Fig. 2.

a Flicker fusion frequencies of all five birds (Bud, Lucky, Bart, Pippi, Milou) in tests with four luminances. Open circles: results with low background intensities; crosses results with higher background intensities (see Table 1). b Average FFF of budgerigars. Note that the luminance scale here is logarithmic. Gray open triangles show the results obtained with two birds in experiments performed and published by Ginsburg and Nilsson (1971). Black open triangles show averages obtained with the birds (except Bud, whose data are excluded because he only cooperated in one test) tested in our study (results with low and high background luminances are given as separate points when the results were statistically different). Filled triangles show average values obtained with six human subjects under the same conditions