Blood profiles of captive Kagu in our study were intermediate between those of wild Kagu and captive Kagu studied by Vassart (1988) although haemoglobin concentration, mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration exceeded (sometimes by far) the highest values recorded for birds. Conversely, the blood profiles of wild Kagu were within the range for birds (see review in Hawkey et al. 1991 in Table 1). The unusual blood profile of captive Kagu is, therefore, not an adaptation of the species to the specific environment of New Caledonia but rather has a pathological origin. Especially, the low counts of red and white blood cells in captive birds point at an anaemia. Low red blood cell counts usually co-occur with low haemoglobin concentration (Jones et al. 2002). However, very high haemoglobin concentration combined with low red blood cell counts could be associated with a haemolytic anaemia, as haemoglobin from destroyed red blood cells would still be present in the plasma, and could be detected by analytical procedures. Although there were no indications of destroyed blood cells during the blood analyses, we cannot exclude the possibility that red blood cells deteriorated in an earlier stage and were, therefore, not detectable during the cell count. Accordingly, the observed unusually high values of mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration in captive Kagu might be artefacts and a by-product of the calculation methods.
The most common causes of macrocytosis and haemolytic anaemia are inadequate diet (especially deficiency of vitamin B12), bone marrow and liver disorders or infections (Shaw et al. 2009). The diet of Kagu kept in the Parc Zoologique et Forestier in Nouméa mainly consists of bovine heart and could be the reason for the pathological blood pattern. The low frequency and small volume of defaecation by captive Kagu and the discoloration of their bills and legs that we observed also point at a deficiency in their diet. Further veterinary studies, however, would be necessary for a precise diagnosis of the mechanisms causing the observed blood anomaly.
High, but physiological haemoglobin concentration, haematocrit and red blood cell count are a good proxy for the condition of birds as they correlate with body mass and body fat (Minias 2015). Therefore, our results imply that wild Kagu were in a better condition than captive birds (Fair et al. 2007), probably due to more appropriate nutrient contents of the natural diet and more physical exercise associated with foraging. This also complies with the absence of haemoparasites and negligible infestation of endemic blood ectoparasites (Beugnet et al. 1995) that we observed in the studied wild population of Kagu.