Patrick mentions is a comment below about the powers of 1, 3 and 7 — and so 13, 37, and 137. Sevens definitely rule as there are so many rules of seven. In poetry, I use the Welsh seven syllable line of Dafydd ap Gwilym (14th century) as a basic building block. The idea is that the odd syllable count adds drama in that you never quite get a series of iambs going unless you emjamb to get there. If you do — you being the poet, but the reader is of course taken on the same ride — your rhyme hits in ways that break the Frankenstein tendencies of the iamb to modern ears while still keeping the expectations alive and occassionally even fulfilled.
Two other poems I’ve built this way are “Dividing Waters” and “Lent 2003″. The last was written for the Poets Against the War and is not build on seven lines and seven verses as “Birds and Fishes” and “Dividing Waters” are. “Birds” and “Waters” have other brothers in their set which I will make available later (or after they are publishing or abandoned).