A sequence of photos showing New Caledonian crows mobbing a raptor.

<p>Photo credit Mick Sibley.</p

a & b. Mean number of stones passed on the cooperative apparatus in the social and asocial conditions.

<p>(a) total, 20 trials (b) first trial in each social or asocial block, 5 trials. Error bars ± S.E.</p

Summary of pairs and their performance on the spontaneous stone passing task (Test 1a & 2a).

<p><i>Note</i>: Eight subjects were naïve to stone passing as they had not performed the role of ‘dropper’ before their test. Six of the pairs were likely to be related (subjects were caught together), and six were likely to be unrelated (caught at different sites, at different times). Criterion: 3 out of 4 stones passed in one block.</p><p>Summary of pairs and their performance on the spontaneous stone passing task (Test 1a & 2a).</p

Mean number of stones passed in an inequity test by group 2 (Test 2c).

<p>Disadv. = Disadvantageous, Adv. = Advantageous, DF = dog-food, M = meat.</p

Model-predicted probability of stone passing in relation to the total volume of food in the apparatus (Test 1b).

<p>Crows increased their passing rate as food volume increased. Circles = equity conditions, squares = inequity conditions. Error bars = 95% confidence intervals.</p

Experimental setup and apparatus used by group one.

<p>The diagram of the box shows the cooperative apparatus. Stones dropped through the hole on the surface of the box would hit a baited central platform, causing the platform to swing downwards and release the food. The large diagram shows the setup of the box between two cages. The crow on the left (‘passer’/subject) had to pick up a stone and place it next to one of two gaps in the wire mesh divider (passing locations). The crow on the right (‘dropper’/partner) could then drop the stone into the apparatus and release food into both cages. For group two, the hole in the apparatus was on the left.</p

Pairwise comparisons from the logistic model of the inequity conditions experienced by group one (Test 1b).

<p>Note:</p><p>***<i>p</i> < .001</p><p>Pairwise comparisons from the logistic model of the inequity conditions experienced by group one (Test 1b).</p

Diagrams of the apparatus used in each of the 6 experiments.

<p>In each experiment birds dropped objects into tubes to obtain an out of reach food reward. Each experiment involved either a choice of two tubes or a choice of two objects. The apparatus was presented on a table in the centre of a large testing cage, as pictured. A: Experiment 1, Sand-filled tubes v Water-filled tubes, B: Experiment 2, Sinking v Floating objects, C: Experiment 3, Solid v Hollow objects, D: Experiment 4, Narrow v Wide tubes, E: Experiment 5, High v Low water levels in Narrow and Wide tubes, F: Experiment 6, U-tube, a concealed connection links one of the outer tubes with the rewarded central tube.</p

Average performance in all six experiments.

<p>Mean proportion of choices made to the correct option, over 20 trials, in each experiment. (1: Sand v Water, 2: Sinking v Floating, 3: Solid v Hollow, 4: Narrow v Wide, 5: High v Low water levels, 6: U-tube.) Error bars are ±2 SE. *  =  significantly different from chance (binomial tests, <i>p</i>&lt;0.001).</p

Water vs. sand experimental set up.

<p>Water vs. sand experimental set up.</p