Data from: With a little help from my kin: barn swallow nestlings modulate solicitation of parental care according to nestmates’ need

In altricial species, offspring competing for access to limiting parental resources (e.g. food) are selected to achieve an optimal balance between the costs of scrambling for food, the benefits of being fed and the indirect costs of subtracting food to relatives. Since the marginal benefits of acquiring additional food decrease with decreasing levels of need, satiated offspring should be prone to favour access to food by their needy kin, thus enhancing their own indirect fitness, while concomitantly reducing costs of harsh competition with hungry broodmates. We tested this prediction in feeding trials of barn swallow (Hirundo rustica) nestlings by comparing begging behaviour and food intake of two similar-sized nestmates, one of which was food-deprived (FD). Non-food-deprived (NFD) offspring modulated begging intensity depending on their nestmate’s need: when competing with FD nestmates, NFD nestlings reduced both the intensity and frequency of begging displays compared to themselves in the control trial before food deprivation. Hence, NFD nestlings reduced their competitiveness to the advantage of FD nestmates, which obtained more feedings and showed a three-fold larger increase in body mass. Moderation of individual selfishness can therefore be adaptive in presence of a needier kin, because the indirect fitness benefits of promoting its condition can outweigh the costs of forgoing being fed, and because it limits the cost of begging escalation against a vigorous competitor.

在晚成雏物种（altricial species）中，为争夺有限的亲代资源（如食物）的后代会受到自然选择的筛选，以在争抢食物的代价、获取食物的收益，以及抢夺亲属食物所产生的间接代价之间达成最优平衡。由于获取额外食物的边际收益会随自身需求水平的降低而下降，饱食的后代更倾向于优先让有迫切需求的亲属获得食物，以此提升自身的间接适合度，同时也能减少与饥饿同窝幼鸟激烈竞争所带来的成本。我们通过家燕（Hirundo rustica）幼鸟的摄食实验检验了这一预测：对比两只体型相近的同窝幼鸟的乞食行为与摄食量，其中一只为食物剥夺组（FD）。非食物剥夺组（NFD）幼鸟会根据同伴的需求状态调整乞食强度：当与FD幼鸟竞争时，与食物剥夺前的对照试验相比，NFD幼鸟的乞食展示强度与频率均显著降低。因此，NFD幼鸟降低了自身的竞争力，使FD幼鸟获得了更多喂食，体重增幅达到对照组的三倍。综上，当存在需求更迫切的亲属时，个体抑制自私行为具有适应性意义：一方面，促进亲属身体状态所带来的间接适合度收益，能够超过放弃自身进食的成本；另一方面，这也限制了与强劲竞争者展开乞食升级竞争的成本。