Post-hatching changes in brain mass were investigated in mallards and four stocks of domesticated ducks, using growth curve analysis and allometry. The birds varied in age between hatching and 154 days. Percentage brain size at hatching in ducks varies between 22.5% and 28% which fits well into the precocial category. Brain growth shows a sigmoid course. The point of inflection is very early when compared to other organs. However, growth is slow after passing the growth rate maximum. Pekins show higher absolute brain masses than mallards. However, the growth patterns with respect to time are very similar among stocks. Strongly negative allometry is found with allometric exponents between 0.31 and 0.37 with the highest value in mallards. Allometrically, there is an increasing reduction of brain mass in Pekins when compared to mallards to about 12.5% in adults. No differences were found between mallards and either Muscovies or the Muscovy × Pekin cross. The reduction in relative brain size in domesticated animals when compared to their wild ancestors is generally attributed to a decrease in functional demands resulting from the artificial environment. Because brain growth is more conservative and less influenced by selection than body weight, we assume that this reduction is, at least in part, a result of the constancy of brain growth patterns.