The role of commensal birds in the epidemiology of pathogens in poultry farms remains unclear. Our study aimed to identify potential key species for interactions with domestic ducks on one free-range duck farm in southwest France. Methods combined direct individual observations on duck outdoor foraging areas, network analysis, and general linear mixed models of abundances. Results showed a wide diversity of wild bird species visiting foraging areas, heavily dominated in frequency by White wagtails (Motacilla alba) and Sparrows (Passer domesticus and Passer montanus). These also were the only species seen entering duck premises or perching on drinkers in the presence of ducks. Moreover, White wagtails were the species most frequently observed on the ground and in close proximity to ducks. Network analysis suggested the role of White wagtails and Sparrows in linking ducks to other wild birds on the farm. The abundance of White wagtails was positively associated with open vegetation, with the presence of ducks and particularly in the afternoon, while the abundance of Sparrows was positively associated only with the fall-winter season. By precisely characterising interactions, the study was able to identify few wild bird species which should be prioritized in infectious investigations at the interface with poultry.

Recent outbreaks of highly pathogenic avian influenza in southwest France have raised questions regarding the role of commensal wild birds in the introduction and dissemination of pathogens between poultry farms. To assess possible infectious contacts at the wild-domestic bird interface, the presence of Mycoplasma gallisepticum (MG) was studied in the two sympatric compartments in southwest France. Among various peridomestic wild birds (n = 385), standard PCR primers targeting the 16S rRNA of MG showed a high apparent prevalence (up to 45%) in cloacal swabs of European starlings (Sturnus vulgaris, n = 108), while the MG-specific mgc2 gene was not detected. No tracheal swab of these birds tested positive, and no clinical sign was observed in positive birds, suggesting commensalism in the digestive tract of starlings. A mycoplasma strain was then isolated from a starling swab and its whole genome was sequenced using both Illumina and Nanopore technologies. Phylogenetic analysis showed that it was closely related to MG and M. tullyi, although it was a distinct species. A pair of specific PCR primers targeting the mgc2-like gene of this MG-like strain was designed and used to screen again the same avian populations and a wintering urban population of starlings (n = 50). Previous PCR results obtained in starlings were confirmed to be mostly due to this strain (20/22 positive pools). In contrast, the strain was not detected in fresh faeces of urban starlings. Furthermore, it was detected in one cloacal pool of white wagtails, suggesting infectious transmissions between synanthropic birds with similar feeding behaviour. As the new Starling mycoplasma was not detected in free-range ducks (n = 80) in close contact with positive starlings, nor in backyard (n = 320) and free-range commercial (n = 720) chickens of the area, it might not infect poultry. However, it could be involved in mycoplasma gene transfer in such multi-species contexts.

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