Ketone body levels in wintering great tits Parus major in sites differing in artificial food availability.

Ketone body levels, among other biochemical blood indices, are important indicators of the physiological condition of birds. Plasma ketone as an indicator of fasting and lipid use is strongly linked to demanding phases in the avian life cycle, such as migration or wintering. The main goal of this study was to check whether ketone body levels differ between the habitats in which individuals stay in winter. To test the above prediction, we used a portable point-of-care device to measure ketone body levels in wintering great tits ( Parus major ). We assumed that wintering in distinct habitats that differ structurally, particularly with respect to food availability, would affect the metabolic performance of birds and their physiological condition. Individual great tits were trapped in mist nets and blood-sampled in three distinct locations within the city: an urban parkland, the deciduous forest and a city centre. As expected, we showed that the mean ketone level was significantly higher in the area where artificial feeding was irregular than in two areas of regular feeding, indicating the more intense fasting state there. We also checked if the level of ketone bodies differs with respect to the sex of an individual, but we found no such difference.