Role of mental representations in quantity judgments by jackdaws (Corvus monedula).

The representation of quantity by the preverbal or nonverbal mind is a question of considerable interest in the study of cognition, as it should be generally adaptive to most animals to be able to distinguish quantity. We already know that some primate species and human infants represent and enumerate objects in similar ways. Considerable data also exist concerning such abilities in birds. Our aim in this study has been to find out whether jackdaws (Corvus monedula) are capable of performing relative quantity judgments based on mental representations, and if so, what are the limiting factors to their abilities. In our setting the birds were required to make a choice between two visibly and sequentially placed set of food items which, at the moment of choice were not visible to the subjects. We investigated all the number combinations between 1 and 5. Our results show that jackdaws are able to perform relative quantity judgments successfully, even when temporal cues are controlled for, whereas their performance declines in the direction of larger set size (numerical size effect), and when the difference between the two arrays decreases (numerical distance and ratio effect). These signatures are usually interpreted as evidence for the "accumulator" model of mental representation of quantity. Our control results suggest that jackdaws do not use temporal cues, but may well use total volume as basis for discrimination, perhaps among other attributes (choice may be based on multiple cues).