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Heavy-Metal Exposure Coincided With Altered Behaviour in Eurasian Tree Sparrows

Eurasian tree sparrow in Nepal
Research · Behaviour

Wild Eurasian tree sparrows (Passer montanus) living beside a long-polluted metal-smelting area showed different behaviour and more signs of developmental instability than birds from a nearby comparison site, according to a new study from Henan Province in China. The findings link greater exposure to several heavy metals with measurable differences in activity, sociability and responses to handling, but the two-site study cannot establish that pollution alone caused those differences.

The researchers compared Eurasian tree sparrows from Mantoushan Village, where industrial wastewater from metal smelting has contaminated the environment, with birds from Geliaofan Village, a relatively unpolluted site 17 kilometres away. Fieldwork conducted between 2021 and 2024 included 140 wild birds: 40 adults and 30 juveniles from each site, divided evenly between males and females.

Polluted-site birds carried more copper, lead, cadmium and chromium

Samples from the birds were analysed for copper, zinc, lead, chromium and cadmium. Birds from the polluted site generally carried higher concentrations of copper, lead, cadmium and chromium in several tissues. They also had lower calcium levels and greater fluctuating asymmetry—small differences between the left and right sides of paired body structures. Such asymmetry is often used as an indicator of developmental instability, although it does not identify a single cause.

The team assessed several aspects of personality under temporary captive conditions. Heart rate during handling was used as one measure of stress responsiveness. Exploration and activity were recorded in a new environment, while reactions to a mirror were used to estimate sociability. The birds were colour-ringed and released after testing.

Some behavioural differences depended on age and sex. Adults and juvenile males from the polluted site were more active, while adult and juvenile males also showed greater sociability in the mirror test. Birds from the comparison site had higher handling heart rates and, in several groups, remained stationary for longer periods. Measures related to boldness and exploration also differed, but not in one uniform direction across every age and sex group.

No simple “polluted personality”

The relationships between individual metals and behaviour were complex. Higher levels of chromium, cadmium, lead or copper were often associated with lower scores for exploration, boldness or sociability, whereas zinc was positively associated with some behaviours. These patterns did not produce a single personality profile that applied to all birds.

The authors suggest several possible biological pathways. Heavy metals can interfere with the nervous system, hormone regulation and energy balance, while reduced calcium may affect physiological function. Behavioural change could in turn influence feeding, predator avoidance, competition and reproduction. However, these mechanisms were not directly tested in this study.

Two sites cannot isolate pollution from every habitat difference

The strongest caution is the study design. Only one polluted site and one comparison site were examined. Because the two villages may differ in food availability, vegetation, predators, human disturbance or other unmeasured conditions, the results cannot separate the effects of metal exposure from every habitat difference. The authors also caution that the study used a modest sample over a limited sampling period, so individual associations should be interpreted cautiously.

Sources and image credit

  1. Ding, J., Li, Q., Li, X., Wang, S., Feng, S., Chen, Y., & Yu, F. (2026). Is the personality of Tree Sparrows (Passer montanus) in heavy metal-polluted environments undergoing silent change? Avian Research, 17(1), 100355. https://doi.org/10.1016/j.avrs.2026.100355Primary research article. The journal’s official article page lists both Available Online and Published Date as 6 March 2026. The study compared 140 wild Eurasian tree sparrows from a metal-polluted site and a comparison site 17 km away in Henan, measuring heavy metals, fluctuating asymmetry and four personality traits: exploration, boldness, activity and sociability.

Image: Nrik kiran — Eurasian tree sparrow (Passer montanus) in Nepal, photographed 10 February 2018; Wikimedia Commons; representative species image, not a bird from the Henan study · CC BY-SA 4.0 · Source

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