4.1 | The basic composition of wintering robins in subtropical forest of southwest China
Studies on the sex ratio and its relationship with bird demography, physiology, and other life history traits helps us better understand the secret life of birds (Donald, 2007). In this study, the number of blue plumaged individuals (2yr+ males) was much lower than the number of olive-brown plumaged individuals (females and yearling males), while the sex-ratio was just slightly skewed to males among olive-brown robins. The low number of 2yr+ males was consistent with previous studies of wintering robins in east-central China, which inferred a higher predatory risk and mortality among blue plumaged males (Li et al., 2016). Meanwhile, the whole group composition also can be explained by habitat segregation and differences in life history strategy among robins with different colored plumages.
Habitat segregation between different sexes during the non-breeding season could be a general ecological trait of many migratory birds, usually caused by social hierarchy dynamics or differential tolerance to ecological factors (Catry et al., 2006; Coppack and Pulido, 2009). Our results showed that, in the case of orange-flanked bush-robins wintering in southwest China, sexual segregation might exist among robins with different colored plumages. In contrast, birds with similar colored plumages tended to show convergence in habitat selection. As a dichromatic bird with DPM, the yearling males of this robin species share the same plumage color as females until their second breeding season. Though the specific reasons and functions of the DPM phenomenon in this robin species remain unknown, it has been shown in previous studies that a female-like appearance helps to reduce conflicts between yearling males and blue-plumaged adult males, suggesting that plumages can be signals of social status and therefore regulate bird behavior (Morimoto et al., 2006). For further studies, concurrent surveys at different locations and more age-related evaluation related to plumage appearance and upper mandible color (Hellström and Norevik, 2013) are recommended to construct a more detailed population composition and further reveal the driving factors and mechanisms of DPM.