Disentangling the effects of multifunctional forestry practices on the abundances of birds and their invertebrate prey.

European forests are increasingly managed to harmonize production goals with biodiversity conservation, through practices such as retention and close-to-nature forestry. Forest birds may benefit from these practices, but it remains unclear how the effects of different management practices compare, and whether responses to management are driven by changes in the availability of invertebrates, a crucial element of bird diets during the breeding season. To answer these questions, we carried out bird point counts on 135 1-ha plots in southwestern Germany from 2017 to 2022, and measured the abundance of invertebrate groups in the lower forest strata using flight interception traps and pitfall traps. We used N-mixture models and Bayesian generalized linear models (GLMs) to estimate, respectively, how abundances of 32 bird species and 20 invertebrate groups respond to predictors representing forest management, structure, composition, and the abiotic environment. We then compared the responses of birds and invertebrates, and employed piecewise structural equation models (SEMs) to disentangle the causal links between forest structure and abundances of bird guilds and invertebrate groups. Bird abundances responded to predictors representing retention and close-to-nature forestry practices, but the direction of effects varied across species and facets of management. Moreover, the effects of retention practices were weaker than those of close-to-nature practices, especially those of admixing broadleaf trees. Hence, these management practices likely need to be applied in tandem with others (e.g., gap creation) to secure a diverse forest bird assemblage. Invertebrate abundances responded to both management types, but responses did not clearly align with those of bird species, and SEMs did not support direct links between bird and invertebrate abundances. Still, we revealed parallel positive responses of birds and invertebrate groups to the same habitat features, such as broadleaf share, suggesting that these may function as cues for high food availability during habitat selection by birds. Therefore, forest management that aims at increasing bird populations should address other potential limiting factors, such as nest site availability, in addition to fostering high invertebrate abundances, which may safeguard habitat quality for birds.