SHORT COMMUNICATIONS 
791 
hematocrit varies across different nutritional 
regimes. Hematocrit is clearly not a reliable 
measure of condition in all avian species. 
Breeding adult male and female European 
Starlings did not differ in their hematocrit. Fair 
dal. (2007) reviewed the published literature, and 
found no differences in hematocrit between males 
and females in 36 avian species. Some studies 
reported males having significantly higher hemal 
ocrit than females (e g.. American Robins | Turclus 
migraiorius], Carey and Morton 1976: Brown¬ 
headed Cowbirds [ Molothrus clier\. Keys et al. 
1986), while others found females had higher 
hematocrit than males (e.g.. Great Tits \Pants 
major], Ots et al. 1998). 
Hematocrit of European Starlings increased 
significantly with age: breeding adults had 
significantly higher hematocrit than 5- and 11 - 
day old nestlings, and I I-day old nestlings had 
significantly higher hematocrit than 5-day old 
nestlings. These findings likely reflect the in¬ 
creased muscular activity and demand for oxygen 
(Saino et al. 1997), and the increased production 
of red blood cells (Campbell 1994) that occurs 
with age. Hematocrit is also positively correlated 
with energy expenditure (Viscor cl al. 1985. Saino 
el al. 1997). Five-day old European Starling 
nestlings are unable to thermoregulate, while I I 
day old nestlings are almost 100% sell sullicient 
at thennoregulation (Feare 1984). Older nestlings 
expend more energy than younger nestlings 
through thermoregulation and increased activity. 
Older nestling Eurasian Tree Sparrows (Passer 
muntanus ) had higher hematocrit than younger 
nestlings (Kostelecka-Myrcha et al. 1997). He¬ 
matocrit also increased with age in Chinstrap 
Penguins ( Pxgoscelis anlarclinis)', adults had 
higher hematocrit than juveniles which had higher 
hematocrit than nestlings (Merino and Barbosa 
1997). Fair et al. (2007) reported published 
studies found either an increase in hematocrit 
w 'lh age, or no association; no study detected a 
decrease in hematocrit with age. Norte et al. 
• 2009) found hematocrit increased with age 
within adult Great Tils. 
Adult and nestling hematocrits were not 
correlated with Julian date of sampling, ambient 
temperature, or relative humidity in our study. 
Christe et al. (2002) found hematocrit of breeding 
adult Common House Martins (Delichon urbi- 
c um) decreased between first and second broods. 
Hematocrit of 5-day old nestlings in our study did 
not differ between early and late broods in 
accordance with our finding no association 
between hematocrit and Julian sampling date. 
Dubiec and Ciehon (2001), however, found Great 
Tit nestlings in second broods had lower hemat¬ 
ocrit than those in first broods, and proposed that 
it reflected poorer body condition in second 
broods. Williams et al. (2004) reported hematocrit 
in breeding adult European Starlings was unaf¬ 
fected by ambient temperature, which concurs 
with our findings. Hematocrit ol 5-day old 
nestlings increased with time of day. hut we did 
not find this association in adults or older 
nestlings. Few studies have examined potential 
hematocrit changes throughout the day. Dawson 
and Bortolotti (1997) reported a negative correla- 
lion between hematocrit and time of sampling in 
adult American Kestrels (Falco sparverius). They 
proposed this decline in hematocrit through the 
day might he due to dehydration from fasting 
overnight, and then replenishing plasma water 
through their diet as the day progressed. Con¬ 
versely, an increase in hematocrit with time ot day 
as seen in our 5-day old European Starling 
nestlings might he attributable to either vigorous 
begging at this age. resulting in a loss ol plasma 
water through evaporation, or that their diet 
consists of items that dehydrate them as the day 
progresses. Further examination into the relation¬ 
ship between hematocrit and meteorological 
variables is warranted, as we did not take ambient 
temperature and relative humidity readings al our 
study site, but relied on a weather station 35 km 
distant. 
Our study supports those that caution against 
using hematocrit as an indicator ot condition. Fair 
et al. (2007) suggest hematocrit can be uselul 
when combined with other indices, but only when 
potentially confounding factors such as energy 
requirements, season, and disease are considered. 
ACKNOWLEDGMENTS 
We ihank all those who helped with data collection 
including A. M. C. Ouedraogo. M A. W. Hornsby. J. W. G. 
Slade. M. E. Lalouf, M. T. McCubbin. and L. K. Zou. This 
studv was funded by a Discov ers grant to C. A. Barber from 
the Natural Sciences and Engineering Research Council, 
and a Dean of Science Research Award to K. D. Smith. J. 
R. Foote provided invaluable help with statistical analysis 
for which we are grateful. We sincerely appreciate the 
insightful comments given by two anonymous reviwers, 
and the editorial improvements made by C E. Braun. We 
thank Saint Mary's University for logistic support and for 
allowing us to conduct this study on campus. 
