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
"Fresh plants could protect chicks from infection by pathogenic microorganisms. Infection by pathogens represents a significant risk, especially when energy is limited because nestlings have to balance growth against immune function...In a Corsican population of hole-nesting blue tits Cyanistes caeruleus, nests are infested by very high numbers of Protocalliphora blood-sucking blow fly larvae that have detrimental effects on nestling development, fledging mass, hematocrit and post-fledging survival...female blue tits actively incorporate fresh fragments of aromatic plants into the nest cup (e.g. Lavandula stoechas, Achillea ligustica, Helichrysum italicum, Mentha suaveolens) during the whole breeding process...these plants significantly improve chick growth and condition at fledging, although they have no significant effect on blow fly infestation. Many of the plant species used by blue tits are already known to have in vitro antibacterial properties (e.g. inhibition of bacterial growth." (Mennerat et al. 2009:850).
"Aromatic plants reduced cultivable bacterial richness on nestlings...Cultivable bacterial density on nestlings was strongly positively related to blow fly infestation intensity, in control nests but not in aromatic-treated nests...Aromatic plants in blue tit nests significantly affect bacterial communities on nestlings. In particular, they reduce cultivable bacterial richness on nestlings. They also limit the increase in cultivable bacterial density related to ectoparasite infestation of nestlings...The strong positive association, observed in control nests, between ectoparasite infestation intensity and bacterial density on nestlings had not been previously described." (Mennerat et al. 2009:852).
"The plants brought by female blue tits to their nests, while not being directly effective against nest ectoparasites, reduced both bacterial richness and bacterial density on nestlings, especially under high ectoparasite infestation." (Mennerat et al. 2009:854).