Friday, 23 November 2007

Orokonui Ecosanctuary (column)

The Orokonui Valley may now be free of possums. Jess, a trained possum
dog belonging to Operations Manager Elton Smith has not detected any
for the last six weeks. The fuchsia is flowering profusely this year
and without being munched by possums will have much more leaf and
nectar to sustain the birds.

It is likely that there will be a spillover effect from the
ecosanctuary as bird populations build up and some species are likely
to appear in adjacent areas. With this in mind I thought it was timely
to find out about the bird predators in our backyards. Yolanda van
Heezik has been studying one of these predators and I invited her to
come along to the November meeting.

Her research has been into the impact of cats on the bird populations
of Dunedin. Information was collected from cats wearing GPS collars
and the prey items they brought home. It was fascinating.

Statistics from various studies show that:
• The most common prey taken by cats is rodents (60-80%), followed by
birds (20-35%).
• Cats may take approximately three times more prey than what they bring home.
• Young cats (under a year old) take more prey than older cats.
• 34% of the cats from Yolanda's study brought home no prey, 45%
caught prey infrequently and 21% were frequent takers (one cat brought
home 60 prey items over the year).
• Silvereyes were the most common native bird caught.

If the number of prey caught by cats in the study is extrapolated to
the entire estimated urban population of cats, then about 50% and 20%
of the estimated fantail and bellbird populations are likely to be
killed by cats each year respectively. This is a pretty significant
chunk of these populations killed every year considering their
breeding patterns have not evolved to compensate for mammalian
predation.

On the good side though Yolanda's work estimated that 20 838 rats and
33 346 mice were also likely to be killed by cats over a year and
therefore possibly lowering the numbers of other bird predators.
Whether this is a significant number compared to actual rodent
populations is the subject of a research project for 2008.

2007 has seen a major step forward for the ecosanctuary with the
construction of the fence. The next big milestone hoped for is
pest-free status in 2008. Early signs are encouraging with the only
footprints on the monitoring tunnel ink pads so far being those of a
skink.

For further information on the ecosanctuary see our box at the
Blueskin Bay Library or visit www.orokonui.org.nz. Merry Christmas!

by Sue Hensley


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Warrington, Evansdale, Waitati, Doctors Point), Dunedin, New Zealand.
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