Urban areas, characterized by high housing densities, extensive impervious surfaces, and few green spaces 1,2 are globally increasing in extent. As of 2018, 55% of the global population and 74% of Europeans are residing in cities, and the numbers are rising 3. Urban expansion alters ecosystems, leading to the rapid transformation or degradation of natural habitats into typically less optimal ones 4–10 affecting both, biotic and abiotic factors, depending on the level of urbanization. Therefore, urbanization causes shifts in species ranges and abundances, resulting, for example, in a decline of specialized bird species and a predominance of generalists 1,11,12, which impact ecosystem functions 13.
Physical changes from urban-to-rural environments, such as alterations in resource availability, or increasing impervious surfaces can influence organismal responses 14,15. Species reactions to urbanization are species-specific and are mediated by factors like temperature variation, noise, light, air pollution 16–18, or variation in parasite and predator densities 16,19–26. For instance, urban birds tend to breed earlier than those in rural areas, possibly due to higher urban temperatures, leading to mismatches between insect prey availability and bird nesting periods 14,16. This, combined with lower caterpillar biomass in urban areas, for example, can adversely affect body condition due to constrained food resources 14, because the nestlings critically depend on caterpillars as food 27. Combined, urbanization can reduce body condition, due to changes in temperature and the implied food limitations 14.
Higher urban temperatures may also proliferate the development of avian blood parasites (i.e. Haemosporidians) within vectors, however, specific upper and lower temperature-thresholds can inhibit this development 26–30. Higher temperatures towards city centers are a direct consequence of increased impervious surfaces and reduced green spaces. Increased levels of urban pollution (e.g., heavy metals), through immune-toxic effects, can directly impair birds' immune responses, exacerbating the effects of parasites 24. Thus, the effects of pollution and parasites can be costlier in urban habitats than in forests 31, decreasing the reproduction of birds and their body condition 32,33. In general, urban habitats may impose greater reproductive and physiological costs on birds 29,30. However, the dynamics of breeding, immune response, and the influence of parasites along urbanization gradients remain poorly understood.
Habitat use of birds is likely influenced by the interplay between blood parasites, vectors, and hosts 20,34–36. Environmental conditions can affect haemosporidian development within both insect vectors and bird hosts 28,37. Haemosporidian prevalence and transmission will therefore depend on the vectors' abundance, but also the vectors' reproductive-habitat conditions 21,34, while the bird host responds to altered environmental conditions 38. For example, land-use changes can decrease immune response 9,31, affect body condition 14, or reproduction rates 32. As land-use can negatively affect a bird's immune response 31, positive effects on Haemosporidian development within the birds occur. As a consequence, the host's body condition, which is a first approximation to the birds' health status 39, further decreases 34. Hence, urbanization as one of the most intense land-use forms, can cause immunosuppression in birds 22–24. Although urban habitats tend to show lower Haemosporidian prevalence, the role of vectors along urbanization gradients is unclear 20.
Habitat variations, such as those brought by urbanization, indirectly shape the distribution of avian blood parasites, influencing species or strains and affecting overall prevalence and intensity within the host population 34,38,40,41. The presence of blood-feeding dipterans like Culicidae, Ceratopogonidae, and Simuliidae, crucial for Haemosporidian transmission, is often reduced in urban settings due to fewer water bodies for reproduction, potentially lowering vector abundance and, consequently, blood parasite pressure on bird hosts 21,31,34,42,43. Although Haemosporidian prevalence in urban habitats is generally lower compared with natural habitats 31,42, the role of vectors as transmitting agents along urbanization gradients has not yet been assessed together with health of hosts, blood parasite prevalence, and life history parameters 44,45.
During a Haemosporidian infection, hosts can experience severe symptoms, yet the severity and nature of these symptoms are dependent on the specific blood parasite species or strain and the infected host species 34,46. Survivors of an acute infection typically harbor low levels of blood parasites, resulting in a chronic infection with minimal or no pathological signs 21,34. The infection activates the immune system, leading to symptoms marked by elevated levels of heterophils and leucocytes 47. Despite some controversy, the heterophil to lymphocyte ratio (H/L-ratio) is frequently utilized as an initial indicator of the immune response to parasitic infection and environmental stressors in birds 48. However, little is known about how the H/L-ratio is affected by the level of urbanization.
One factor used to evaluate the effect of habitat variation on the immune response on avian health is body condition, often referred to as variation in stored energy reserves 39,49. A robust body condition is presumed to enhance immune response capabilities, thereby increasing the likelihood of surviving a Haemosporidian infection and consequently increasing fitness, reproductive success, and chance of fledgling survival 50,51. While body condition as a solitary metric is somewhat coarse, its utility is enhanced when combined with the evaluation of body asymmetries and scaled mass indices, providing a more comprehensive assessment of the influence of habitat on bird health 52–56.
Urban environments expose birds to chronic stressors like pollution, higher disturbance rates, and increased temperatures, which can collectively suppress the immune system, potentially easing the spread of parasite infections 16,25,26,31,57. The prevalence of Haemosporidians is intricately linked to vector abundance. Thus, higher incidences are predicted in areas with greater vector populations, typically in less urbanized habitats 21,34. These environmental and ecological factors are expected to result in a spectrum of immune responses in birds, with those infected exhibiting a heightened immune response, especially in nestlings where a chronic stage of infection has not yet been established due to the short interval between infection and blood sampling 34.
Our research aims to assess the variation of avian Haemosporidians in Parus major nestlings along a urban-to-rural gradient concerning host health, transmission risk, and land use (a proxy for resource availability). We hypothesized that haemosporidian intensity in Parus major nestlings correlates with the level of urbanization due to reduced immune responses and lower body condition, as well as a higher transmission risk from greater vector abundance in city centers 14,16,24–26,31,34,50,55,58. We also anticipated that body condition deteriorates with increasing parasite intensity, leading to greater body asymmetries and lower scaled mass indices in highly urbanized areas 31. Lastly, we evaluated the relationship between life history traits, reproduction, and urbanization levels, and integrated all discussed aspects 31.