The risk of brood parasitism does not affect levels of extra-pair paternity in a cuckoo host

In socially monogamous bird species, extra-pair paternity is common, but
its level varies considerably within and among species. For hosts of brood
parasites, the risk of brood parasitism may influence the costs of
engaging in extra-pair copulations (e.g., leaving the nest undefended) and
the benefits of having extra-pair young (e.g., spreading the risk of nest
predation). However, whether brood parasitism risk affects extra-pair
behavior in host species has rarely been tested. We investigated whether
variation in the risk of brood parasitism affected levels of extra-pair
paternity in Daurian redstarts, a socially monogamous passerine. As a
common host of the common cuckoo, the redstart is unique in that its first
clutch of the year takes place before the arrival of cuckoos. As a
consequence, redstarts experience large seasonal variation in cuckoo
parasitism risk, with no parasitism in the first egg-laying period and a
high risk in the second period. We genotyped 995 nestlings (and unhatched
eggs) from 181 broods and found moderate levels of extra-pair paternity,
with 34.8% of broods containing at least one extra-pair young and 11.7% of
all offspring being sired by an extra-pair male. However, levels of
extra-pair paternity did not differ between the first and the second
egg-laying period. Also, experimentally simulating parasitism risk during
the first egg-laying period by broadcasting cuckoo vocalizations and
presenting taxidermic models of cuckoos did not affect levels of
extra-pair paternity in redstarts. Our results, therefore, suggest that
the risk of brood parasitism does not affect host extra-pair paternity.

在社会一夫一妻制鸟类类群中，额外父权（extra-pair paternity）现象较为普遍，但不同物种间乃至同一物种内部的该现象水平均存在显著差异。对于巢寄生（brood parasitism）宿主而言，巢寄生风险可能会影响个体进行婚外交配（extra-pair copulation）的代价（例如弃置巢穴无人防卫），以及抚育额外子代的收益（例如分散巢被捕食的风险）。然而，巢寄生风险是否会影响宿主物种的婚外交配行为，相关实证检验仍较为匮乏。本研究以社会一夫一妻制的雀形目鸟类北红尾鸲（Daurian redstart）为研究对象，探讨巢寄生风险的季节变化是否会影响其额外父权水平。作为大杜鹃（common cuckoo）的常见宿主，北红尾鸲的独特之处在于其年度首窝产卵发生于大杜鹃抵达繁殖地之前。因此，该物种面临的大杜鹃巢寄生风险存在显著的季节差异：首个产卵期无寄生风险，而第二个产卵期则面临较高寄生风险。我们对181个繁殖窝的995只雏鸟（及未孵化卵）开展了基因分型分析，结果显示北红尾鸲的额外父权水平处于中等区间：34.8%的繁殖窝至少包含1只额外父权子代，且11.7%的全部子代由婚外雄性所育。但首个与第二个产卵期的额外父权水平并无显著差异。此外，在首个产卵期通过播放大杜鹃鸣叫声、展示大杜鹃剥制标本的方式模拟寄生风险，并未对北红尾鸲的额外父权水平产生显著影响。综上，本研究结果表明，巢寄生风险并不会对宿主物种的额外父权水平产生影响。