Résumé
1. Primeval forests offer a reference baseline to understand the origins and evolution of mating systems, as their relatively undisturbed environment provides a glimpse into how ecological interactions and natural selection play out in their original context.
2. We established rates of extra-pair paternity (EPP) for two bird species referential to evolutionary ecology: blue tits and great tits, breeding in natural cavities in Białowieża National Park, the sole remnant of European lowland primeval forests.
3. Genotype-by-sequencing of 889 blue tits and 1256 great tits revealed that 48% of blue tit broods and 39% great tit broods were of mixed paternity, with 15% blue tit and 14% great tit nestlings not sired by their social father. These referential levels align with median values of EPP reported in nestbox studies in secondary and managed forests, suggesting that certain reproductive strategies are advantageous across environments. Observed EPP levels did not affect reproductive success or parental investment, indicating no or limited adaptive value for EPP in the context of a primeval forest ecosystem. 4. Our study challenges the assumption that EPP is maintained by direct or indirect fitness benefits. Rather than indicating recent adaptive responses, the observed patterns may be consistent with long-standing and resilient mating strategies that have persisted under increasing global ecological pressures.