About 4% of the 82 Mesetan individuals were non-local. At least three of four non-local individuals from the Los Berrocales site were women, but they probably came from the nearby Guadarrama sierra. More on that in a moment...
Based on δ18O values of the more recent molars, dairy products may have been important to the Mesetan-Madrid adult diet. But as δ13C values continue to climb, the authors suggest that a C4 plant, such as millet, or a CAM plant, often desert plants, is needed to square everything since the other main culprit, marine proteins, are not sufficiently available in the high steppe plateau of Spain. (More on photorespiration from Khan Academy)
Other possibilities are left open, such as the use of fertilizers, fallow grazing and the 'canopy effect', but these are not likely given elevated results in other environments of Spain. So, of the two remaining culprits, millet looks to be a good candidate.
The authors consider the implication of millet production to be reduced or no seasonal mobility, given the growing season of this plant.
Lastly, going back to the migrant percentages.
Almost all of these individuals are directly dated to the Chalcolithic to Bronze Age:
|Archaeol Anthropol Sci (Diaz del Rio, 2017)|
We may learn more about these specific individuals in an upcoming study, but the insular nature of this group doesn't seem to indicate strong family ties with outside region. It could also be the Meseta geology is so large that the Madrid group, to whatever degree may be foreign or native, is better insulated against first generation movers.
"Diet and mobility patterns in the Late Prehistory of central Iberia (4000–1400 cal bc): the evidence of radiogenic (87Sr/86Sr) and stable (δ18O, δ13C) isotope ratios"
Díaz-del-Río, P., Waterman, A.J., Thomas, J.T. et al. Archaeol Anthropol Sci (2017). doi:10.1007/s12520-017-0480-y [Link]