Certain settling decisions result in the formation of clusters of kin in space. Two primary mechanisms underlying the formation of these associations in adult kin are (1) philopatric tendencies and (2) kin associations (Van der Jeugd et al. 2002; Sonsthagen et al. 2010). In the former, kin clustering is merely a biproduct of the common preference of related individuals to settle in proximity to their natal territory (Greenwood 1980). This drive allows individuals to maximize advantages associated with site familiarity (Wright 1943; Rathbun 1979; Beletsky and Orians 1991). Conversely, kin associations are direct efforts by animals to surround themselves with other genetically similar individuals. By doing so, the emergence of within-group cooperative and altruistic behaviors is facilitated (Reyer 1984; McAllister and Roitberg 1987) and opportunities for kin selection are enhanced (O’hara and Blaustein 1981; Nituch et al. 2008). Interactions between kin can result in numerous benefits including increased survivorship (Lambin and Krebs 1993), local recruitment (Stoen et al. 2005), and reproductive success (Andersson and Ahlund 2000; Nielsen et al. 2006). Philopatry and kin associations are by no means mutually exclusive and it is even suggested that natal philopatry is a precursor for the emergence of kin associations (Greenwood 1980).
In recent years, due to the increased accessibility of genetic markers, the spatial structuring of kin, that is, genetic-spatial autocorrelation (GSA), has been shown to be widespread across various animal taxa; from mammals (Girman et al. 1997; Taylor et al. 1997; Coltman et al. 2003), to arthropods (Burgman and Williams 1995; Loeb et al. 2000; Uesugi et al. 2009), to birds (Piertney et al. 1998; Shorey et al. 2000). Given the philopatric nature of female waterfowl (Doty and Lee 1974; Cooke et al. 1975), it is expected that colonially nesting species, for example, common eiders (Somateria mollissima) and red-breasted mergansers (Mergus serrator), exhibit some degree of GSA (Pearce 2007). Kin associations have also been reported to occur among certain waterfowl (Andersson and Ahlund 2000; Nielsen et al. 2006); and accordingly, such behaviors should also contribute to the presence of GSA (Van der Jeugd et al. 2002; McKinnon et al. 2006; Waldeck et al. 2008; Sonsthagen et al. 2010). Kin associations within a colony are expected to result in genetic structuring at the local scale (Sonsthagen et al. 2010); however fine-scale GSA is not an exclusive indicator of this phenomenon. The presence of (1) high levels of intracolonial relatedness and/or (2) extreme philopatric tendencies, will also produce this signal (Van der Jeugd et al. 2002; Sonsthagen et al. 2010). In the first case, fine-scale GSA can occur randomly if there is a high level of background relatedness (Fowler et al. 2004). For example, this was believed to have occurred among greater white-fronted geese (Anser albifrons frontalis) where nests of kin are occasionally found in tight clusters (Fowler et al. 2004). High intracolonial relatedness can result from a high proportion of females with philopatric tendencies (Greenwood 1980; Ratnayeke et al. 2002). Second, the extent of the philopatric tendencies exhibited by colonial females will have consequences on the degree of GSA observed (Van der Jeugd et al. 2002; Sonsthagen et al. 2010). For instance, first-order relatives (i.e., mother–daughter or sister –sister) exhibiting fidelity to a particular site (e.g., a nest bowl) would result in a stronger, more acute signal of GSA than females returning merely to a given region. While the manifestations listed above may not be easily deconfounded, complementing observations of fine-scale GSA with other ecological data, specifically that of brood synchrony, can bring greater meaning to its interpretation by further qualifying the relationships between kin (i.e., not just with respect to geographical distance).
The red-breasted merganser (M. serrator) is a medium-size sea duck that breeds across the Holarctic range. Strong philopatric tendencies of breeding females have been reported for colonial populations, including on Tern Islands (TIs) at Kouchibouguac National Park, New Brunswick through mark-and-recapture studies (R. D. Titman, pers. comm., see appendix of Craik 2009). While, it is therefore expected that at least some degree of genetic structuring exists (Pearce 2007), the extent and scale at which this occurs is currently unknown. Furthermore, the potential presence of kin association between nesting females remains untested.
The main objective of our study is to examine patterns of spatial-genetic organization among nesting hens on the TIs at various scales. Using a multivariate approach, we first assessed the degree of genetic structuring with respect to the island females were nesting on (i.e., broad scale). Second, we examined the correlation between the geographic and genetic distances between nesting females (i.e., local scale). Finally, in order to assist with our interpretation of GSA, we measured the timing of a female's nesting activities and related these observations to both the geographic and genetic distances to nearby females. To our knowledge, this is the first study to (1) look at the levels of genetic structuring with a red-breasted merganser colony and (2) examine GSA directly in conjunction with the synchronization of nesting activities.