Body condition influences activities and processes affecting fitness. Erythrocyte variables, particularly haemoglobin concentration and haematocrit, are increasingly being used as avian condition indicators. However, some potentially confounding factors should be considered when adopting this approach and there are some conflicting findings concerning such variables’ reliability as condition indicators. We examined whether whole blood haemoglobin concentration (Hb), haematocrit (Hct) and four other erythrocyte and one leukocyte variable were significantly associated with body mass and four commonly used body condition indices (BCI) in 12-day old Welcome Swallow Hirundo neoxena nestlings. Haematocrit and Hb were significantly associated with body mass and all four BCIs and had significant repeatabilities (0.426 and 0.433, respectively); total erythrocyte count, mean corpuscular volume, mean cell haemoglobin, mean cell haemoglobin concentration and total leukocyte count were not associated with mass or the BCIs, despite mostly having significant repeatabilities (0.129–0.433) and being reliable condition indicators in some other bird species. The lack of association between four of these other erythrocyte variables and nestling condition indicated that some caution was appropriate in interpreting the functional significance of the Hct/Hb–condition relationships. It seems advisable to measure a suite of erythrocyte variables when trying to determine whether the commonly measured Hct and Hb are associated with body condition in nestling birds.