Unexpectedly, birds with a higher natural parasitaemia showed a reduced RMR, a pattern also found in immunologically naive canaries when experimentally infected with Plasmodium . As the nocturnal oxygen consumption in most birds in our study showed marked fluctuations during the migration period, it is unclear whether the lower RMR in the high-parasitaemia group was a result of hypometabolism per se or was a consequence of reduced nocturnal restlessness, a behaviour well known in migrating birds .