Validation of the use of phenylhydrazine hydrochloride (PHZ) for experimental manipulation of haematocrit and plasma haemoglobin in birds

The levels of haematocrit (Hct) and plasma haemoglobin (Hb) vary markedly through the annual cycle of birds, as well as among individuals at all life‐stages (embryos, chicks, adults). It is thus surprising that the functional, fitness‐related consequences of this variation are poorly understood. Putative ‘adaptive’ variation in these haematological traits has been associated with varying demands for aerobic capacity and oxygen transport, for example during migration, winter acclimatization, with increasing altitude, or during parental care. It has also been proposed that changes in Hct might reflect ‘costs’ of parental investment, for example during ‘reproductive anaemia’. However, almost all studies to date have been correlative. Here we describe a series of experiments that validate the use of phenylhydrazine hydrochloride (PHZ) for the transient, reversible experimental manipulation of Hct and Hb in birds. A single bolus injection (12.5 μg PHZ/g body weight delivered via intra‐muscular injection) caused a rapid decrease in Hct and plasma Hb within 24 h, from pretreatment values of 50–54% to 40–44% in non‐breeding Zebra Finches Taenipoygia guttata and European Starlings Sturnus vulgaris, and to 35% in breeding female Zebra Finches, changes within the normal physiological range. Hct and Hb returned to pre‐injection levels within 5–10 days of treatment. Changes in plasma Hb paralleled those for Hct. We suggest that PHZ treatment provides a widely applicable technique for use in experimental work to establish relationships between haematological status, aerobic capacity, workload (e.g. migration, parental care, thermoregulation), individual quality (of both adults and chicks) and trade‐offs such as costs of reproduction.