Laying-sequence variation in yolk carotenoids and egg characteristics in the red-winged blackbird Agelaius phoeniceus

In many bird species with asynchronous hatching, smaller, later-hatched nestlings are out-competed for food by their larger, earlier-hatched siblings and therefore suffer increased mortality via starvation. It is thought that female birds can either maintain or reduce the survival disadvantage of later-hatched nestlings by differentially allocating maternal resources across the eggs of a clutch. Carotenoid pigments are an example of resources that female birds allocate differentially when producing a clutch, but laying sequence patterns for these pigments remain poorly studied in North American songbirds. We examined intraclutch variation in yolk carotenoids and egg metrics in 27 full clutches of red-winged blackbird Agelaius phoeniceus eggs collected from eight wetlands in central Alberta, Canada. We predicted that carotenoids would decrease across the laying sequence, as in this species, later-hatched, marginal nestlings suffer greater mortality than earlier-hatched, core nestlings. We found nine carotenoid pigments in red-winged blackbird egg yolks, including two that have never been described from avian yolks: α-doradexanthin and adonirubin. As predicted, concentrations and amounts of most carotenoids decreased across the laying sequence, suggesting that female red-winged blackbirds depleted their carotenoid resources as they laid more eggs. However, egg mass and yolk mass both increased across the laying sequence, suggesting that female red-winged blackbirds may use other maternal resources to compensate for the size and survival disadvantage experienced by later-hatched, marginal nestlings.