Sources of variation in haematocrit in birds

Abstract

The validity of the haematocrit or packed cell volume as an indicator of condition in wild birds has recently been questioned. We reviewed over 300 published papers on haematocrit values for wild birds. These studies show that changes in haematocrit could be caused by a number of different natural factors that include age, sex, geographical elevation, energy expenditure, parasitism, nutrition and genetics. Haematocrit also increased with age from hatching, due to increased erythropoiesis, so that adult birds generally have greater haematocrit values than nestlings or juveniles. Haematocrit values were either independent of elevation or increased with elevation. A meta‐analysis of 36 studies showed no difference in haematocrit between the sexes. Relationships between haematocrit value and both energy expenditure and parasitic infection vary between studies. In temperate climates, haematocrit tended to be higher in winter than in summer, which may be due to dehydration or increased oxygen demand caused by thermogenesis, moult or acquisition of reproductive status. Our review indicates that the use of haematocrit as a sole indicator of condition or health could lead to incorrect conclusions if natural factors that can affect haematocrit are not taken into consideration.

Seasonal stress physiology and body condition differ among co-occurring tropical finch species

“…Small increases in CORT are likely to drive these individuals to forage more and expend more energy, and this could explain their relatively high haematocrit concentrations (mean > 60). High haematocrit concentrations in birds are often attributed to extreme dehydration or increases in energy demands, such as cold stress and increased activity during breeding or migration (Carpenter 1975;Breuer et al 1995;Horak et al 1998;Box et al 2002;Fair et al 2007). However, all finch species showed no significant seasonal differences in haematocrit, and without obvious changes in haematocrit above or below mean concentrations, interpretation of the reason for relatively high haematocrit values is not possible (Fair et al 2007).…”

“…Haematocrit of temperate bird species varies seasonally: lower concentrations are typically found late in the breeding season or during moult, whereas higher values occur in the winter, early breeding season, and during migration, all times when energy requirements are expected to be elevated (Morton 1994;Box et al 2002;Gayathri and Hegde 2006;Owen and Moore 2006). More research is needed to fully describe this variation, as few studies have looked at possible seasonal differences in haematocrit in tropical species (Fair et al 2007).…”

Mothers adjust offspring sex to match the quality of the rearing environment

“…Haematocrit, or packed cell volume in relation to the whole volume of blood, reflects the erythrocyte content of blood, and is an increasingly used index of avian condition and physiological performance [39][40][41][42]. Haematocrit was calculated as the ratio of packed cell volume of whole blood (%) from the two centrifuged capillaries; within-individual measurements were highly repeatable (r ¼ 0.98, F 47,96 ¼ 46.32, p , 0.001).…”

Spatial variation in haemoglobin concentration of nestling Blue Tits (Cyanistes caeruleus): a long-term perspective

“…All those features support the use of the haemoglobin level as a relatively reliable indicator of physiological condition in field and experimental studies on birds (Stevens 1996;Kilgas et al 2006a;Kaliński et al 2009Kaliński et al , 2012Pyrke and Rollins 2012;Lill et al 2013;Minias 2014;Minias et al 2014;Glądalski et al 2015a). But it is important to emphasize that some recent studies recommend caution when using blood characteristics to evaluate physiological condition in some species (Fair et al 2007;Lill et al 2013;Minias 2015;Johnstone et al 2015). Also the use of haemoglobin concentration as a body condition index in a particular population should not be automatically extended to other populations (Fair et al 2007;Smith and Barber 2012;Lill et al 2013;Minias 2015;Glądalski et al 2015a).…”

“…But it is important to emphasize that some recent studies recommend caution when using blood characteristics to evaluate physiological condition in some species (Fair et al 2007;Lill et al 2013;Minias 2015;Johnstone et al 2015). Also the use of haemoglobin concentration as a body condition index in a particular population should not be automatically extended to other populations (Fair et al 2007;Smith and Barber 2012;Lill et al 2013;Minias 2015;Glądalski et al 2015a). Knowledge of phenotypic and ecological factors that directly influence the level of haemoglobin in free living birds is relatively poor Minias 2015).…”