Consequences of experimental addition of fresh, aromatic plants into nests of blue tits ( Cyanistes caeruleus ) on the physiological condition of nestlings

The blue tit ( Cyanistes caeruleus ) is one of a few known passerine birds that incorporate fresh fragments of aromatic plants in their nest during the nesting period. In this study, we experimentally tested whether aromatic plants in blue tit nests affect physiological condition of nestlings as indicated by hemoglobin and glucose in the blood. Nestlings in nests supplemented with fresh, aromatic plants had elevated blood hemoglobin, and therefore improved physiological condition, as compared with control nestlings. There was no difference between nestlings in two contrasting habitats (urban parkland and forest). Although mean levels of blood glucose did not differ between treatment and control nestlings, the blood of forest nestlings contained more glucose than the blood of parkland nestlings, which indicates poorer health for forest nestlings. In general, knowledge of the effects of incorporating fresh green, aromatic plant material into avian nests on physiological condition of nestlings is still fragmentary and more experimental studies are needed. Significance statement The blue tit is one of a few known passerine birds that incorporate fresh fragments of aromatic plants in their nest during the nesting period. Little is known about the impact of this material on nestling body condition, and there is a need to understand this behavior and its effects. We tested whether nestlings in nests supplemented with fresh, aromatic plants improved their physiological condition in comparison with control nestlings. The experiment showed that nestlings from supplemented nests improved their physiological condition by exhibiting elevated hemoglobin.