In 1991, eggs of the common tern (Sterna hirundo) were collected at eight different locations and incubated artificially. The residual yolk sacs of the hatchlings from the highly polluted colonies, located in the main sedimentation area of the Rhine and Meuse rivers, contained on average 16 ng TEQ (toxicity equivalency) per gram lipid, which was fivefold higher than the concentrations in the reference colony. The hepatic EROD activity was induced fourfold compared to the reference colony (p < 0.005). At the individual level, a significant relationship was found between log TEQ and both log EROD and log PROD (r = 0.568, p < 0.05 and r = 0.661, p < 0.02, respectively). Average TEQ concentration in chicks that hatched after 23 d of incubation were twice the concentration of those that hatched after 21 d (p < 0.05). No concentration‐related effects on morphology and physiology were found. The non‐ortho‐ and mono‐ortho‐PCBs were predominant regarding the total TEQ, while the PCDDs and PCDFs contributed less than 10%. The PCB patterns were highly similar among the individual birds and among different locations. In contrast, PCDD and PCDF patterns were not similar at all locations, and two distinct patterns could be recognized and related to sediments that were deposited during different time periods.

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