Marine animals equipped with data recording devices sometimes show changes in their foraging behaviour, which can indicate an adverse effect of the instrument. We attached time-depth recorders (TDRs) and radio-transmitters to study foraging behaviour in Humboldt Penguins Spheniscus humboldti and observed an increase in the duration of foraging trips. We suggest that further tagging of this endangered species be approached with caution since this increase in trip duration may negatively affect reproductive and foraging success.