|Do Little Penguins respond to the Lunar Cycle?
Beth Hawkins and Philip Wallis
© Earthcare St Kilda
Little penguins generally leave their burrows before dawn, forage at sea during the day, and return to the colony after night-fall (Stahel and Gales, 1987; Reilly, 1994). This diurnal foraging pattern reflects the fact that penguins rely primarily on vision to locate their prey (Cannell and Cullen, 1998). However, some little penguins remain at sea over-night, or embark on long-term trips of three or more days away from the burrow (Weavers, 1992; Collins et al., 1999). It is possible that the number of penguins remaining at sea over-night could increase under certain environmental conditions, in particular, under increased light levels associated with a full moon. If a full moon provided sufficient light, penguins might stay at sea longer to forage. It is known that some seabirds, such as albatross do increase their foraging activity during a full moon. For example, a study into environmental factors influencing by-catch rates of Japanese fishing vessels in Australian waters, found that the chance of catching seabirds was five times greater on full moon nights than new moon nights (Klair and Polacheck, 1998). It is possible that little penguins could also forage more extensively during full moon nights, however, the necessity to return to the burrow to mate, incubate eggs or feed chicks might prevent this from occurring during the breeding season. Similarly, moulting penguins must stay in their burrows (Stahel and Gales, 1987). To test the hypothesis that more penguins stay out at sea during full moon nights, data from the little penguin colony at St Kilda pier, collected between 1986 and 2003, was analysed. If more penguins remain at sea on full moon nights, there should have been less penguins recorded at the colony on those nights, taking into account differences between the breeding/moulting and non-breeding season.
Little penguin numbers have been recorded regularly (approx. twice a month) at St Kilda pier since 1986. Using this data, I determined the average number of adult penguins caught during each moon phase. Each sampling date was assigned to a particular moon phase (i.e. first quarter, full, last quarter or new), according to which it fell closest to. (Moon phase dates were sourced from http://www.timeanddate.com/calender/). Each sampling date was also classified as falling within the breeding/moulting season, or the non-breeding season, in order to determine whether breeding and/ or moulting activities affected the numbers of penguins remaining at sea. A two factor Analysis of Variance (ANOVA) was then used to determine the effect of moon phase and season (and any interaction of these factors) on penguin numbers at the colony.
The number of penguins caught at the colony on full moon nights was not significantly less than on other nights (F= 1.33, p=0.264). There was an average of 11.9 adult penguins caught on full moon nights, compared with 13.6 during first quarter, 12.8 during last quarter and 12.7 on new moon nights (Figure 1).
Figure 1: The average number of adult penguins caught at the St. Kilda pier colony during each moon phase: full (f), first quarter (fq), last quarter (lq), and new (n).
There were, however, significantly more adult penguins at the colony during the breeding/moulting season, averaging 14.3, compared to just 10.4 penguins during the non-breeding season (F=33.65, p<0.001). There was no significant interaction between moon phase and seasonal factors (F=0.04, p=0.99).
Discussion and Conclusion
Based on the St Kilda pier colony data, little penguins do not appear to respond to the lunar cycle. However it may be the case that other factors, such as relative cloud cover have obscured this response. Cloudy nights would counteract the effect of a bright moon, causing penguins to return to their burrows when they might otherwise have stayed at sea. The tendency of little penguins to undergo long-term trips of three or more days during the non-breeding season, further complicates the analysis (Weavers, 1992; Collins et al., 1999). Such penguins would be absent from the colony, regardless of whether or not they are foraging in response to the moon. However, a study by Cannel and Cullen (1998) into the foraging behaviour of Little Penguins at different light levels, suggests that there would be insufficient light for penguins to forage at night, even during a full moon. This study found that little penguins require a minimum level of light between 0.6-6.1 lux to locate prey, while light intensity on a cloudless night with a full moon is only approximately 0.12-0.22 lux (Andreadis, 1997; Cannell and Cullen, 1998). If there is insufficient light to locate prey during a full moon, there is unlikely to be any extra incentive for little penguins to remain at sea on those nights.
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