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Fig. 2. Standardised BMR residuals in relation to the time when the BMR value was
obtained for individual snow petrels (N=65). Time is expressed in h
(0:00–24:00) and rad (0
–2
). The regression line represents
a significant effect of the term cosine (time of BMR) on BMR (P=0.05)
and indicates a cyclic diurnal rhythm in BMR with the lowest predicted values
at 12:00 h and the highest predicted values at 00:00 h. `Night' and `day'
measurements refer to BMR measurements that were started during the night (at
22:10 h) and the day (at 11:30 h), respectively. The BMR residuals were
obtained from the model in Table
1 (BMR controlled for body mass and body condition). Standardised
residuals are divided by an estimate of their standard deviation (s.d.), and
have mean=0 and s.d.=1.