 Brown creepers are known to reject long-tailed cuckoo eggs whereas yellowheads and whiteheads, who are close relatives of brown creepers, tend not to.The research showed that both the brown creeper and yellowhead lacked a short-wavelength sensitive (SWS1) opsin gene which has large effects on the range of light that can be seen.This can influence how far in or out of the UV range an organism can see, which in terms of the Mohoua spp.