RESUMO
Specific fatty acids (FA) such as unsaturated (UFA) and saturated (SFA) fatty acids contained in foods are key factors in the nutritional ecology of birds. By means of a field and experimental approach, we evaluated the effect of diet on the activity of three esterases involved in FA hydrolysis; carboxylesterase (CE: 4-NPA-CE and a-NA-CE) and butyrylcholinesterase, in two South American passerines: the omnivorous rufous-collared sparrow (Zonotrichia capensis) and the granivorous common diuca-finch (Diuca diuca). The activity of the three esterases was measured in the intestines of freshly caught individuals over two distinct seasons and also after a chronic intake of a UFA-rich or SFA-rich diet in the laboratory. In turn, we assessed the feeding responses of the birds choosing amongst diets contrasting in the kind of specific FA (UFA- vs. SFA-treated diets). During summer, field CE activities (4-NPA-CE and a-NA-CE) in the small intestine were higher in the rufous-collared sparrow (25.3 ± 3.3 and 81.4 ± 10.8 µmol min(-1) g tissue(-1), respectively) than in the common diuca-finch (10.0 ± 3.0 and 33.9 ± 13.1 µmol min(-1) g tissue(-1), respectively). Two hour feeding trial test indicated that both species exhibited a clear preference for UFA-treated diets. On average, the rufous-collared sparrow consumed 0.46 g 2 h(-1) of UFA-rich diets and 0.12 g 2 h(-1) of SFA-rich diets. In turn, the consumption pattern of the common diuca-finch averaged 0.73 and 0.16 g 2 h(-1) for UFA-rich and SFA-rich diets, respectively. After a month of dietary acclimation to UFA-rich and SFA-rich diets, both species maintained body mass irrespective of the dietary regime. Additionally, the intestinal 4-NPA-CE activity exhibited by birds fed on a UFA-rich or SFA-rich diet was higher in the rufous-collared sparrow (39.0 ± 5.3 and 44.2 ± 7.3 µmol min(-1) g tissue(-1), respectively) than in the common diuca-finch (13.3 ± 1.9 and 11.2 ± 1.4 µmol min(-1) g tissue(-1), respectively). Finally, the intestinal a-NA-CE activity exhibited by the rufous-collared sparrow was about two times higher when consuming an UFA-rich diet. Our results suggest that the rufus-collared sparrow exhibits a greater capacity for intestinal FA hydrolysis, which would allow it to better deal with fats from different sources.