Free-living birds also increased body mass over the study period, but not to the same extent as in captives (figure 3). Fat scores also increased in free-living birds over the course of our measurements, but never reached the levels of captive birds that had access to food ad libitum (electronic supplementary material, figure S2). Pectoral muscle scores were less variable between stages in free-living birds, but were higher during the migration stage (t = 3.23, p = 0.002; electronic supplementary material, table S2). Importantly, parasitaemia had no effect on any of these morphological variables at any stage we sampled (electronic supplementary material, tables S1 and S2).