Egg size and asymmetric sibling rivalry in red-winged blackbirds.
How big to make an egg is a life history decision that in birds is made coincident with a series of other similar decisions (how many eggs to have, whether to fortify them with maternally derived hormones or immune system boosters, whether to hatch the eggs synchronously or asynchronously). Though within-population variation in egg size in birds has been well studied, its adaptive significance, if any, is unclear. Here we examine within-population variation in egg size in relation to asymmetric sibling rivalry in a 17-year study of red-winged blackbirds (Agelaius phoeniceus), an altricial songbird. Egg mass showed a twofold range of variation, with roughly 80% of the variation occurring across clutches. By commencing incubation before the clutch is complete, mothers create advantaged core and disadvantaged marginal elements within their brood. Previous work on this system has shown that sibling competition is asymmetric, and that core offspring enjoy priority access to food, and as a consequence show higher growth and lower mortality than marginal offspring. Here we examine the effect of initial egg size on nestling growth and survival in relation to these competitive asymmetries. Egg mass was strongly linked to hatchling mass, and remained significantly related to the mass of both core and marginal nestlings; the effect of egg size was stronger for core offspring early in the nestling period, but the disparity between core and marginal nestlings narrowed as they approached fledging age, and slower growing marginals fell victim to brood reduction. The effect of egg mass on survival differed dramatically between core and marginal nestlings. Egg mass was significantly related to the survival of marginal but not core nestlings: below average egg mass was associated primarily with very early mortality. Asymmetric sibling competition is clearly a strong determinant of the consequences of egg size variation.
Forbes
Scott
S
Department of Biology, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB, R3B 2E9, Canada. s.forbes@uwinnipeg.ca
Wiebe
Mark
M
D000818:Animals; D052284:Clutch Size; D003163:Competitive Behavior / Q000502:physiology*; D005247:Feeding Behavior / Q000502:physiology; D005260:Female; D008954:Models, Biological; D009425:Nesting Behavior / Q000502:physiology*; D010063:Ovum / Q000166:cytology / Q000254:growth & development*; D035781:Siblings; D020308:Songbirds / Q000033:anatomy & histology / Q000254:growth & development* / Q000502:physiology*; D015996:Survival Rate
Evolution. 1981 Sep;35(5):898-910
28581044
Oecologia. 2004 Jun;140(1):52-60
15118900
Biol Rev Camb Philos Soc. 1994 Feb;69(1):35-59
8193216
Trends Ecol Evol. 1995 Mar;10(3):130-4
21236982
Trends Ecol Evol. 1997 Jul;12(7):270-4
21238065
Comp Biochem Physiol A Physiol. 1996 Jul;114(3):271-6
8759148
Am Nat. 2008 Mar;171(3):358-65
18217858
Physiol Biochem Zool. 2003 May-Jun;76(3):367-74
12905123
Biol Rev Camb Philos Soc. 2002 Feb;77(1):1-26
11911371