Delayed plumage maturation explains differences in breeding performance of Saffron Finches

Theory predicts that, even within the same population, birds may vary their breeding investment with age. This may be due to an improvement in individual quality of old birds in body condition, foraging and in nesting experience. Birds commonly signal age and individual quality to potential mates and competitors through plumage colour. Young male and female breeders of Saffron Finches (<i>Sicalis flaveola</i>) have dull plumages that change to yellow with age. We asked whether older, yellow birds are in better condition and invest more in breeding than dull, younger birds. In southern Brazil, we monitored Saffron Finches breeding in nest-boxes over three breeding seasons. We found that yellow birds were in better condition: females had more furcular fat and males were heavier. Yellow parents bred earlier, had larger clutch sizes, shorter incubation periods, re-nested more and ultimately fledged twice as many offspring as dull parents. Egg mass, nestling mass and nest survival probability were similar between plumage colours. Female colour predicted clutch sizes and incubation periods better than male colour. Our results highlight the importance of accounting for female quality and reproductive investment decisions within the context of sexual selection.

演化理论预测，即便在同一种群内部，鸟类的繁殖投入也会随年龄产生变化。这一现象可能源于老龄鸟类个体在身体状况、觅食能力与筑巢经验方面的提升。鸟类通常通过羽色向潜在配偶与竞争者传递自身年龄与个体质量的信号。黄丝雀（Sicalis flaveola）的年轻雌雄繁殖个体羽色黯淡，随年龄增长会逐渐转为黄色。本研究旨在探究：相较于羽色黯淡的年轻个体，老龄黄色个体是否身体状况更佳、繁殖投入更高。在巴西南部地区，我们对使用人工巢箱繁殖的黄丝雀开展了三个繁殖季的监测工作。研究结果显示，黄色个体身体状况更佳：雌性个体的锁骨间脂肪含量更高，雄性个体体重更大。黄色亲鸟的繁殖起始时间更早，窝卵数更大，孵化期更短，二次筑巢繁殖的频次更高，最终育雏出飞的后代数量是黯淡羽色亲鸟的两倍。不同羽色个体的卵重、雏鸟体重与巢存活率均无显著差异。雌性羽色对窝卵数与孵化期的预测效果优于雄性羽色。本研究结果凸显了在性选择框架下，考量雌性质量与繁殖投入决策的重要性。