Egg spots are important cues for egg recognition in barn swallows

The study areas are located in Danzhou (19°11′–19°52′ N, 108°56′–109°46′ E) and Dongfang (18°43′ –19°18′ N, 108°36′–109°07′ E) in the western part of Hainan Island. Hainan Island is located in southern China, spanning an area of 33,900 km2. It is at the northern edge of the tropics, and has a tropical monsoon climate with a wide variety of plants. The average temperature is 22.5–25.6°C with an annual precipitation of 1,500–2,500 mm (Yang et al. 2019). Barn swallows (Hirundo rustica) in Hainan Island mostly build nests under the eaves of residential houses, with the breeding period occurring between April and July (Yang et al. 2022). Our study was conducted in scattered villages in Danzhou (April to July, 2020) and Dongfang (April to July, 2021), where we searched for barn swallow nests with eggs along the eaves of residential houses to conduct egg recognition experiments.The barn swallow, as one host of the common cuckoo (Cuculus canorus), has demonstrated a certain level of egg recognition ability. However, it remains unclear whether eggshell spots play an important role in their egg recognition. To this end, this study examined the role of eggshell spots in egg recognition in two populations of barn swallows. Our results revealed that in both Danzhou and Dongfang populations, the rejection rate of barn swallows for spotted eggs was significantly lower than that for eggs without spots, suggesting that eggshell spot signaling plays a pivotal role during the process of egg recognition in barn swallows. Our findings also indicated that egg shape (ovoid vs. round) did not affect egg recognition by barn swallows. This was most likely because the round shape did not deviate significantly from egg-shaped properties, causing the inability of barn swallows to distinguish the difference in shape.