Habitat-specific clutch size and cost of incubation in eiders reconsidered |
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Authors: | Markus Öst Mikael Wickman Edward Matulionis Benjamin Steele |
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Institution: | 1.ARONIA Coastal Zone Research Team,Eken?s,Finland;2.Eken?s,Finland;3.Perkinsville,USA;4.Department of Natural Sciences,Colby-Sawyer College,New London,USA |
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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. |
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Keywords: | Energetic incubation constraint hypothesis Female and clutch safety trade-off Nest cover Partial clutch predation Somateria mollissima |
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