Our study revealed some degree of genetic structuring among females in relation to where they nested, despite the close proximity of the two islands (Fig. 4). Positive fine-scale spatial autocorrelation of kin was revealed with this colony, albeit at low frequencies and only on the more densely populated TI-A (Fig. 5A). Such trends on TI-B were absent.
The colony-scale structuring observed suggests that certain females, along with their brood mates, prefer to nest on their islands of origin. Such inferences are further supported by numerous field observations (R. D. Titman, unpubl. data). Exhibiting fidelity to one's natal island may be a rule-based mechanism to maximize the likelihood of choosing good-quality nesting and brood-rearing habitat (Schjorring 2001). In several species, juvenile experience can serve as a basis for future settling decisions (Brown and Brown 1992; Osorio–beristain and Drummond 1993; Schjorring et al. 2000).
The strength of fine-scale GSA is expected to be weak (Sonsthagen et al. 2010). Interference with the signal of GSA can be the result of ecological and/or sampling-related factors. Ecological factors include: (1) constraints on nest-site selection and (2) mechanism of recognition. In the first case, constraints on nest-site selection can diminish the presence of GSA, since where an animal nests has important fitness consequences resulting from access to resources and varying degrees of exposure to predation, brood parasitism, and the elements (Clark and Shutler 1999). It is known that vegetation type and structure, in fact, have a strong influence on nest-site selection in this colony of mergansers (Craik and Titman 2009). The second factor is only applicable where GSA is attributed to the presence of kin associations. Kin associations require that the individuals involved are capable of recognizing each other (Waldman 1988; Andersson and Ahlund 2000). If mechanisms of kin recognition are based on familiarity as opposed to phenotype matching (e.g., Petrie et al. 1999; Van der Jeugd et al. 2002), females may actually be attempting to nest in proximity to brood mates and not siblings, per se. Therefore, the presence of high rates of intraspecific brood parasitism within a colony may disrupt the signal of GSA, since not all of one's neighbors are necessarily first-order kin. This consideration is noteworthy because brood parasitism within the TIs can reach up to 40% in some years (Young and Titman 1988). Alternatively, sampling-related interference may have resulted from (1) our underlying assumption that the females (or feathers) apprehended from a given nest are endemic to it and (2) a paucity of genetic samples obtained from nests. While it cannot be discounted as a possibility that the signal on TI-B was lacking due to the sampling efforts (i.e., only approximately 40% of nests were sampled across the colony), it is noteworthy that there are not even vague patterns of positive correlation observed within the correlogram for that island (Fig. 5A). In any case, as the ratio of sampled verse unsampled nests on both islands was virtually the same, it could be reasoned that the degree of structuring on TI-B, if any at all, is much less than on TI-A. One possible explanation for why no GSA was detected in the 0–10 m distance interval might relate to our finding that no nesting synchrony exists between kin (see discussion below). If this is the case, proximally nesting kin most probably colonized a given area in multiple waves. As a function of time, the vacancy of suitable nest sites will be reduced due to the nesting efforts of non-kin and thereby posing a heightened constraint to how close together kin are able to settle.
As stated in the introduction, the detection of fine-scale GSA is ecologically inconclusive. Extreme philopatry, high intracolonial relatedness, and kin associations are three possible phenomena, which the clustering of kin at the local scale can be attributed to. Sonsthagen et al. (2010) were able to disqualify the possibility that extreme philopatry was the mechanism underlying the local-scale clustering of kin in colonies of Pacific common eiders (S. m. v-nigrum). They reasoned that because of the seasonal instability of nests caused by perpetual movements of driftwood, the likelihood that females exhibit fidelity to a particular bowl is minimal. Instead, the patterns of association they observed could more likely be attributed to the presence of kin associations (Sonsthagen et al. 2010). While it was observed in our study that at least some females exhibited fidelity to their natal island, the results are contrary to what is expected if every female had the proclivity to return to a specific natal site (i.e., extreme philopatry), namely that fine-scale GSA was only detected on TI-A. Therefore, the patterns of GSA observed within the TIs are unlikely to be solely the result of extreme philopatry. Furthermore, due to the permutation-based statistical methods employed in our study, the assessments made had effectively accounted for the likelihood of observing a given pattern of the spatial distribution of nests. Consequently, the possibility that the pattern observed is only an artifact of high levels of intracolonial relatedness can be largely discounted.
