Data from: Alien eggs in duck nests: brood parasitism or a help from Grandma?

Intraspecific brood parasitism (IBP) is a remarkable phenomenon by which
parasitic females can increase their reproductive output by laying eggs in
conspecific females' nests in addition to incubating eggs in their
own nest. Kin selection could explain the tolerance, or even the selective
advantage, of IBP, but different models of IBP based on game theory yield
contradicting predictions. Our analyses of 7 polymorphic autosomal
microsatellites in two eider duck colonies indicate that relatedness
between host and parasitizing females is significantly higher than the
background relatedness within the colony. This result is unlikely to be a
by-product of relatives nesting in close vicinity, as nest distance and
genetic identity are not correlated. For eider females which had been
ring-marked during the decades prior to our study, our analyses indicate
that (i) the average age of parasitized females is higher than the age of
non-parasitized females, (ii) the percentage of nests with alien eggs
increases with age of nesting females, (iii) the level of IBP increases
with the host females' age, and (iv) the number of own eggs in the
nest of parasitized females significantly decreases with age. IBP may
allow those older females unable to produce as many eggs as they can
incubate to gain indirect fitness without impairing their direct fitness:
genetically-related females specialize in their energy allocation, with
young females producing more eggs than they can incubate and entrusting
these to their older relatives. Intraspecific brood parasitism in ducks
may constitute cooperation among generations of closely-related females.

种内巢寄生（Intraspecific Brood Parasitism, IBP）是一种显著的现象，寄生雌鸟除了在自身巢中孵化卵外，还会在同种雌鸟的巢内产卵，以此提升繁殖产出。亲缘选择理论可解释IBP的耐受性甚至选择优势，但基于博弈论的不同IBP模型却得出相互矛盾的预测结果。我们对两个绒鸭种群中7个多态性常染色体微卫星标记的分析表明，宿主雌鸟与寄生雌鸟之间的亲缘关系显著高于种群内的背景亲缘水平。这一结果不太可能是亲缘个体邻近筑巢的副产物，因巢间距离与遗传同一性无相关性。对于本研究前数十年已环志的绒鸭雌鸟，分析显示：（i）被寄生雌鸟的平均年龄高于未被寄生个体；（ii）含外来卵的巢占比随筑巢雌鸟年龄增长而上升；（iii）IBP水平随宿主雌鸟年龄增加而升高；（iv）被寄生雌鸟巢中自身卵数量随年龄增长显著减少。IBP或让无法产出与孵化能力匹配卵量的老年雌鸟，在不损害直接适合度的前提下获得间接适合度：遗传相关的雌鸟存在能量分配特化，年轻雌鸟产出超出自身孵化能力的卵，并托付给老年亲缘个体。鸭类的种内巢寄生可能构成紧密亲缘雌鸟间的代际合作。