Divergent Egg-Rejection Strategies Between Laying and Incubation Periods in the Green-Backed Tit (Parus monticolus)

Egg recognition and rejection constitute essential host defenses against brood parasitism, with rejection decisions reflecting evolutionary adaptations shaped by cost-benefit trade-offs. Although long-term studies have established correlations between egg rejection behavior and parasitism risk, it remains unclear whether hosts can dynamically adjust their defenses in response to stage-specific parasitic threats across different breeding periods. We investigated temporal variation in egg recognition and rejection latency in the green-backed tit (Parus monticolus), analyzing both seasonal patterns and stage-specific responses within breeding cycles. Our findings reveal that while rejection behavior remained consistently precise throughout the breeding season without seasonal variation, we observed a striking stage-dependent pattern: complete acceptance of parasitic eggs during laying followed by near-total rejection during incubation. This biphasic response contrasts sharply with patterns reported in most other host species. We propose this unique recognition strategy stems from the tit's egg-covering behavior during laying-an adaptation likely driven by nest predation pressure that temporarily suppresses anti-parasitic defenses. These results demonstrate that green-backed tits have evolved independent adaptive responses to distinct selective pressures: nest predation during egg-laying and brood parasitism during incubation. This study provides new insights into the plasticity of host defenses and suggests that species with egg-covering behaviors may develop specialized anti-parasitic strategies. Our findings offer important implications for understanding the evolutionary dynamics of host-parasite interactions and establish a framework for investigating recognition behaviors in other cavity-nesting hosts.