Animals frequently use metabolites produced by symbiotic bacteria as agents against pathogens and parasites. Secretions from the preen gland of birds are used for this purpose, although its chemicals apparently are produced by the birds themselves. European hoopoes Upupa epops and green woodhoopoes Phoeniculus purpureus harbour symbiotic bacteria in the uropygial gland that might be partly responsible for the chemical composition of secretions. Here we investigate the antimicrobial activity of the volatile fraction of chemicals in hoopoe preen secretions, and, by means of experimental antibiotic injections, test whether symbiotic bacteria living within the uropygial gland are responsible for their production. Hoopoes produce two different kinds of secretions that differ drastically in their chemical composition. While the malodorous dark secretions produced by nestlings included a complex mix of volatiles, these chemicals did not appear in white secretions produced by non-nesting birds. All volatiles detected showed strong antibacterial activity, and a mixture of the chemicals at the concentrations measured in nestling glands inhibited the growth of all bacterial strains assayed. We found support for the hypothesized role of bacteria in the production of such antimicrobial chemicals because experimental clearance of bacteria from glands of nestlings with antibiotics resulted in secretions without most of the volatiles detected in control individuals. Thus, the presence of symbiotic bacteria in the uropygial gland provides hoopoes with potent antimicrobials for topical use.

Summary: 1. Although a growing body of evidence supports that olfaction based on chemical compounds emitted by birds may play a role in individual recognition, the possible role of chemical cues in sexual selection of birds has been only preliminarily studied.2. We investigated for the first time whether a passerine bird, the spotless starling Sturnus unicolor, was able to discriminate the sex of conspecifics by using olfactory cues and whether the size and secretion composition of the uropygial gland convey information on sex, age and reproductive status in this species.3. We performed a blind choice experiment during mating, and we found that starlings were able to discriminate the sex of conspecifics by using chemical cues alone. Both male and female starlings preferred male scents. Furthermore, the analysis of the chemical composition of the uropygial gland secretion by using gas chromatography–mass spectrometry (GC–MS) revealed differences between sexes, ages and reproductive status.4. In conclusion, our study reveals for first time that a passerine species can discriminate the sex of conspecifics by relying on chemical cues and suggests that the uropygial gland secretion may potentially function as a chemical signal used in mate choice and/or intrasexual competition in this species.

In the middle of the 20th century, the production of plastics exploded worldwide because of their low cost and the versatility of their applications. However, since plastic debris is highly resistant to environmental degradation, a growing presence of plastics in all the ecosystems has been confirmed. Among them, plastic particles < 5 mm, also known as microplastics (MPs), are of special concern because they are dispersed in aerial, terrestrial and aquatic environments, being the soil the main environmental sink of these contaminants. Due to their large specific surface area and hydrophobicity, MPs are considered good adsorbents for other environmental organic pollutants also present in terrestrial ecosystems, such as pharmaceuticals, personal-care products or pesticides with which they can interact and thus modify their environmental fate. In this review article, we examine the recent literature (from 2017 to 2022) to get a better understanding of the environmental fate of pesticides in soil (adsorption, mobility and/or degradation) when they are simultaneously present with MPs and the ecological risks on living organisms of the interactions between MPs and pesticides in soil. More studies are needed to fully understand the toxicological impact of the copresence in soil of pesticides and MPs.