Element concentrationsa (mean ± SE) of reference samples of wintering adult great tits (excrement) used as an internal standard, and certified reference material ERM-CE278 (mussel), run in batches with samples from different years.

The certified values for mussel tissue (cert value) obtained from the Institute for Reference Materials. Sample sizes are given in brackets.a μg g-1; dwNA Not analyzedNI Element not included in certified reference materialElement concentrationsa (mean ± SE) of reference samples of wintering adult great tits (excrement) used as an internal standard, and certified reference material ERM-CE278 (mussel), run in batches with samples from different years.