<b>Intra- and inter-clutch variation in egg characteristics of a cuckoo host</b>

The study area is a mosaic of land types, including villages, arboreal forests, and croplands.</p><p dir="ltr">Located in the eastern extension of the Wumeng Mountains, Liuzhi features a predominantly karst landscape with elevations ranging from 1,070 to 1,657 meters above sea level and experiences a subtropical monsoon climate. The study area comprises a mosaic of land cover types, including villages, shrublands, croplands, and barren slopes (Liu et al., 2023).</p><p dir="ltr">Exploring the evolution of adaptive traits of different species in response to varying natural environments and ecological pressures is fundamental to understanding evolutionary dynamics.

This study focused on the grey bush chat (<i>Saxicola ferreus</i>) in southwestern China, comparing the patterns of intra- and inter-clutch variation in egg characteristics between populations in Guizhou and Yunnan under varying levels of parasitism pressure and geographic environments. The results showed that both populations showed a pattern of greater inter-clutch variation than intra-clutch variation in egg phenotypic traits, consistent with the evolutionary strategy of host anti-parasitism behavior.

In addition, the Guizhou population, which experienced higher brood parasitism pressure, exhibited lower intra-clutch variation and higher inter-clutch variation in multiple egg phenotypic variables. In terms of spatial variation, the geographic environment significantly influenced the egg parameters. The eggs in the Guizhou population were significantly heavier and larger in volume than those in the Yunnan population, likely due to differences in environmental temperature and food resource availability.

This study demonstrates the combined effects of parasitism pressure and geographic environment on the phenotypic variation of host eggs, improving our understanding adaptive evolution in cuckoo hosts.</p>