In saucepan cook onion and chicken in butter,stirring 5 min.
Drain tomatoes and measure 1 cup packed, reserving remainder for another use.
Stir tomatoes into chicken mixture with water, cayenne, and cumin, simmer covered 1 hour.
Pull meat off bones, return it to pan, simmer, uncovered until most liquid is evaporated, about 20 min. Stir in salt and cool mixture.
Slice cucumber lengthwise and crosswise and seed. Cutpieces lengthwise into very thin slices. Toss cucumber with 1 tbls lime juice and season with salt and pepper.
Halve, peel, and pit avocado, cut into 1/4 inch slices, toss with remaining lime juice, salt and pepper.
Arrange 1 tortilla and smear a small piece of avocado lengthwise along edge of tortilla ( this will help hold the wrap together)
Spread 1/4 of chicken mixture across lower fourth of tortilla. Top chicken with 1/2 cup cilantro sprigs, add 4-5 avocado slices and 1/4 of the cucumber slices.
Tightly roll up.
May be made 6 hours ahead and chilled individually wrapped in plastic
Serve chilled or at room temperature