Abstract
Selective pressures imposed by pathogenic microorganisms to embryos have selected in hosts for a battery of antimicrobial lines of defenses that includes physical and chemical barriers. Due to the antimicrobial properties of volatile compounds of green plants and of chemicals of feather degrading bacteria, the use of aromatic plants and feathers for nest building has been suggested as one of these barriers. However, experimental evidence suggesting such effects is scarce in the literature. During two consecutive years, we explored experimentally the effects of these nest materials on loads of different groups of bacteria (mesophilic bacteria, Enterobacteriaceae, Staphylococcus and Enterococcus) of eggshells in nests of spotless starlings (Sturnus unicolor) at the beginning and at the end of the incubation period. This was also explored in artificial nests without incubation activity. We also experimentally increased bacterial density of eggs in natural and artificial nests and explored the effects of nest lining treatments on eggshell bacterial load. Support for the hypothetical antimicrobial function of nest materials was mainly detected for the year and location with larger average values of eggshell bacterial density. The beneficial effects of feathers and plants were more easily detected in artificial nests with no incubation activity, suggesting an active role of incubation against bacterial colonization of eggshells. Pigmented and unpigmented feathers reduced eggshell bacterial load in starling nests and artificial nest boxes. Results from artificial nests allowed us to discuss and discard alternative scenarios explaining the detected association, particularly those related to the possible sexual role of feathers and aromatic plants in starling nests. All these results considered together confirm the antimicrobial functionality mainly of feathers but also of plants used as nest materials, and highlight the importance of temporally and geographically environmental variation associated with risk of bacterial proliferation determining the strength of such effects. Because of costs associated to nest building, birds should adjust nest building effort to expected bacterial environments during incubation, a prediction that should be further explored.
Highlights
Bird nests are infected by numerous parasites that affect dramatically their reproductive output [1,2,3]
The expected associations between eggshell bacterial loads and nest materials were most obvious for nest lining feathers’ than for green plants’ experiments
Some of our bacterial quantifications do not distinguish between potentially pathogenic and non-pathogenic bacteria, we detected evidence of expected associations for Enterobacteriaceae and Enterococcus, two groups of bacteria that include embryo pathogens. We discuss these results that varied depending on the study year and location under the hypothesis that nest lining feathers and green plants have antimicrobial functions
Summary
Bird nests are infected by numerous parasites that affect dramatically their reproductive output [1,2,3]. Microorganisms are common in nests [6], some of them being highly pathogenic for developing embryo [7]. They can cross the eggshell [8], cause diseases in embryos [9] and, reduce egg viability [10]. Eggs have numerous defensive traits against pathogens like the eggshell and antimicrobial contents [11,12,13,14,15]. Birds can modulate their incubation behaviour in order to reduce humidity and conferring protection from precipitation or water that favour bacterial penetration [18,19]
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