I recently learned that when a true-adjective could be placed in front of a noun it can usually be combined with the noun by dropping the 'i' ending. For example, 'aoi sora' meaning 'blue skies' can become 'aozora' with the same meaning. I noticed, however, that slight spelling changes often accompany this abbreviation. Upon further investigation, it seemed apparent that when these changes were to occur, it was most often the first mora of the noun which became voiced if not. Yet, I still haven't much of an idea of what exactly the rules for when these changes should occur and what they should be are. Any help would be greatly appreciated.
crow + thief = squid
烏 + 賊 = 烏賊