Effects of experimental nest treatment with herbs on ectoparasites and body condition of nestlings

Nest fumigation behavior involves the incorporation of fresh green plant fragments that contain ectoparasite-repellent volatile compounds into birds’ nests. This behavior is relatively rare among bird species, and there is ongoing debate about whether it benefits parental breeding success. In this study, we experimentally tested whether the inclusion of aromatic-herbal plant fragments in the nests of great tits Parus major affects the physiological condition of nestlings, as indicated by blood levels of hematocrit, hemoglobin, glucose, and body condition indices, such as weight and wing length. We divided the nests into two groups, adding aromatic herbs to the test group’s nests and non-aromatic plants to the control group. After the nestlings fledged, all nest material was collected to extract, identify, and count arthropod ectoparasites. Nestlings in nests supplemented with aromatic plant fragments had elevated levels of hematocrit and hemoglobin, indicating improved physiological condition compared to the control group. Ectoparasites were present in both groups, although ticks (Ixodidae) occurred less frequently in nests with aromatic plants. The experimental treatment did not affect fledging success. Further experimental studies are needed to explore the effects of incorporating aromatic plant fragments into tit nests within the frameworks of both the nest protection hypothesis and the drug hypothesis.

Methods
During the breeding season, nestboxes were visited every day to record laying date and the number hatchlings. From 26 nests of great tits with incubated clutches and without fresh plant material supplemented by birds on their own, we established pairs of nests according to their hatching date and brood size (±2 nestlings). Thirteen nests were randomly assigned to the experimental treatment and 13 were control broods. Three nests from the experimental treatment were lost (when the nestlings were 5-8 days old) due to predation. Starting on day two and then every other day until day 16 after hatching, a 1.5 g portion of fresh aromatic plant fragments (approximately 0.45 g of lavender Lavandula angustifolia, 0.35 g of mint Mentha sp., 0.35 g of the Immortelle Helichrysum italicum, and 0.35 g of the Common Yarrow Achillea millefolium) was supplemented in test nests and a 1.5 g portion of fresh grass Poaceae spp. was added to control nests. Both aromatic herbs and grass were placed on three edges of the lining layer of the nest (near back wall, left wall and right wall). All plant species used in the experiment normally occur in the geographical distribution of the great tit and are found in natural nests of blue and great tits (Petit et al. 2002, Dubiec et al. 2013, Mainwaring et al. 2017). These aromatic plants were not available in the bird territories or in the forest study area. At our study sites only some blue tits and very occasionally use Lamium galeobdolon and Veronica sp. Unlike blue tits, great tits will not introduce new herbs during the experiment, allowing for better control of experimental conditions. All the aromatic herb species used were acquired in garden stores about two weeks before the experiment and were grown in appropriate conditions until the end of the experimental procedures. Grass was obtained from the faculty botanic garden. In the morning, before visiting the study site, fresh plant fragments were cut off the aromatic plants, weighed and put into plastic pouches and a similar treatment was given to the grass.