A role for pancreatic hormones in the regulation of autumnal fat deposition of the garden warbler (Sylvia borin)?

Glucagon and insulin were measured by heterologous immunoassays in plasma samples of 17 garden warblers (Sylvia borin) kept under constant ad libitum or fasting-refeeding conditions during the migratory season from September to May. Plasma levels of key metabolic indicators (glucose, triglycerides, cholesterol, and free fatty acids) were measured every 2 weeks. Measurements of the two hormones concur with the general assumption of a higher glucagon:insulin ratio, indicating a more pronounced catabolism in birds than in mammals. The concentrations of both hormones varied (insulin: 0.7-7.7 microIU/ml, n = 66; glucagon: 0.4-4.5 ng/ml, n = 99), but differences between mean values per month were significant only for glucagon. Neither hormone titer correlated with either the seasonal or a fasting-refeeding-induced body mass cycle. However, there was a positive correlation between food intake, changes in body mass, and plasma triglycerides and insulin; in contrast, there was a negative relationship with the glucagon:insulin ratio. Glucagon showed only a small negative relationship to plasma glucose and cholesterol, but correlated directly more closely with plasma free fatty acids. The present data support the fact that glucagon is more lypolytic in birds than in mammals. Pancreatic hormones are suggested to participate in the regulation of premigratory hyperphagia and hyperlipemia.