Factors determining the timing of host response towards parasitic eggs: a meta-analytical approach

Egg rejection is the most widespread defence strategy used by hosts of avian brood parasites to mitigate the fitness costs imposed by parasitism. Although the timing of this response (that is the latency between parasitism and egg rejection) is a critical component of host defences, the variation in rejection latency across different brood parasite–host systems remains poorly understood. Here, a meta-analysis of experimental studies on egg rejection was conducted to identify the key predictors of rejection latency. Results showed that hosts from sympatric populations, where parasites are present, tend to reject eggs more rapidly than hosts from allopatric populations. Egg characteristics strongly influenced response timing: mimetic eggs were ejected later; similarly sized eggs showed the longest latencies and smaller eggs were rejected most quickly. In addition, higher rejection rates were associated with shorter latencies. The duration of nest monitoring also showed a significant effect on measured latency, highlighting a major methodological factor that can bias latency estimates. Our findings indicate that the characteristics known to trigger egg rejection also shape its timing and emphasize the need for methodological standardization (particularly in nest monitoring protocols) to accurately assess egg-rejection behaviours. This work provides new insights into the proximate and ultimate factors driving the timing of host responses in avian brood parasite systems. • Sympatric host populations reject faster than allopatric ones. • Egg colour and size mimicry significantly influenced rejection latency. • Experimental artificial eggs were likely to be rejected faster than natural ones. • Nest architecture had no significant effect on rejection latency. • The duration of nest monitoring significantly affected rejection latency.