Blue tits use medicinal plants to disinfect their nests, scientists have discovered.

The birds line their nests with aromatic plants such as mint or lavender, which kill bacteria, that creates a more sterile environment for chicks, which in turn grow faster and have a better chance of survival. However, individual blue tits are quite picky about which plants they use, and it is not clear how they pass their knowledge on to other birds.

 A number of European bird species add aromatic plants to their nests. Starlings, for instance, usually bring back aromatic plants as part of a courtship display designed to attract females. But they stop doing so as soon as a female lays her eggs, suggesting the plants are for display only.

On Corsica, Blue Tits (Cyanistes caeruleus) also incorporate fresh fragments of aromatic plants including lavender, mint and curry plants into their nests. It has been hypothesised (Adele Mennerat, a biologist now at the University of Bergen in Norway) that aromatic plants used by blue tits had some anti-parasite properties, because most of these plants, or close species of the same genus, are traditional Mediterranean plants with well-known medicinal properties. 

Mennerat and colleagues from France's National Centre of Scientific Research, based in Montpellier, and the University of Toulouse initially tested whether these plants deterred blow fly larvae that commonly live in tit nests and feed on chicks' blood, significantly damaging their health. Despite repeated attempts it was never find any effect of these plants on blow fly infestation. So the researchers tested the effects of these plants on the bacteria living on birds and describe their results in the journal Oecologia. They found that aromatic plants, including lavender (Lavandula stoechas), apple mint (Mentha suaveolens), the curry plant (Helichrysum itlaicum) and Achillea ligustica significantly change the composition of bacterial communities living on blue tit nestlings. Mennarat observed that they reduce the number of different bacterial species, and the total number of bacteria, especially on chicks that are most vulnerable because they are both highly infested by blow fly larvae and carry great amounts of bacteria on their skin.

It is unclear exactly how volatile compounds produced by the aromatic plants kill the bacteria. But blue tit chicks living in nests adorned with aromatic plants grew faster and had a higher proportion of red blood cells, which is known to be a strong indicator of a chick's future chances of survival after fledging. Mennerat cannot yet be sure but she suspects that living in a disinfected nest enables the chicks to allocate less energy to their immune systems and more to growing physically.

Much remains to be explained, however. No other bird has yet been shown to use aromatherapy in the same way, while related species such as the great tit or coal tit do not even decorate their nests with such plants. So it is unclear how the behaviour originated in blue tits. Different birds also prefer different aromatics, regardless of their local availability, the researchers found.

One of the most unexpected findings was that female blue tits display individual preferences in their use of aromatic plant species. For example, in a territory with big bushes of lavender, for some reason blue tits at this site still collect mint that can only be found far away from their nests. We still don't know why and how blue tits have such individual preferences.

 The researchers are also keen to discover how these personal aromatic preferences are passed on between birds.

References:
- Earth News by BBC - "Blue Tits Embrace Aromatherapy" written by Matt Walker

- Oecologia - "Aromatic Plants in nests of the blue tit Cyanistes Caeruleus protect chicks from bacteria" written by Adèle Mennerat & Team

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