No. 1, March 2009 phology before or after their first complete (adult) prebasic molt. Specifically, we examined whether (1) short-distance altitudinal migrants have longer more pointed wings and shorter tails than residents, (2) yearlings have shorter more rounded wings than adults, and (3) if residents, that consistently have higher reproductive success (Gillis et al. 2008), are larger than migrants.