On Mon, Aug 13, 2012 at 11:53:57AM +1000, Carl Clifford wrote:
> There is a piece in the Sydney Morning Herald's web site regarding
> a study on Common Myna and their effects on native bird species.
> The study was based partly on data collected by the Canberra
> Ornithologist's Group. Now that we know they are a pest, it would
> be nice to come up with a control measure - that"s the rub.
As Philip suggests its worth reading the paper rather than the SMH piece:
The SMH article contains errors - Crimson Rosella & Sulphur-crested
Cockatoo have increased, not decreased, in Canberra, the paper claims
that their model shows mynahs have slowed this increase.
I haven't read in detail Philip's criticisms of their use of GBS data but
even if you accept it I think this modelling tells you very little.
Their model is equivocal about mynah impact on cavity nesting birds
showing positive effects for 3 species and negative effects for 4 species.
The model outputs don't appear to be convincing evidence of impact
on cavity nesters collectively or any of the individual species.
I'm not saying such impacts are non-existant or unimportant, just
thisn't good evidence.
Their model effectively assumes mynahs are the only bird species which
can impact other bird species. This is a huge limitation.
For example the model effectively assumes the tripling of cockatoo density
over the period has had no impact on other species even though cockatoo
density is similar to mynah density and cockatoo biomass is significantly
larger than mynah biomass.
The model does show a consistent negative impact on small birds.
While this decline may correlate with mynah introduction, causation is a
very different matter. Its easy to posit factors not included in their
model, for example Noisy Miner density.
They also find consistent positive impacts on large birds (incidentally
their categories overlap in body weight) - and again causation is hard
Interestingly Common Myna density is not used in the model, instead
they use years since arrival - which of course steadily increases through
the period. Canberra mynah density peaked just over a decade ago and
has generally declined since in the last decade. Noisy Miner density
is omitted from the paper entirely but I gather there has been a large
increase over the last 15 years.
I'm betting the variable they use as a proxy for Common Myna impact
will correlate better with Noisy Miner density than it does with Common
Myna density. In other words if I'm right you could take the paper
and cross out Common Myna and write in Noisy Miner. Given Noisy Miner
impact has a high profile its a puzzle how the referees didn't insist
this was addressed.
Incidentally its easy to over-interpret models. For example, as I read
their outputs, Grey Fantails & King-parrots benefit from increases in
"modified-urban grassland" but Starling don't benefit - maybe I'm missing
something about veg categories but none of the 3 effects seems plausible.
Its also easy to critcise models - all models are wrong some are useful,
but unfortunately this is not one of them.