Is there a role for aromatic plants in blue tit (Cyanistes caeruleus) nests? Results from a correlational and an experimental study

Abstract

The utility of fresh green material in avian nests is still not fully understood. Potential explanations include the effects of plants' volatile compounds on parasite reduction (nest protection hypothesis) or direct beneficial effects on nestling condition (drug hypothesis). We used correlative data collected during 2020 and 2021 in a Mediterranean population of blue tits (Cyanistes caeruleus) as well as experimental data (aromatic nest content manipulation) to assess the effects of aromatic plant use on nestling physiological condition and survival, nest-dwelling ectoparasitic pressure and its relationship with breeding parameters. We found that aromatic plants were disproportionally used in relation to their abundance in the environment and that their use was positively related to egg mass (but only in 2020). Blowflies and facultative parasitic mites were more frequent in nests with aromatics compared to nests without aromatics, but obligatory parasitic mites were less abundant in nests with aromatics. However, no effects of aromatic plants were observed on nestling haemoglobin levels nor erythrocyte sedimentation rate or other physiological health metrics, but the heterophil to lymphocyte ratio was higher in nests with the highest quantity of aromatics. The artificial addition of mint reduced the flea abundance, but 7-day old nestlings showed significantly lower colour saturation and brightness in the mouth flange. Nestling survival to fledging was not related to aromatic plant use. Therefore, our results partially support a beneficial effect of aromatic plants in blue tit nests because some ectoparasite groups were reduced. Immediate effects on nestling physiology or survival could not be established. Significance statement: Some avian species place fresh aromatic plant material in their nests, and several non-mutually exclusive hypotheses have been proposed to explain its potential functions. In this study, we use both correlational and experimental data from a blue tit population to test two hypotheses, namely the 'nest protection hypothesis' and the 'drug hypothesis'. The first one proposes that aromatic plants have direct repellent effects against ectoparasites, while the latter poses that these plant components benefit nestling condition through the stimulation of some components of the immune system. Our results suggest that some ectoparasites, such as fleas and obligatory parasitic mites, were less abundant in nests where aromatic plants were artificially or naturally added, respectively, but no relationships were detected with nestling physiology or survival, which could be partially explained by our small sample size. Our study partially supports a beneficial effect of aromatic plants in avian nests.