modelling of Common Myna impact in Canberra

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Subject: modelling of Common Myna impact in Canberra
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Date: Mon, 20 Aug 2012 08:01:49 +1000
Hi All,

It's interesting where I live, hearing the reaction to the SMH article, and the 
PLoS One paper. I live on Magnetic Island off Townsville, where there are 
usually no mynas. Right now, there are at least 6 that have made it over from 
the mainland, presumably accompanying the ferries, since the ferry terminal is 
where the birds are stationed. The overwhelming reaction among my good friends, 
who are typically conservation minded, has been that the SMH article 
(misconstrued as it is compared to the less clear conclusions of the original 
publication) is justification for what they've always "known" about mynas, i.e. 
they are a serious pest and culling should occur. I agree, we should try to 
cull them on the island, provided it's not at the expense of some other 
conservation outcome. However, my experience of mynas in northern Australia is 
very different to "rats of the sky", "cane toads of the air" type sentiments 
sometimes applied. I (and my immediate colleagues) undertake systematic surveys 
of fauna (and flora) across north Queensland and have been involved in over 
1600 permanent fauna (sometimes just birds) monitoring sites (1 ha each), many 
visited repeatedly. We conduct 8 bird counts per hectare site. Our surveys 
typically avoid anthropogenic influence as much as possible. We obtain 
somewhere between 10,000 and 50,000 bird records annually (not necessarily that 
many individual birds of course), and we have counted exactly zero mynas in our 
travels. And they've been in north Qld a loooong time.

I seriously doubt that they represent a significant threat to any Australian 
bird species, and where a threat may occur, it is in an urban, periurban or 
agricultural setting, where the mynas are riding the habitat modification. Try 
finding a myna away from serious anthropogenic habitat alteration in 
mid-eastern to northern Australia. From my (significantly less) experience 
around Canberra and further south, it's the same there - except the challenge 
is turned on its head - i.e. try finding an area away from serious 
anthropogenic habitat alteration.

I think mynas are a lesson in human psychology - many of us just don't like 
them. I think that's ok as long as it's acknowledged for what it is and not 
taken to be part of an important conservation outcome.

I think noisy miners (and yt miners) may be a problem to other native birds in 
some circumstances. They are not especially common in our surveys, but we do 
record them from time to time in seemingly intact woodlands and forest. Those 
sites are invariably depauperate in a select set of woodland birds. Correlation 
or causation? I think causation, but I'm not sure.

Eric Vanderduys

-----Original Message-----
 On Behalf Of Andrew Taylor
Sent: Sunday, 19 August 2012 2:33 PM
Subject: modelling of Common Myna impact in Canberra

A recently published paper [1] claimed to have established that Common Mynas 
had harmful effects on Canberra populations of 11 bird species - the first time 
this has been demonstrated.

I posted some quick comments on birding-aus but as these claims have been 
widely publicized [2] and they'll be used  to justify myna control programs, 
I've written a more detailed explanation of why I think the paper's claims are 
unsupported and the paper provides no useful evidence of myna impacts.

The paper [1] describes a model built on  COG Garden Survey Data.
The authors divide Canberra into 4 regions and convert the survey results into 
biannual population estimates for 20 species of birds over the 29 years the 
survey has run.

The external model variables are essentially population, dwellings, extent of 5 
vegetation types, mynas and year.

The model attempts to explain the changes in 20 bird populations over
1981-2009 in the 4 regions using these model variables.

The demographics of Canberra myna populations, as estimated in the paper vary 
between regions but overall there is a peak 5-15 years ago followed by a 
decline.  In two regions the decline is recent and in 2009 myna populations are 
still near 50% of their peak.  In the other two regions the peak is earlier and 
by 2009 myna populations are below 20% of their peak.

The effects mynas are suspected to cause are largely density-dependent 
(proportional to the number of mynas) so the obvious model input to estimate 
myna impact is myna density.  The model instead uses a proxy variable, years 
since myna establishment.  This yields a variable which linearly increases from 
1991, 1993, 1981 & 1989 in the respective regions.

The paper doesn't compare this variable to myna density but its clear there 
will be a large disparity.  For example from eyeballing the graph, the region 3 
1986 myna density looks to be about double the 2009 myna density, but the proxy 
variable will be 5 in 1986 and 28 in 2009.
In other words 2009 myna density is about half 1986 but the modelled myna 
impact is a factor of over 5 times larger.

The justification in the paper for this  choice of variable makes no numerical 
or ecological sense to me.  This variable choice is crucial because all the 
claims in the paper are based solely on attributing changes in the COG data to 
this myna variable.

