Mean corpuscular volume (MCV) as a measure of condition in birds

Haematological parameters, particularly haematocrit, are frequently used in assessing condition in birds, Mean corpuscular volume (MCV) overcomes some of the drawbacks associated with measuring haematocrits and therefore shou ld be a better condition index. We investigated the relationship between th ese blood parameters and the breeding performances of great skuas Catharact a skua. As predicted from activity budgets of the sexes, male MCVs were sig nificantly positively correlated with hatch date and the MCVs of both sexes were negatively correlated with the number of chicks fledged. No correlati ons existed between these breeding parameters and haematocrit. Since males provision females and chicks, they probably influence breeding performance via foraging abilities, whereas in females nutritional investment may be mo re important. This is the first study of a free-living/nondomesticated vert ebrate population in which these kinds of relationship have been demonstrat ed and we conclude that MCV is a useful measure of condition.