Do aromatic plants enhance the immune system? Testing the ‘drug hypothesis’ in adult Eurasian Blue Tits

Some bird species place fresh green plants, often aromatic, in their nests, but their function is still not fully understood. The drug hypothesis proposes that volatiles emitted by aromatic plants stimulate some components of the immune system, benefiting nestling health and development. Although the nest aromatic environment is shared between nestlings and parents, the same hypothesis has not been tested for parents. We used correlative and experimental data (nest content manipulation, mint Mentha sp.) to assess the effects of aromatic plant use on parental immune system and condition in a Eurasian Blue Tit Cyanistes caeruleus population during the nestling period. In non‐manipulated nests, Blue Tits did not consistently incorporate plant material, and the presence and quantity of aromatic plants were not associated with parental condition and immune parameters. The experimental addition of mint also did not significantly affect parental condition and immune parameters, although we observed a tendency for a higher number of heterophils in aromatic‐treated nests. Our results do not support the hypothesis that aromatic plants stimulate the immune system of parent Blue Tits.