Background: Managed forests can become a seminatural habitat by allowing natural vegetation to establish under the tree canopy. The aim of this study was to examine how this natural understory vegetation in pine plantations affects bird community composition and diversity. We compared bird communities in three types of pine forest plantations that differed in the composition of their understory vegetation. Results: One thousand four hundred seventy-six birds of 33 species were observed in the three different forest habitats over two years. We found that the presence and composition of understory vegetation had a major role in determining bird community composition, richness, complexity and diversity. Conclusions: To ensure pine forest with high biodiversity in the eastern Mediterranean region, the understory needs to be rich in shrubs that are attractive to birds and other natural fauna.

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Pine-forest bird communities are affected by the forest structure and the density and composition of its understory vegetation. In this study, we focused on the combined effects of vegetation structure and caller identity on the mobbing behavior of birds in breeding and non-breeding seasons. We examined the effect of the understory structure and the density of three types of conifer forest habitats on bird behavior by broadcasting three different types of mobbing calls: Those of two all-year resident species in the state Israel (Sardinian warbler Sylvia melanocephala Gmelin and great tit Parus major L.) and one European species (coal tit Periparus ater L.), which is absent from these habitats. The mobbing call attracted 689 birds of 17 species, which represented 31% to 95% of the forest bird species that we detected in point counts at the same study plots. Bird reactions to mobbing calls were affected by the status and season, depending on forest type. Our results show that responses were stronger in forests with developed understory in comparison to forests with no understory, especially in winter. The highest number of responders and highest species richness of responders were observed in winter. P. major calls generated more interspecific than intraspecific responses, whereas S. melanocephala calls generated equal levels of inter- and intraspecific reactions. Both species generated different response patterns across the three forest habitat types. The response level of responders is higher when the mobbing calls are issued by local species rather than the P. ater. In winter, the response of non-resident species was higher than of resident species. Based on these results, we encourage the managers to maintain well-established understory vegetation, with special attention to the shrubs layer, to promote species diversity and rich behavioral responses of birds in the conifer forests in the eastern Mediterranean region.