Age-related brood parasitism and egg rejection in magpie hosts

When the strength or nature of a host -parasite interaction changes over the host life cycle, the consequences of parasitism can depend on host population age structure. Avian brood parasites reduce hosts' breeding success, and host age may play a role in this interaction if younger hosts are more likely parasitized and/or less able to defend themselves. We analyzed whether the age of female magpies (Pica pica) hosts is associated with parasite attack or their ability to reject foreign eggs. We recorded parasitism and model egg rejection of known-age individuals over their lifetime, and established whether likelihood of parasitism or egg rejection changed with age or longevity. Parasitism probability did not change with female age and there was a trend for longer lived females to be more likely to be parasitized. However, model egg rejection probability increased with age for each individual female, and longer-lived females were more prone to reject. Most females in the population were young, and the majority of them accepted model eggs, suggesting that brood parasites exploiting younger host individuals are benefitting from a lower defensive level of their hosts. Our results stress that the intensity of selection by brood parasites may be mediated by the age-structure of host populations, a so far neglected aspect in brood parasite-host research.

当宿主-寄生虫互作（host-parasite interaction）的强度或性质随宿主生命周期发生变化时，寄生效应的后果可能取决于宿主种群的年龄结构。鸟类巢寄生者（avian brood parasites）会降低宿主的繁殖成功率，若年轻宿主更易遭受寄生、或自身防御能力更弱，则宿主年龄可能在该互作中发挥关键作用。
我们针对雌性喜鹊（Pica pica）的年龄是否与寄生攻击及其外来卵拒斥能力相关展开了研究。我们记录了已知年龄个体在其生命周期内的寄生情况与模拟卵拒斥行为，并分析了寄生概率或拒卵概率是否随年龄或寿命发生改变。
寄生概率并未随雌性年龄产生显著变化，但存在趋势表明寿命更长的雌性更易被寄生。然而，每只雌性个体的模拟卵拒斥概率均随年龄增长而升高，寿命更长的雌性拒卵倾向也更强。种群中的多数雌性为年轻个体，且其中大多数会接受模拟卵，这表明利用年轻宿主个体的巢寄生者可从宿主更低的防御水平中获益。
本研究结果表明，巢寄生者施加的选择强度可能由宿主种群的年龄结构所介导，这是当前巢寄生-宿主（brood parasite-host）互作研究中迄今被忽视的一个重要方面。