Different teeth form at different points in childhood. First molars begin forming before birth and complete their crown formation in the first year or two of life. Second molars form somewhat later. Third molars (wisdom teeth) form later still. Comparing strontium ratios across different teeth can, in principle, reveal whether a person moved during childhood and approximately when.
The strontium analysis of the Bj 581 individual used three mandibular (lower jaw) molars. The values obtained were lower than those typical of most other Birka individuals, placing them outside the local geological baseline for the Björkö region. This means the individual did not grow up in the immediate Birka area.
More specifically, the analysis detected different strontium ratios in the first and second molars — a difference that the researchers interpreted as indicating a change in geographic location during childhood, possibly before the age of three. The individual grew up somewhere else, moved during early childhood, and eventually came to Birka, where she died and was buried.
The interpretation of these strontium values requires careful handling. Strontium isotope analysis can establish that someone’s childhood environment was geographically distinct from their burial location. It cannot, from a strontium ratio alone, identify a specific origin location — the same ratio might match multiple geological regions. The researchers’ characterization of the probable geographic origin as resembling areas under Viking Age cultural influence, possibly including southern Sweden or Denmark, is an inference from the isotope values combined with comparison data from other sites and regions.
But the core point — that this individual was mobile, that she had moved geographically, that she was not a lifelong resident of the Birka region — is supported by the isotope evidence and is consistent with a picture of elite social status in the Viking Age. High-ranking individuals in this era traveled: as warriors, as traders, as people whose political and economic networks extended beyond their home region. The strontium evidence does not prove any of these specific scenarios. It is consistent with all of them.