Fig. 6. Clouds experiment. (A) 180° fisheye view of the celestial hemisphere on a cloudy day. The mean degree of polarization is reduced since polarized light from patches of blue sky mixes with partly or totally unpolarized light from clouded sky regions. (B) Polarotactic response as a function of the mean degree of polarization () for a compound stimulus. For a wide (92°) zenithal stimulus, was reduced from 100% to 0% by changing the proportion of polarized to unpolarized light. Data are from 12 individuals. Tests are indicated by black symbols (=1% to 74%, N=17–19), motivation controls by gray symbols (=100%, N=162) and zero controls by white symbols (=0%, N=17). Note: a reduction in polarization to =49% did not impair the polarotactic response. With lower degrees of polarization, response values declined, but the orientation to polarized light was statistically significant at least down to a -level of 10% (P<0.01).