Hi John,
It's possible that it could be due to the establishment of vineyards in the
Lower South-west.
Under natural situations, Silvereyes in the Marri forests feed on insects in
winter, whereas in summer they are principally nectarivores, feeding
principally on the nectar of the Marri flower. With the establishment of
vineyards near the Marri forests, Silvereyes descend there in their
thousands at the end of the summer to feed on the ripening grapes. The
Silvereyes don't enter the vineyards every year to consume the grapes, but
once every 4-5 years, when the marri trees have few or no flowers.
Consequently, banding studies have shown that Silvereyes in these areas have
longer life-spans, surviving up to 15 years, because there is no shortage of
food.
So there may be two reasons for the overgrown bills:
1. Silvereyes consuming grapes at the end of summer are less dependent on
feeding on insects. I'm guessing that foraging and feeding on insects causes
more abrasion of the bill than does the consumption of grapes. Therefore,
individual Silvereyes that are more dependent on feeding on insects are less
likely to have overgrown bills.
2. They are older and/or better nourished birds, as a result of feeding on
the grapes, providing better resources for bill growth.
There are other possibilities of course: genetic mutations (either natural
or induced by pesticides), disease, or malnourishment as a result of a
deficiency in their diets.
Dr Ian Rooke (WA Dept of Agriculture) and Professor Don Bradshaw (University
of WA) co-ordinated the studies of the Silvereyes around Margaret River in
the 1980s, which resulted in the publication of several papers on their
ecophysiology in the Australian Journal of Zoology. A comprehensive summary
and synthesis of those studies is presented in Bradshaw (2003).
Hope this information helps.
Regards,
Stephen Ambrose
Reference
Bradshaw, S.D. (2003). Vertebrate Ecophysiology: An Introduction to its
Principles and Application (Cambridge University Press, Cambridge).
-----Original Message-----
From:
On Behalf Of John Graff
Sent: Thursday, 7 October 2010 2:40 PM
To: Birding-Aus
Subject: RFI: Silvereyes with overgrown bills
Hi everyone,
I'm posting this on behalf of Rose Ferrell and the Lower South West Bird
Group. See forwarded message below - does anyone have any information about
what could cause this? I can provide a photo if anyone is interested
Location is south-west WA
Thanks in advance,
John Graff
Perth, WA
> Hi Erica, John & Robyn,
> The lower south west bird group outing last month took us to a private
> farming property. The owner has several feeders, and pointed out to us
> the silvereyes. Several had deformed beaks. At least two were long and
> decurved, and several others seemed longer than usual, but more or less
> straight.
>
> We are seeking information about this. I am hoping you may be able to help
by forwarding this to BAWA / other
> appropriate people so we can get some information about this condition.
> The landowner wants to know if he should stop feeding them rolled oats /
> other in his feeder.
>
> I would be grateful for any information you can send!
> Thanks for your help,
> Warm regards,
> Rose Ferrell
> (Lower South West Bird Group)
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