Factors affecting frontline host defences against avian brood parasitism: a meta-analysis López-Luengo, Mari Carmen Samas, Peter Ruiz-Raya, Francisco Soler Cruz, Manuel Honza, Marcel Aggression Dummy Mobbing This work was supported by MICIN/AEI/10.13039/501100011033 and FSE+, as part of the grant PRE2021-097543, associated to PID2020-115950GB-I00. Computational resources were provided by the e-INFRA CZ project (ID: 90254), supported by the Ministry of Education, Youth and Sports of the Czech Republic. This work was funded by the Czech Science Foundation (grant number: 22-26812S). Funding for open access charge: Universidad de Granada/CBUA. Obligate avian brood parasitism is a reproductive strategy in which a bird lays its egg in the nest of other bird (host), thereby forcing the host to raise the parasitic offspring as its own. This behaviour triggers a coevolutionary arms race, driving the evolution of host defences and parasite counter defences. Hosts can defend against brood parasitism at all stages of the breeding cycle, with frontline defences (those aimed at preventing a parasite from laying its egg) representing the earliest and critical stage of defence. Despite their importance, the main factors driving frontline (prelaying) antiparasitic behaviours remain unclear. Here, a meta-analytical approach was used to quantify the influence of various factors on host aggression, as an important and widespread frontline defence, towards adult brood parasites. In general, host aggression is assessed in field experiments by presenting brood parasite models near nests and observing the host response. Results show that host aggression towards brood parasites was influenced by the geographical overlap between host and parasite, as well as the distance of the experimental model from the host nest. In particular, aggression was higher in host populations living in sympatry with the parasite. In addition, hosts showed greater aggression towards parasite models placed closer to their nests in field experiments. Other factors, including differences in host—parasite body size, nest type (enclosed or open) and exposure time to the experimental model, did not affect host aggression. Our findings indicate that host aggression towards brood parasite models is primarily influenced by the sympatry—allopatry distribution between brood parasites and hosts. Furthermore, our study highlights the key methodological aspects that should be considered in future field experiments involving artificial brood parasite models. 2026-02-02T13:00:45Z 2026-02-02T13:00:45Z 2026 journal article López-Luengo, M. C., Samas, P., Ruiz-Raya, F., Soler, M., Honza, M. (2026) Factors Affecting Frontline Host Defences Against Avian Brood parasitism: A meta-analysis. Animal Behaviour, 233, 123459. https://doi.org/10.1016/j.anbehav.2025.123459 https://hdl.handle.net/10481/110591 10.1016/j.anbehav.2025.123459 eng http://creativecommons.org/licenses/by-nc-nd/4.0/ open access Attribution-NonCommercial-NoDerivatives 4.0 Internacional Elsevier