Asian koels exhibit host-specific egg mimicry

Study areaHuangpi District (114°09′–114°37′ E, 30°40′–31°22′ N) is located in the north of Wuhan City, Hubei Province, central China. It has a subtropical monsoon climate and spans a total area of 2256.70 km2, with a resident population of 1.2427 million. In this study area, azure-winged magpies mainly nest in green trees along roads, surrounding houses, ponds, and around farmland (Zhou et al. 2025).Egg color quantificationIn May–August 2025, systematic searches were conducted in Huangpi District for breeding nests of azure-winged magpies, located and geotagged using GPS. The date of the first egg laying, egg color and morphology, clutch size, and signs of brood parasitism were recorded for each nest.A spectrometer (AvaSpec-2048-SPU2, Avantes, The Netherlands) was used to acquire spectral reflectance data from the eggs in the 300–700-nm waveband for objective egg color measurements. The spectrometer was equipped with an Avalight-Hal-S halogen light source, a reflection probe (FCR-7UV200-2-ME), and a reflection probe holder (RPH-1). Before measurement, the probe was fixed at a 90° angle on the RPH-1 probe holder to keep it perpendicular to the eggs. Measurements were first standardized using white and black reference tiles for diffuse reflectance made of PTFE-based material (WS-2). Each egg was then measured at three regions (i.e., top, middle, and blunt) (Yang et al. 2011), yielding a total of six measurement points (three of each for background and spot colors). The data was summarized using the average values for each egg. All egg color measurements were performed in the same dark environment (Yang et al. 2011).Avian visual modeling was applied to describe the brightness-, chroma-, hue-, and ultraviolet (UV) ratio-related similarities of mimetic eggs. Although two distinct visual systems, i.e., the UV- and violet-sensitive (UVS and VS, respectively) (Cuthill 2006; Hart and Hunt 2007), are reported in birds, evidence suggests that corvids possess the latter (Ödeen and Håstad 2003; Ödeen et al. 2011). Hence, the VS visual system of the common peafowl (Pavo cristatus) was selected for spectral analysis.The Asian koel (Eudynamys scolopaceus) is a generalist brood parasite of multiple hosts in Asia, with no evidence of exhibiting host-specific egg mimicry. In this study, by using spectrometry and avian visual modeling, we quantified egg parameters of Asian koel and its host, the azure-winged magpie (Cyanopica cyanus), to verify whether Asian koels mimic eggs of azure-winged magpies and exhibit host-specific egg mimicry. Our results demonstrated that the eggs of Asian koel and azure-winged magpie displayed consistent spectral reflectance both for background color and egg spots. Moreover, we observed that background color and spot brightness, chroma, and hue in Asian koel and azure-winged magpie eggs were highly mimetic. The subtle difference in egg color between the two species was mainly due to the ultraviolet (UV) value of the background color, with azure-winged magpies retaining a significantly higher UV value in their egg background color than Asian koels. Finally, the results of our avian visual modeling revealed that azure-winged magpies cannot distinguish between the two egg types based on the chromatic (dS) and achromatic (dL, luminance) differences in background color and spots. Therefore, this study indicates that Asian koel eggs are highly mimetic of the host azure-winged magpie eggs. This is the first reported case showing that Asian koels exhibit host-specific egg mimicry.