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A set of string pulling tasks was used to compare the cognitive abilities of birds with different levels of brain complexity, which was judged using Portmann's index for the hemispheres. Varying the number and relative position of strings and bait, we investigated whether birds of different species (hooded crows (Corvus cornix), red crossbills (Loxia curvirostra), Eurasian blue tits (Parus caeruleus), and great gray owls (Strix neb ulosa)) are capable of comprehending the logical structure of such tasks based on cause-effect relationships. Among the observed representatives of Passeriformes, only hooded crows, which possess the most complex and highly differentiated brain, were shown to be able to solve correctly the most difficult versions of the string pulling tasks and, therefore, to understand their logical structure. Small passerine birds and owls, which are characterized by a similarly high value of Portmann's index, proved to be incapable of compre hending the cause-effect relationship between the components of the tasks.

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