Developmental impairment in Eurasian dipper nestlings exposed to urban stream pollutants

Avian studies of endocrine disruption traditionally have focused on reproductive impairment, given that many environmental contaminants affect sex steroid hormones. There is also increasing interest in altered thyroid function, and associated early development, particularly in altricial species with extended developmental windows. Both types of effect are relevant under the complex pollutant conditions created in streams draining urban areas, but case studies are scarce. Therefore, the authors measured breeding performance, as well as nestling growth, condition, and plasma thyroid hormones, in 87 Eurasian dipper (Cinclus cinclus) nests on 36 urban and rural streams in south and mid-Wales (UK); invertebrate prey data were also collected. The objective of the present study was to evaluate whether urban stream pollution or food scarcity might affect reproduction or development in this specialized aquatic songbird. Clutch sizes and egg fertility were similar on rural and urban streams, whereas nest success was actually higher at urban sites and food abundance was not significantly reduced. However, subtle but important differences were apparent. Urban nestlings were significantly lighter than rural nestlings for their body size (condition index), and brood sex ratios were increasingly male biased with increasing urbanization. The nestling thyroid hormone profile closely reflected urban land use, whereas depressed triiodothyronine (T3) hormones and poorer body condition were associated with higher exposure to polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) at urbanized sites. These data suggest that PCBs, PBDEs, and/or accompanying contaminants in urban streams could be affecting dipper nestling development, with potential consequences for the birds' fitness.