Eye size and morphometric measurements
We estimated the eye size of bird species that were recorded in our sites by measuring skull specimens of museum collections (Chen, 2010). We measured orbital morphometric parameters, including the maximum length along the long and short axes of the orbit socket, and the length and width of skull. Eye size in volume (ES, mm3) was estimated using ES = 2 × 1.33πa 2 b (Garamszegi et al., 2002), where a and b each represents the long the short axial length of an eye socket. This estimate was strongly correlated (r = 0.97, p < 0.05; Chen, 2010) to that obtained from the approach of applying a series of spherical plastic balls of different sizes to best fit with the eye socket of the birds (Brooke et al., 1999; Thomas et al., 2006). For few species where proper skull specimens were inadequate or completely lacking, we adopted live bird measurements as a substitute. Birds were mist-netted along trails surrounding our study area, and we measured the maximum lengths along the long and short axes of eye socket (to 0.1 mm) using an electronic caliper (SV-03 Digital Vernier Caliper, E-BASE, Yunlin, Taiwan). Our netting complied with the legal requirements of Taiwan and the guidelines for the use of animals in research (Sherwin, 2006) throughout our procedures. The orbital measurements (mm) from skull specimens were correlated to direct eye measurements of live birds in both the long axis (r = 0.52, p < 0.05) and the short axis (r = 0.80, p < 0.05; Chen, 2010), respectively. We corrected our eye measurements using these correlations, but kept the use of this measure to a necessary minimum for a better consistency. We used log-transformed eye size (LES) and its relative value (RES), log (eye size)/log (body mass), for further analyses. The latter considered the allometric effect and was corrected for body size (Berg et al., 2006).