Effect of fog on the arrival time of little penguins Eudyptula minor: a clue for visual orientation?

We report a dramatic change in the arrival time of little penguins Eudyptula minor at their breeding colony when a heavy low-level fog was present on the southern beaches of Phillip Island, Australia. On the evening of 21 December 2002, only 5% of penguins arrived at the expected time, and the peak time at arrival that night was 2 h behind the peak arrival times of previous and subsequent days. It appears that little penguins were unable or unwilling to return to the colony under the low visibility caused by the fog. Only half (52%) of the average number of penguins returned to the colony on the evening of the fog. We propose two explanations for our observation. First, penguins could have been close to the colony but did not come ashore in conditions of poor visibility to avoid predation. Secondly, the extensive fog may have affected the orientation of the penguins and they were unable to find their way back home. Our results could serve as indirect evidence for both hypotheses.