A review of the nest protection hypothesis: does inclusion of fresh green plant material in birds’ nests reduce parasite infestation?

Abstract

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.

Sexual and natural selection in the evolution of extended phenotypes: the use of green nesting material in starlings

“…In support of this idea, Gwinner & Berger () found fewer bacteria in nests treated with green plants in the European starling. However, most experiments with starlings have not found that green nesting material reduces ectoparasite loads in nests (Fauth et al ., ; Gwinner et al ., ; Brouwer & Komdeur, ; reviewed in Scott‐Baumann & Morgan, ). Furthermore, males did not carry more green plants when nests were experimentally contaminated with ectoparasites (Brouwer & Komdeur, ).…”

“…PGLS and analysis of transition rates) and also with previous studies (Clark & Mason, ). Because cavities might be more susceptible to be colonized by parasites (Collias & Collias, ; Marshall, ), the use of green plants in cavity‐nesting species suggests that this behaviour likely has a sanitary function in this group (Scott‐Baumann & Morgan, ). In support of this idea, Gwinner & Berger () found fewer bacteria in nests treated with green plants in the European starling.…”

“…Thus, the experimental addition of green plants to nests attracted competing females (Polo et al ., ) and reduced offspring recruitment rates (Polo et al ., ). Female owners also remove green nesting material from nests (Veiga & Polo, ), and therefore, recent reviews have argued that this male extended phenotype likely evolved via sexual selection in starlings (Scott‐Baumann & Morgan, ). Therefore, green plants may have originally provided a sanitary function in cavity nests and had a positive effect on nestlings’ health (Gwinner, ), becoming a reliable signal to females, which could have favoured the exaggeration of this trait via sexual selection (Veiga et al ., ).…”

“…For example, female blue tits, Cyanistes caeruleus , incorporate plants in the nest from the end of nest construction until fledgling, favouring nestling condition and immune response (Mennerat et al ., ; Tomás et al ., ). In contrast, most studies of starlings suggest that males carry green plants to nests to attract females during courtship, rather than to control ectoparasite and bacterial loads during the nesting period (Dubiec et al ., ; Scott‐Baumann & Morgan, ). Moreover, male starlings generally incorporate green plants only during the beginning of the breeding period (European starling, Sturnus vulgaris , Fauth et al ., ; and spotless starling, Sturnus unicolor , Veiga et al ., ), and then, females remove green plants from nests after beginning egg laying (Veiga & Polo, ).…”

“…Moreover, male starlings generally incorporate green plants only during the beginning of the breeding period (European starling, Sturnus vulgaris , Fauth et al ., ; and spotless starling, Sturnus unicolor , Veiga et al ., ), and then, females remove green plants from nests after beginning egg laying (Veiga & Polo, ). These observations contradict what would be expected if these plants provided a beneficial function to their offspring (Polo et al ., ; Scott‐Baumann & Morgan, ). Moreover, males carry more plants when females are close to the nest box (Brouwer & Komdeur, ), and polygynous males incorporate more green plants to each of their nests than monogamous males (Veiga et al ., ), both of which further support a sexually selected function for the use of green plants.…”