Figure 1.

Task protocol and example stimuli. (a) Delayed match-to-numerosity task. The crow starts a trial by moving its head into the light barrier. After 600 ms, a pseudo-random sample stimulus is shown for 800 ms followed by a 1000 ms delay. Then, in 50% of the cases a ‘match’ follows, which contains the same number of dots as the sample stimulus, but differs in appearance. If a match appears, the crow has to respond by pecking at the touch screen to receive a reward. If a ‘non-match’ appears, the crow has to refrain from answering but wait for 800 ms until the non-match disappears and a match stimulus appears. (b) Example stimuli for both stimulus sets. Standard stimuli and control stimuli (equating total black area and dot density across numerosities) are shown. The number space between 1 and 30 was covered by 7 numerical values (1, 2, 4, 7, 12, 20 and 30).