In a series of choice tests, we determined the relative preferences of Hous
e Finches (Carpodacus mexicanus) for equicaloric aqueous solutions of hexos
es (1:1 mixture of fructose and glucose) and sucrose. At 2% (m/v), birds co
nsumed each sugar solution equally and in amounts similar to plain water. C
onsumption of hexose but not sucrose increased at 4% sugar concentration. A
t 6% and 10%, finches displayed consistent, strong preferences for the hexo
ses over sucrose. In other passerine species, strong hexose preference has
been linked to the absence of sucrase, the enzyme needed for digestion of s
ucrose. Fecal sugar readings from the House Finches, however, indicated app
roximately equal assimilation of hexose and sucrose, so the hexose preferen
ce apparently is not due to sucrase deficiency. Rather, energetics may dete
rmine the finches' sugar preferences: hexoses are rapidly processed because
the 6-carbon sugars are readily assimilable whereas sucrose must first be
hydrolyzed.