| Summary: | Avian studies of endocrine disruption have focussed traditionally 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, we 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, also collecting invertebrate prey. Our objective was to evaluate whether urban stream pollution or food scarcity might affect reproduction or development in this specialised aquatic songbird. Clutch sizes and egg fertility were similar on rural and urban streams, while 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, while 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.
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