The third tentative factor that may contribute to the detection of GSA is the presence of kin associations. Feasible forms of kin association within this colony include: (1) lowered aggressive tendencies toward neighboring kin; (2) prehatch brood amalgamation (pre-HBA); (3) posthatch brood amalgamation (post-HBA); and (4) cooperative defense. Reduced agonism between kin can be important in the recruitment of related individuals into a given area (MacColl et al. 2000; Hoglund and Shorey 2003), especially under high-density conditions where the frequencies of interaction between individuals are high (Sonsthagen et al. 2010). Generally speaking, it has been demonstrated that the greater the proximity between neighbors, the greater their vulnerability to thievery (Wojcieszek et al. 2007), cannibalism (Brown 1967; Yom–Tov 1974 and references therein), and brood parasitism (Reyer 1984; McRae 1998); presumably as a function of increased nest access. Most relevant to the TIs is likely to be intraspecific brood parasitism (i.e., pre-HBA), which is incidentally known to have occurred at some of the highest rates known among ground-nesting waterfowl (Young and Titman 1988). Aggression toward foreign females would be expected if brood parasitism has a negative effect on host fitness (e.g., Milonoff et al. 2004) and under such circumstances, both recognition and discriminatory behavior toward kin would serve as adaptive traits (Andersson 2001; Lopez–Sepulcre and Kokko 2002). Under such a scenario, by helping kin to obtain a nearby nesting territory, the costs associated with parasitism are offset by inclusive fitness obtained from either (1) the offspring that are reared directly by the recruited individual or (2) as a result of increased level of relatedness to parasitic eggs. While a higher level of relatedness between donors and recipients has been described in some populations of goldeneyes (Bucephala clangula) (Andersson and Ahlund 2000) and common eiders (Andersson and Waldeck 2007; Waldeck et al. 2008), the relationship between the two in this colony of red-breasted mergansers has yet to be described. The prospect that females on the TIs exhibit increased tolerance toward related females is intriguing, although with no direct line of support. However, the observation that positive GSA was prominent only on TI-A, which also had the higher nesting density, is consistent with the hypothesis that there is an increased tolerance by females toward kin who are attempting to establish adjacent nests. Conversely, the prospect of kin-based post-HBA in this colony has not been well explored. Aside from a few accounts (e.g., Bukacinski et al. 2000; Kraaijeveld 2005), the evidence that a higher degree of relatedness exists between nonparental females and the young they are tending, has not been well substantiated. Instead, more evidence seems to implicate accidental fusion (Gorman and Milne 1972; Savard 1987) and abandonment (Pöysä 1995; Öst 1999; Kilpi et al. 2001) as the two principal factors driving adoption and post-HBA. Öst et al. (2005) showed explicitly that female relatedness was not a factor influencing patterns of brood amalgamation among eider hens. This assertion cannot be made here, as the possibility of relatedness being a factor is unclear. While on one hand, there was a lack of incubation synchrony observed between kin (Table 2C), the brood-rearing period of red-breasted mergansers is known to be long (≈7 weeks) and amalgamated broods of mixed cohorts have been observed (S. R. Craik, pers. obs.). Finally, the possibilities of kin-based cooperative defense operating within this colony are discussed below.
In this study, we report synchrony of incubation between close-nesting females throughout the entire colony (Table 2C), albeit not necessarily amongst kin (Table 2B). While these observations are consistent with the hypothesis presented above that females are engaging in a cooperative-defense strategy (i.e., through deliberate synchronization of nesting activity), it cannot be ruled out that the synchrony observed is in fact just a biproduct of the gradual thawing of suitable habitat.
Nesting in groups has several defensive advantages including benefits from (1) the vigilance and predator-detection abilities of group mates; (2) the defensive actions taken against predators by group mates; and (3) predator satiation from the offspring of group mates (Schmutz et al. 1983; McKinnon et al. 2006). Synchronizing breeding with neighboring females essentially maximizes the period of nesting overlap and, therefore, simultaneously maximizes the period of time when such advantages are obtained (Schmutz et al. 1983). Even though the payoff from this type of cooperative behavior is augmented if directed toward kin (O’hara and Blaustein 1981) (and is for all forms of cooperation), if females are surrounded by a significant proportion of nonrelated neighbors, it may then be best to pursue this strategy apart from considerations of genetic relatedness. Furthermore, constraints such as obtaining a sufficient accumulation of endogenous reserves may be more critical to a female's reproductive success as she is deciding when to initiate nesting (Devries et al. 2008).