Estimation of fat reserves of Robins (Erithacus rubecula) migrating through the southern coast of the Baltic Sea in spring

Abstract

Fat is the main source of energy during bird migration, and the visual fat scoring is widely used in the studies on, e.g., stopover ecology of migratory species. Yet, visual fat scoring can lead to wrong conclusion due to potentially different pattern of fat distribution in species that differ in size or even individuals being during active migration and those at stopovers. To overcome limitations of this method, we developed the predictive equation for estimating fat reserves in spring-migrating Robins-one of the most numerous migratory species in Europe. The equation allows estimating the amount of fat reserves based on body mass of birds, which was the best predictor of fat mass accumulated by Robins. The predictive value of the equation was reasonable with the coefficient of determination equal to 0.56 and the standard error of estimation SEE =0.36. This equation was tested on the sample of adult and immature Robins captured during spring migration. The studied species is a typical short/medium distance migrant employing short step migration, which does not require large fat reserves. The mean estimated fat mass in spring-migrating Robins varied from 0.7g in individuals with the lowest fat reserves (fat score 0) to almost twofold higher amount, i.e., 1.38 g, in birds with the high fat score (fat score 4). In the fattest Robins, fat composed about 8% of their total body mass.