Chunking during serial learning by a pigeon: III. What are the necessary conditions for establishing a chunk?

What are the minimal conditions for the formation of chunks by a pigeon learning an arbitrary list? Experiment 1 compared the acquisition of two types of chunkable list (each composed of colors and achromatic geometric forms): A----B----C----D'----E' (or A'----B'----C'----D----E) and A----B----C'----D'----E' (or A'----B'----C----D----E). The first type of list was acquired more rapidly than the second. On both lists, however, evidence of chunking did not emerge until the four-item phase of training (e.g., pauses at the end of one category of list item). In Experiment 2, chunking was shown to occur on four-item lists in which colors and forms were segregated (A----B----C'----D' and A'----B'----C----D), but not on lists in which the two types of items were interspersed (A----B'----C'----D and A'----B----C----D'). As in Experiment 1, evidence of chunking (pauses at chunk boundaries) did not appear until the fourth item was added.