A budgerigar sat on a perch from which it viewed a pair of lights about three feet away.One was the yellow training light, the other a variable mixture of red and green.

• 停在栖木上头的长尾鹦鹉，可以看到约90公分外的两个光源，其中一个是用来训练鹦鹉的黄光，另一个则是各种红绿比率的混合光。