If a large amount of birds fit into these two groups it is making the data collection much easier since variations with in each groups is considered being very small:
"Diurnal birds use four retinal cone types for color vision and are classified into two distinct groups by the peak sensitivity of the short-wave cone (Endler and Mielke, 2005; Ödeen and Hảstad, 2003). In the U-system group, peak sensitivity is shifted to the ultraviolet part of the spectrum and in the V-system it is shifted to the violet part of the spectrum (Ödeen and Hảstad 2003). Each group contains frugivorous birds. Crows have been assigned to the V-system group, based on molecular data (Ödeen and Hảstad, 2003). Knowledge of cone sensitivities in different bird species is limited (Hart, 2001), but interspecific variability in each group is considered relatively small (Håstad et al.,6
2005). Therefore, our results are likely to apply to other frugivorous species with the V-system. We caution against applying them to species with the U-system."
From, http://www.biologie.uni-freiburg.de/data/bio1/schaefer/pdf/pre-print.pdf.