Memory for what, where, and when in the black-capped chickadee (Poecile atricapillus).

Episodic memory is the ability to remember personally experienced past events (Tulving in Organization of memory. Academic Press, San Diego, pp. 381-403, 1972). In non-human animals, the behavioural criterion for episodic-like memory is remembering "what" occurred in conjunction with "when" and "where" (Clayton and Dickinson in Nature 395:272-274, 1998). We conducted tests for "what, where, and when" memory in a food-storing bird, the black-capped chickadee (Poecile atricapillus). In Experiment 1, chickadees found sunflower seeds and mealworms in concealed sites in their home cage. Birds later re-visited these sites after either a short (3 h) or long (123 h) retention interval. Chickadees normally prefer mealworms, but at the long retention interval mealworms were degraded in taste and appearance. Chickadees showed some memory for what kind of food they had encountered and where, but no memory for when food had previously been found. Experiment 2 followed a similar procedure, except that chickadees searched for hidden sunflower seeds and mealworms in trees in an indoor aviary. These more natural conditions increased both the spatial scale of the task and the effort required to find food. In this experiment, birds showed evidence for all three components of what-where-when memory. Unlike some previous studies of episodic-like memory, birds' behaviour cannot be accounted for by differential memory strength for more recent events. The results show that memory for what, where, and when occurs in food-storing birds outside the corvid family, does not require food caching or retrieval, and that remembering "when" can depend on the nature of the task.