Complex effects of ectoparasites on avian fitness: a meta‐analysis

Ectoparasites temporarily or permanently feed on their hosts and, hence, they are expected to exert a direct or indirect negative impact on host condition and fitness. Here, we conducted a meta‐analysis to examine the effects of ectoparasites on fitness‐related traits of avian hosts. We compiled ~850 effect sizes from available literature reflecting interactions of diverse ectoparasite taxa with a broad spectrum of avian hosts distributed worldwide. We found evidence for complex and multifaceted negative effects of ectoparasites on bird fitness. Host condition‐related traits (e.g. haematology) were more strongly affected by ectoparasites compared to key fitness components, likely reflecting direct impact of ectoparasites on avian condition and health. In contrast, the effects of ectoparasites on avian survival and reproduction showed much greater heterogeneity, supporting strong stochasticity of associations between ectoparasitic pressure and fitness components. Finally, we found conspicuous variation in the impact of different ectoparasite groups on avian fitness, where mites, hemipteran bugs, and blowfly larvae yielded the strongest negative effects, whereas ticks, fleas, and chewing lice showed the weakest effects. We suggest that this variation may likely be driven by key life history traits and life cycles of ectoparasites. Our study builds a comprehensive framework for further ecological research on host–ectoparasite interactions, and it lays the foundation for a broader understanding of evolutionary responses of hosts to ectoparasite‐mediated selection. We also identify major gaps in our knowledge and recommend that future scientific effort focus on enhancing (the still limited) diversity of the host–ectoparasite systems under study.