Excerpt from the TPWD's online bulletin of September 1, 2007,
and titled Bag and Possession Limits for Ducks:

"The daily bag limit shall be 5 ducks with the following 
species and sex restrictions - two scaup, two redhead, and 
two wood duck; only one from the following aggregate bag: one 
hen mallard, or one pintail, or one canvasback, or one "dusky 
duck" (mottled duck, Mexican-like duck, black duck, and their 
hybrids); all other ducks not listed - five."