Factor affecting blood chemistry values can be correlated with variables of ecological interest. Blood parameters can be used as a very useful complementary technique when approaching ecological issues. Here, I report some examples of application of blood chemistry to the resolution of ecological problems. The study of circadian variation in blood values indicate the existence of important differences between diurnal and nocturnal birds in the circadian rhythms of blood chemistry values, and the distinctive patterns of variation seem to be affected by particular activity times of each group. Metabolic adaptation to food deprivation are very important in the life history of many species of birds. Fluctuation in food availability may impose fasting for more or less predictable periods of time. Fasting endurance have an adaptive value. Urea and uric acid levels in blood increase with fasting and decrease slowly with refeeding,  therefore being good indicators of nutritional conditions n species of birds with poor fat reserves as raptors.