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Do
Little Penguins respond to the Lunar Cycle?
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Beth
Hawkins and Philip Wallis
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© Earthcare
St Kilda
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| Introduction |
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. |
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| Methods |
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. |
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| Results |
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). |
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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). |
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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). |
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| 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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| References |
Andreadis, P. (1997).
"A lunar rhythm in the foraging activity of Northern water snakes."
In Scott, A., Hamilton, E., Chester, E. and White, D. (Eds) Proceedings
of the Seventh Symposium on the natural history of lower Tennessee and Cumberland
River valleys.
Cannell, B. and Cullen, J.
(1998). "The foraging behaviour of Little Penguins Eudyptula minor
at different light levels." Ibis 140: 467-471.
Collins, M., Cullen, J. and
Dann, P. (1999). "Seasonal and annual foraging movements of little
penguins from Phillip Island, Victoria." Wildlife Research 26: 705-721.
Klair, N. and Polacheck, T.
(1998). "The influence of environmental factors and mitigation measures
on by-catch rates of seabirds by Japanese longline fishing vessels in
the Australian region." Emu 98: 305-316.
Reilly, P. (1994). Penguins
of the World. Oxford University Press, Melbourne.
Stahel, C. and Gales, R. (1987).
Little Penguin: Fairy Penguins in Australia. NSW University Press, Kensington.
Weavers, B. (1992). "Seasonal
foraging ranges and travels at sea of little penguins Eudyptula minor,
determined by radiotracking." Emu 91: 302-317.
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Contact
Earthcare Ph: Australia 0500 832 784 Postal: PO Box 287 Elwood, 3184
Email
earthcarestkilda@gmail.com
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