The secondary adjustment of clutch size in red-winged blackbirds ( Agelaius phoeniceus )

Forbes, Scott; Glassey, Barb; Thornton, Suzanne; Earle, Lisa

doi:10.1007/s002650100332

Cited by 38 publications

(52 citation statements)

References 33 publications

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“…3): the underlying relationship between egg mass and nestling size thus becomes more apparent among older marginal nestlings. Core offspring, which enjoy priority access to food, enjoy enhanced growth (Forbes et al 1997(Forbes et al , 2001Forbes and Glassey 2000;Hall et al 2010), and as a consequence little early brood reduction (Fig. 4).…”

Section: Discussionmentioning

confidence: 98%

“…Core brood size ranged from one to four; marginal brood size ranged from zero to four, and overall brood size ranged from one to six (Forbes 2009). Partial brood loss was frequent particularly in larger broods and was associated with retarded growth of marginal progeny, and only rarely affected core progeny (Forbes et al 1997(Forbes et al , 2001Forbes 2009). We believe that nearly all partial brood loss arose as a by-product of fatal sibling rivalry.…”

Section: Methodsmentioning

confidence: 98%

“…Larger core sibs enjoy priority access to parentally delivered food and as a consequence are largely buffered from the inimical effects of food shortfalls (Forbes et al 1997(Forbes et al , 2001Forbes 2009Forbes , 2010: the result is an obvious link between egg size and subsequent nestling size. For marginal sibs, access to food is limited by competition with the core brood.…”

Section: Within-clutch Variation In Egg Sizementioning

confidence: 99%

“…But a series of other decisions are superimposed upon this: how many eggs to make (Monaghan and Nager 1997); whether to fortify individual eggs with maternally derived hormones (Schwabl 1996) or immune system complements (Royle et al 2001;Bortolotti et al 2003); when to commence incubation (Clark and Wilson 1981). As in many songbirds, red-winged blackbirds (Agelaius phoeniceus) impose maternal handicaps upon their nestlings-e.g., by asynchronous hatching (Forbes et al 1997(Forbes et al , 2001Forbes 2009); differences in egg size (Reed et al 1999); hormonal titre (Lipar et al 1999); and immune system complement (Hall et al 2010)-that render some offspring more equal than others. The presence of such handicaps structures the sibship into castes of advantaged ''core'' offspring and disadvantaged ''marginal'' offspring (Mock and Forbes 1995;Forbes et al 1997;Mock and Parker 1997;Forbes and Glassey 2000;Hall et al 2010).…”

Section: Introductionmentioning

confidence: 97%

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Egg size and asymmetric sibling rivalry in red-winged blackbirds

Forbes

Wiebe

2010

Oecologia

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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.

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Section: Discussionmentioning

confidence: 98%

Section: Methodsmentioning

confidence: 98%

Section: Within-clutch Variation In Egg Sizementioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

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Egg size and asymmetric sibling rivalry in red-winged blackbirds

Forbes

Wiebe

2010

Oecologia

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“…Nestling blackbirds engage in non-aggressive begging competitions for parentally delivered food and older, stronger nestlings are more successful than their younger nestmates by virtue of their ability to stretch their necks higher, and present themselves as more conspicuous targets for parents with food. Crowding is evident in larger broods as nestlings generally grow more slowly and suffer higher mortality (Forbes, Thornton, Glassey, Forbes, & Buckley, 1997;Forbes & Glassey, 2000;Forbes, Glassey, Thornton, & Earle, 2001). But the costs of crowding do not befall all nestlings equally.…”

Section: Despotism In Sibling Triadsmentioning

confidence: 99%

Sibling Rivalry in the Birds and Bees

Forbes

2010

Handbook of Jealousy

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Families are complex social structures that are forums for often-spectacular cooperation and altruism among close kin. We find myriad examples of selfsacrificial behavior across the full spectrum of nature, from simple social insects where sisters forgo their own reproduction, and sometimes forfeit their lives to aid their close relatives, to mother hippos, bears, and crocodiles that are both fearless and lethal in the protection of their progeny. Nurturing parents and affectionate siblings match our expectations of family harmony, and biologists on the basis of shared genetic interests expect cooperation and altruism among close relatives.Inclusive fitness theory, first proposed by biologist William D. Hamilton (Hamilton, 1964a(Hamilton, , 1964b, provides the conceptual framework for understanding interactions among kin. Close relatives have a high probability of sharing identical copies of genes by recent common ancestry, and promoting the interests of relatives also promotes one's own (shared) genetic interest; the closer the relationship, the higher the probability of sharing identical genes. Thus from a genetic perspective, altruism within the family is easy to understand. But families are also venues for extremes of violence and selfishness, and Robert Trivers (1974), building upon Hamilton's work, showed how parents and offspring might disagree over how resources within the family should be shared, defining the concept of parent-offspring conflict (Trivers, 1974)

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