The use of aromatic plants and their essential oils for ectoparasite treatment is a field of growing interest. Several species of birds regularly introduce aromatic herbs into their nests putatively to reduce parasites. The behaviour is most often seen in cavity nesting birds and after nest building has finished. The plants are included in a non-structural manner and are often strongly aromatic. Various different hypotheses have been proposed regarding the function of this behaviour; from the plants altering some non-living factor in the nest (crypsis, water loss and insulation hypotheses) to them being involved in mate selection (mate hypothesis) or even having a beneficial effect, direct or indirect, on chicks (drug or nest protection hypothesis, NPH). Many studies have been carried out over the years observing and experimentally testing these hypotheses. This review focuses on studies involving the most popular of these hypotheses, the NPH: that plants decrease nest parasites or pathogens, thereby conveying positive effects to the chicks, allowing the behaviour to evolve. Studies providing observational evidence towards this hypothesis and those experimentally testing it are discussed.

Mechanisms affecting patterns of vector distribution among host individuals may influence the population and evolutionary dynamics of vectors, hosts and the parasites transmitted. We studied the role of different factors affecting the species composition and abundance of Culicoides found in nests of the blue tit (Cyanistes caeruleus). We identified 1531 females and 2 males of 7 different Culicoides species in nests, with C. simulator being the most abundant species, followed by C. kibunensis, C. festivipennis, C. segnis, C. truncorum, C. pictipennis and C. circumscriptus. We conducted a medication×fumigation experiment randomly assigning bird's nests to different treatments, thereby generating groups of medicated and control pairs breeding in fumigated and control nests. Medicated pairs were injected with the anti-malarial drug Primaquine diluted in saline solution while control pairs were injected with saline solution. The fumigation treatment was carried out using insecticide solution or water for fumigated and control nests respectively. Brood size was the main factor associated with the abundance of biting midges probably because more nestlings may produce higher quantities of vector attractants. In addition, birds medicated against haemoparasites breeding in non-fumigated nests supported a higher abundance of C. festivipennis than the rest of the groups. Also, we found that the fumigation treatment reduced the abundance of engorged Culicoides in both medicated and control nests, thus indicating a reduction of feeding success produced by the insecticide. These results represent the first evidence for the role of different factors in affecting the Culicoides infracommunity in wild avian nests.

The normal diet of most small passerines contains insufficient calcium for effective eggshell formation and birds must often forage specifically for calcium-rich items in the environment. It has been demonstrated that anthropogenic acid precipitation can significantly reduce levels of available calcium in soil and the abundance of calcareous items such as snail shells. Recent research in areas adversely affected by acid precipitation on Continental Europe has suggested that reduced calcium availability has resulted in an increased incidence of eggshell defects in small birds. In this paper we present data from one of the areas of the U.K. most severely affected by acid precipitation and where snail abundance (0.36 snails per m2) and exchangeable soil calcium levels (0.02 mg g−2) were very low: lower than values at sites in the Netherlands where severe eggshell defects occurred. The provision of supplementary calcium and the examination of eggshells, however, provided no evidence that blue tits Parus caeruleus were constrained in their ability to form effective eggshells as a result of calcium deficiency. We suggest that other factors, besides low calcium availability, may be contributory to the high incidence of eggshell defects seen in other areas.