Blue tits make nests in holes and hollows that can become quite warm and damp as the chicks mature. This creates an optimal environment for the growth of pathogenic bacteria and parasites like blow fly larvae that can create severe health problems for the chicks. To promote healthy growth in the chicks, female blue tits actively seek out fresh fragments of aromatic plants like lavender, yarrow, daisy, and apple mint to place in the nest cup. The complex bouquet of aromatic biochemicals produced by these plants have long been known for their antimicrobial characterstics (though the biochemistry is still poorly characterized). In any case, these plant fragments conferred greater health and survivability upon the chicks by reducing the density of bacterial colonies. Interestingly, they were found to exert a significant effect on bacterial richness in chicks infested with blood sucking blow fly larvae. Sparsely placed bacteria are not able to form biofilms and other communities that increase their pathogenicity. Chicks benefit by being able to devote their nutrient resources for growth rather than for their immune systems. Adult blue tits did not experience any significant decrease in bacterial infection from these plants.Edit Summary
“Birds use fresh plants to limit the growth of nest microorganisms. On Corsica, blue tits (Cyanistes caeruleus) incorporate fresh fragments of aromatic plants into their nests. These plants do not reduce infestation by nest ectoparasites, but have been shown to improve growth and condition of chicks at fledging…Aromatic plants significantly affected the structure of bacterial communities, in particular reducing bacterial richness on nestlings. In addition, in this population where there is a strong association between bacterial density and infestation by blood-sucking Protocalliphora blow fly larvae, these plants reduced bacterial density on the most infested chicks. Aromatic plants had no significant effect on the bacteria living on adult blue tits.” (Mennerat et al. 2009:849).