Data from: Kind to kin: weak interference competition among white stork (Ciconia ciconia) broodmates

Altricial nestlings in structured families show a diverse array of behavioural mechanisms to compete for food, ranging from signalling scrambles to aggressive interference. Rates of filial infanticide are moderately high in white storks. It has been hypothesized that this unusual behaviour is an adaptive parental response to the absence of efficient mechanisms of brood reduction (aggression or direct physical interference) by nestlings. To test this latter assumption, we analyzed video recordings of 41 complete feeding episodes at 32 broods during the first half of the nestling period, when nestlings complete 90% of growth and chick mortality and size asymmetries are highest. Parents delivered food to all nestlings simultaneously by regurgitating on the nest floor. No direct (bill to bill) feeding was recorded. Senior nestlings were never observed to limit their junior nestlings from eating food, either by aggression or physical interference. Experimental feeding tests revealed that heavier nestlings handled prey items more efficiently and ate food at a higher speed. The high degree of tolerance shown by senior nestlings is unusual among birds with similar ecological and phylogenetic affinities, such as herons. Tolerance by seniors cannot be easily explained by absence of parental favouritism or proximate factors known to affect the occurrence of sibling aggression in other species (rate of food transfer, brood size, hatching asynchrony or length of nestling period).

具有结构化家庭的晚成雏（altricial nestlings）演化出多样的行为机制以争夺食物，涵盖信号介导的争夺型竞争（scramble competition）至攻击性干扰两类策略。白鹳的亲子弑幼率处于中等偏高水平。有假说提出，这一异常行为是亲代针对幼鸟缺乏高效育雏缩减机制（即攻击或直接物理干扰手段）所产生的适应性应对策略。为验证该假设，我们对育雏期前半阶段的32窝雏鸟的41次完整喂食录像开展了分析——该阶段雏鸟完成90%的生长发育，且雏鸟死亡率与体型不对称性均达到峰值。亲代通过在巢底反刍的方式同时向所有雏鸟投喂食物，未观察到喙对喙的直接喂食行为。研究未发现优势雏鸟通过攻击或物理干扰的方式限制弱势雏鸟进食的情况。人工喂食实验结果显示，体重更大的雏鸟处理猎物的效率更高，进食速度也更快。与鹭类等具有相似生态与系统发育亲缘关系的鸟类相比，优势雏鸟所表现出的高度容忍性实属罕见。现有已知会影响其他物种同胞攻击行为的近因因素（包括食物传递速率、窝雏数、孵化异步性以及育雏期时长）以及亲代偏爱的缺失，均无法轻易解释这一容忍行为。