This reduction of changes in myna density to a simple linear increase raises 
concern about correlations with variables not included in the model. Its not 
uncommon for variables to show roughly linear increase or decrease for a period 
of time. For example there conceivably might be a bias in the COG data related 
to observer recruitment, and if it increased or decreased roughly linearly and 
roughly over the same time period as myna establishment, part of its effects 
might erroneously be attributed to mynas.

Its important to consider what variables are included in the model.

One obvious omission is rainfall.  Its  easy to obtain. It has been linked to 
bird abundance and it shows large variation through the time period.
For example, on a quick look at BOM data for the the Botanic Gardens, Sep-Nov 
rainfall appears to vary by a factor of 6 between 1981-2009.
Rainfall may be less important in a suburban environment its still seems some 
of the variation in some species will be explained by rainfall.

Another omission is the effects of birds other than mynas. For example, Pied 
Currawong, Red Wattlebirds and Noisy Miners have all been linked to negative 
effects on other bird species in suburban environments, but the model excludes 
this possibility.  Particularly problematic is the omission of Noisy Miners 
because their Canberra population is roughly constant until 2000 then shows a 
large roughly linear increase [3].

As Noisy Miners aren't included the model its impossible to exclude their 
effects being erroneously attributed to mynas.  There is good reason to suspect 
there might be such effects because a number of researchers have noted such 
effects elsewhere and there has been such a large increase in Canberra's Noisy 
Miner populations.  There is also good reason to suspect such effects could be 
erroneously attributed to mynas in the model because the growth in  Noisy Miner 
population follows a pattern that correlates with the mynas proxy variable.

In other words the concern is if that if the authors had taken the same data 
and same approach to modelling Noisy Miner impact they would have found similar 
impacts but attributed them to Noisy Miners.

There are other species whose omission as model variables is problematic. For 
example, cockatoo numbers triple through the time period.
Their density and the variation in their density is similar in magnitude to 
mynas.  While the authors suggest mynas affect other cavity nesters by 
competing for nest sites, their model assumes cockatoos do not affect other 
cavity nesters by competing for nest sites.  This asymmetry is hard to justify.

So, for example, it is impossible to exclude the negative impact on Galahs the 
model attributes to mynas being actually caused by cockatoos.

Its suspected that mynas have a negative impact on cavity nesters.
However the model's outputs for cavity nesters are equivocal: 4 species 
supposedly negatively affected and 3 positively affected.

3 of the 4 species supposedly negatively affected species populations grew 
through the time period, for example cockatoo numbers tripled, but the model 
output indicate mynas slowed this growth.

The only cavity nester which showed a large decline, starling, supposedly 
benefited from mynas - an unexpected result. The authors suggest this large 
decrease in starling numbers is hiding the negative effect from mynas.  This is 
plausible. The model attributes the majority of this decline to the linear 
pseudo-variable year. If this large  unexplained effect is non-linear then it 
could  mask a negative effect from starlings.

Eyeballing the starling & myna density graphs raises the suspicion that if the 
model had used myna density as a variable they might instead have found the 
expected negative effect.

This problematic presence of large unexplained effects also occurs for other 
species. For example, a large part of the increase in cockatoos is unexplained 
by the model, and is also attributed to the linear pseudo-variable year.  If 
this large effectively unexplained effect is non-linear then part of it could 
be being erroneously attributed to the myna variable.

Note when the model produced an unexpected positive impact from mynas
(starlings) the model output was rejected, but when the  model produced an 
unexpected negative impact (cockatoos) the model output was accepted, and  
conveyed via press release to Australia's major media outlets.

Another concern is the extent to which the model is opaque.  The PLOS paper 
presents only a table of coefficients.  We aren't given the values of the model 
variables, e.g. as graphs, to which these coefficients apply.
The authors discuss only the signs of the coefficients and only for the myna 
variable.  Some of the coefficients look to have implausible values but the 
information needed to interpret them is absent.

It easy to criticise models - all models are wrong, some are useful - and even 
given the lovely 29 years of COG data its no small task to tease apart 
competing effects - but this model fails to do so and fails to support the 
claims made.

Personally I think is likely given the high density they reached mynas did 
affect at least a few Canberra bird species, but this modelling leaves me none 
the wiser as to which species or the magnitude of these effects.

On the other hand 2 recent observational papers [4,5] (both paywalled) on 
Sydney & Newcastle mynas I found while writing this did suggest to me I (and 
others) might be over-estimating myna impacts.


[3] Canberra Bird Notes 37 (2) June 2012 Noisy Miners in the COG Garden Bird 
Survey Martin Butterfield [4] [5]

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