Oral disease is linked to low nestling condition and brood size in a raptor species living in a highly modified environment.

Anthropogenic stressors can favor the occurrence of noninfectious disease that can be worsened by the impact of opportunistic pathogens, making the epizootiology of environmental diseases difficult to unravel. The incidence and impact of oral lesions in nestlings of a facultative scavenger species, the black kite Milvus migrans, were examined over seven breeding seasons in the highly degraded environment close to Madrid, Spain. We found an overall prevalence of 31% of nestlings with oral lesions, with no clear spatial pattern in nests with affected and unaffected individuals. The occurrence and number of oral lesions were negatively associated with nestling body condition and brood size. Broods, where all siblings had oral lesions, were smaller than those where some or all siblings were apparently healthy, suggesting that oral disease could be causing nestling mortality and, consequently, brood size reduction. In turn, nestling body condition was negatively affected by lesion occurrence, brood size, and laying date. Although these relationships were bidirectional, piecewise structural equation modeling analyses showed a greater negative effect of body condition on lesion occurrence than vice versa, indicating that nestlings in poorer body condition were more likely to develop oral lesions (which could contribute to aggravate their state of deterioration) than those in better condition. Nestlings from small broods were also more likely to have oral disease (directly or indirectly through their lower body condition) than nestlings from large broods. Nestlings that hatched last in the broods showed greater development stress than those that hatched first. Anthropogenic stressors could trigger poor body condition, and contribute to microbiota dysbiosis-related diseases. Although further research is needed to determine the consequences for the long-term fitness of individuals, actions should be taken to mitigate adverse conditions that may favor the appearance of environmental diseases associated with peri-urban areas, given their rapid expansion over natural areas.