Seasonal migration of a bird from lowlands to montane habitats should induce an increase in its hematocrit value. This relationship has been studied mainly in domestic species, however, and data on wild birds are mostly correlational. We report our results on variation in hematocrit values of the Citril Finch (Serinus citrinella), which migrates altitudinally. Data were taken from wild birds trapped within the same season at different altitudes and from an experiment in which Citril Finches were transported to different altitudes (600 to 2000 m above sea level). On average, hematocrit values from Citril Finches trapped at higher altitudes in the wild were 3% higher than those of birds collected at lower levels. In the experiment in which Citril Finches were transported to different altitudes, individual birds were measured repeatedly, so that the same individuals acted alternately as controls or experimental subjects. Hematocrit values of experimental and control individuals did not vary initially but varied over the course of the experiment, decreasing when birds were moved to lower altitudes and increasing when they were moved to higher altitudes by an average of 6%. These results indicate that wild birds that migrate altitudinally must contend with an increase in hematocrit values. Transporting the birds to altitudes within the species' range allowed us to exclude the possibility that the increase in hematocrit from lower to higher altitudes was the result of the flight itself and demonstrates an adjustment in hematocrit not induced by the exercise of flight.