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Lyme borreliosis (LB) and West Nile fever (WNF) are among the most common vector-borne human infections. Both infections involve birds in their enzootic transmission cycle. Currently, there is no available data on possible circulation of Borrelia burgdorferi sensu lato (B. burgdorferi s.l.) and West Nile virus (WNV) in wild birds in Bulgaria. Serum samples from 160 wild birds of 18 species collected in May and in September 2021 in two regions of Bulgaria were tested by ELISA for antibodies to B. burgdorferi s.l. and WNV. Specific antibodies were found in 56 (35.0%) of all tested birds. Antibodies to B. burgdorferi s.l. were present in 40 (25.0%) and antibodies to WNV were found in 28 (17.5%) of the birds. Simultaneous presence of antibodies against both pathogens was detected in 12 (7.5%) of the tested birds. Analysis of the results by bird species showed that out of the 18 different bird species, antibodies against at least one of the two tested infectious agents were found in nine: Acrocephalus arundinaceus (Linnaeus, 1758), Cyanistes caeruleus (Linnaeus, 1758), Fringilla coelebs (Linnaeus, 1758), Garrulus glandarius (Linnaeus, 1758), Luscinia megarhynchos (Brehm, 1831), Parus major (Linnaeus, 1758), Passer domesticus (Linnaeus, 1758), Sitta europaea (Linnaeus, 1758) and Turdus merula (Linnaeus, 1758). This study observed high levels of antibodies against the etiological agents of Lyme borreliosis and WNF in wild resident and migratory birds. The highest seropositivity levels against both pathogens were found in three species – P. major, T. merula and L. megarhynchos. The results confirmed active circulation of B. burgdorferi s.l. and WNV in the bird population and defined its importance as a reservoir. [ABSTRACT FROM AUTHOR]