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.