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.

晚成雏物种（altricial species）中，为争夺有限的亲本资源（如食物）而竞争的后代，会经自然选择在争夺食物的成本、获得食物的收益，以及挤占亲属食物份额所产生的间接成本之间达成最优权衡。由于获取额外食物的边际收益会随自身需求水平下降而降低，饱食的后代更倾向于让有进食需求的亲属获得食物，以此提升自身的间接适合度（indirect fitness），同时还能减少与饥饿同窝幼鸟激烈竞争的成本。我们在家燕（Hirundo rustica）雏鸟的喂食实验中验证了这一预测：通过比较两只体型相近的同窝雏鸟的乞食行为与食物摄入量，其中一只被设置为食物剥夺组（FD）。非食物剥夺组（NFD）雏鸟会根据同窝同伴的需求调节乞食强度：当与FD组同伴竞争时，NFD雏鸟的乞食强度与频率均低于其在食物剥夺前的对照实验中的表现。因此，NFD雏鸟降低了自身的竞争力，以帮助FD组同伴获得更多喂食机会，这些FD雏鸟的体重增幅达到三倍。由此可见，当存在需求更迫切的亲属时，克制个体的自私行为具有适应性：一方面，提升亲属生存状态所带来的间接适合度收益，能够超过放弃自身进食机会的成本；另一方面，这种克制还能限制与强劲竞争者展开乞食升级竞争的成本。