Migrating monarchs tend to have darker orange and larger wings than they do during the breeding phase in the summer.[51] The darkness of the orange color in monarch wings appears to be a visual indicator of their migratory ability.[52] The monarchs migrating south are larger in size and weight[53] Two studies have used stable isotopes to infer natal origins of migrating monarchs captured at their wintering sites (eastern and western), and both showed that monarchs migrating longer distances tended to be larger.[54][55] Wing size differs between early and late migrants.