Habitat-specific clutch size and cost of incubation in eiders reconsidered

Abstract

The energetic incubation constraint hypothesis (EICH) for clutch size states that birds breeding in poor habitat may free up resources for future reproduction by laying a smaller clutch. The eider (Somateria mollissima) is considered a candidate for supporting this hypothesis. Clutch size is smaller in exposed nests, presumably because of faster heat loss and higher incubation cost, and, hence, smaller optimal clutch size. However, an alternative explanation is partial predation: the first egg(s) are left unattended and vulnerable to predation, which may disproportionately affect exposed nests, so clutch size may be underestimated. We experimentally investigated whether predation on first-laid eggs in eiders depends on nest cover. We then re-evaluated how nesting habitat affects clutch size and incubation costs based on long-term data, accounting for confounding effects between habitat and individual quality. We also experimentally assessed adult survival costs of nesting in sheltered nests. The risk of egg predation in experimental nests decreased with cover. Confounding between individual and habitat quality is unlikely, as clutch size was also smaller in open nests within individuals, and early and late breeders had similar nest cover characteristics. A trade-off between clutch and female safety may explain nest cover variation, as the risk of female capture by us, mimicking predation on adults, increased with nest cover. Nest habitat had no effect on female hatching weight or weight loss, while lower temperature during incubation had an unanticipated positive relationship with hatching weight. There were no indications of elevated costs of incubating larger clutches, while clutch size and colony size were positively correlated, a pattern not predicted by the 'energetic incubation constraint' hypothesis. Differential partial clutch predation thus offers the more parsimonious explanation for clutch size variation among habitats in eiders, highlighting the need for caution when analysing fecundity and associated life-history parameters when habitat-specific rates of clutch predation occur.

Causes and consequences of fine-scale breeding dispersal in a female-philopatric species

“…4-5 days for each additional kilometre of dispersal is substantial, considering that the interquartile range of annual hatch dates in our study population is only 8.3 (±0.8 SE) days (n = 8 years, 2003-2010, 2,946 nests). Late-breeding eider females have higher rates of mass loss during incubation, lower weights at hatching of the brood, smaller clutches and lower hatching success (Ö st et al 2008a;Ö st and Steele 2010). In our current data, clutch size had a nonsignificant tendency to decrease with increasing breeding dispersal distance (LMM, P = 0.18), controlling for female experience, body condition and previous hatching success (analysis identical to that for residual annual hatch dates; see Table 1).…”

“…However, the relationship between nest success and breeding density is confounded by the fact that habitat and breeder quality are linked. Thus, experienced breeders select more concealed and centrally located nests in the densest parts of colonies, with no apparent female-female competition (Ö st et al 2008a;Ö st and Steele 2010). This non-random spatial age distribution of breeders may contribute to the positive relationship between breeding density and nest success, because experienced breeders have higher hatching success (Ö st and Steele 2010).…”

“…Due to this heterogeneity, eiders have many alternatives when choosing a nest site. We may also hypothesize that this heterogeneity could contribute to island-specific spatial variation in nest predation rates, because well-covered nests are better protected from avian clutch predators but more risky for the incubating females (Ö st et al 2008a;Ö st and Steele 2010).…”

Indeterminate laying and flexible clutch size in a capital breeder, the common eider

“…Here, we study arctic common eiders S. mollissima borealis breeding in Svalbard, with high natural predation risk for the Wrst egg in the clutch, before the female sits permanently at the nest (Mehlum 1991b;Robertson and Cooke 1993;Swennen et al 1993;Hanssen et al 2002;Andersson and Waldeck 2006;Öst et al 2008). After predation of their Wrst eggs, some females did not desert the nest but continued laying in it.…”

The early birds and the rest: do first nesters represent the entire colony?

“…Common eider responds to climate change because females that attain the best body condition in late winter generally become the first nesters in a given year (Bolduc et al 2005;Ö st et al 2008) and earlier nesting generally is positively related to hatching success (Ö st et al 2011), more exogenous protein use for egg formation relative to later nesters (Sénéchal et al 2011), and increased clutch sizes Jónsson et al 2009;Mehlum 2012). Unfavorable winter weather can interfere with accumulation of endogenous reserves for breeding, cause nest desertion and affect levels of yolk hormone, nest site selection or immune function (Robertson 1995;Descamps et al 2010;Love et al 2010;Sénéchal et al 2010;Jónsson and Lúðvíksson 2013).…”