TISSUE-SPECIFIC BIOACCUMULATION OF ZN, CD, NI, FE AND CR IN THE HOUSE SPARROW (PASSER DOMESTICUS) ALONG A RURAL–URBAN GRADIENT IN DISTRICT OKARA, PUNJAB, PAKISTAN

Abstract

Background. Trace-metal contamination is increasing across South Asia, but tissue-specific exposure data for urban passerines in Pakistan remain limited. The sedentary, granivorous House Sparrow (Passer domesticus) is therefore a useful sentinel of localized metal exposure.

Objective. To quantify zinc (Zn), cadmium (Cd), nickel (Ni), iron (Fe) and chromium (Cr) in the liver, kidney and pectoral muscle of House Sparrows occupying paired rural and urban habitats of District Okara, Punjab, and to evaluate whether the species resolves a rural–urban contamination gradient at the tissue level.

Methods. Bird tissues (n = 3/site) were dried, homogenized, HNO₃–H₂O₂ digested, and analysed using certified reference materials. LOD/LOQ were established from 25 procedural blanks. Site differences were assessed using Welch’s t-test, Hedges’ g, and BH-FDR, while CF, log₂FC, MPI, hierarchical clustering, and PCA characterized and integrated multi-element contamination patterns.

Results. Fourteen of the fifteen tissue × element combinations were quantifiable; Ni fell below the detection limit in kidney at both sites. Accumulation ranked Fe > Zn > Cd > Cr in kidney and Fe > Zn > Cr > Ni > Cd in liver and muscle. Every quantified element except Zn in the kidney was higher at the urban site. Enrichment was strongest for Cd (CF = 2.50 in muscle, 2.00 in kidney) and Zn (CF = 1.52 in liver, 1.50 in muscle), and weakest for Fe (CF = 1.01–1.04). MPI rose from rural to urban in all three tissues (kidney +26 %, liver +27 %, muscle +42 %). Principal component analysis separated the strata along PC1 (83.5% of variance), driven by the contrast between the Zn- and Fe-dominated kidney profile and the Fe–Cr–Ni profile of liver and muscle. After correction for multiple testing, no single contrast retained significance, a direct consequence of the small sample (minimum detectable effect, Cohen’s d ≈ 3.1 at n = 3 per group).

Conclusions. Metal burdens remained below published avian toxicity thresholds, indicating no acute toxicity. However, consistent changes in 13/14 contrasts, particularly for Cd, Ni, and Cr, support anthropogenic metal enrichment. P. domesticus may therefore serve as a useful sentinel of urban trace-metal exposure in central Punjab