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claims of this paper are unsupported

Posted by andrewt on 26 Aug 2012 at 11:40 GMT

This is an expansion of the terse individual comments I've previously added to this paper.

The major error in this paper is the claim to be "able to distinguish between the impacts of the Common Myna and other causal factors through the inclusion of urban development and vegetation variables." because their modelling excludes anumber of potential casual factors, several equally as plausible as the factors they include, such as:

1. Climate - e.g spring rainfall has been linked to bird abundance in the region [1] and it shows large variation through the study time period - on quick examination Sep-Nov rainfall appears to vary by a factor of 6 between 1981-2009 at a site near the centre of the study regions.

2. Nest site competition from species other then Common Myna. For example, Sulphur-crested Cockatoo competition for nest sites has been observed elsewhere in Australia [2], their estimated density triples through the study period achieving densities similar to peak Myna density, but the modelling doesn't include Sulphur-crested Cockatoo density as a potential causal factor. Potential competitors also include non-avian species such as feral honey bees.

3. Competition for food, competition for territories and predation by species other than Common Myna. At least three species in the study region, Noisy Miner, Red Wattlebird and Pied Currawong, have been linked to impact on smaller bird species in suburban and other habitats [6,7]. The omission of Noisy Miners is a particular concern because their density in the study region was roughly constant for the first 20 years of the study period and then follows a large roughly linear increase [3].

Grarock et al. cannot preclude the above factors causing the density changes they have observed and hence cannot demonstrate that the density changes are due to the impacts of Common Mynas.

A second concern is Grarock et al.'s modelling of mynah impact as a years since Common Myna establishment. This yields a simple linearly increasing variable. As the effects posited for Common Myna's are largely density-dependent, density would appear more relevant..

In contrast to the model variable, Common Myna density declined in the last years of the study period. A community group culled approximately 25,000 Common Mynas from the study regions & surrounds during the last 4 years of the study period [4] which may be a significant factor in these declines. Grarock et al do not mention this removal and their modelling assumes this removal of 25,000 Common Mynas had no effect.

The modelling of mynah impact as a simple linear increase increases the concern that unmodelled casual factors contain a similar component. Noisy Miner density is of particular concern as its possible the model variable used for Common Mynas reflects Noisy Miner density more closely than Common Myna density.

A third concern is Grarock et al's claim that their modelling yields information on nest-cavity competition given they present no data on nesting and the small size of their 4 study areas compared to bird movements. One of the 7 studied cavity nesters, Australian King Parrot, nests entirely outside Grarock et al's study areas [5] in forested areas where mynas may be present only at low density or absent. Hence Grarock et al have not as they claim modelled nest-cavity competition for this species. For the other 6 cavity nesters its unclear to the degree their density within the study region is dependent on nesting within the study region and hence the extent nest site competition can be evident in Grarock et al's data.

Andrew Taylor

[1] "A Statistical Analysis of Trends in Occupancy Rates of Woodland Wirds
in the ACT December 1998 to December 2005", Jenny Bounds and Nicki Taws,
Canberra Bird Notes 32 (2) June 2007
[2] http://www.publish.csiro....
[3] "Noisy Miners in the COG Garden Bird Survey", Martin Butterfield,
Canberra Bird Notes 37 (2) June 2012,
[4] http://www.indianmynaacti...
[6] Parsons, H., Major, R. E. and French, K. (2006), Species interactions and habitat associations of birds inhabiting urban areas of Sydney, Australia. Austral Ecology, 31: 217-227
[7] K. French, R. Major, K. Hely, Use of native and exotic garden plants by suburban nectarivorous birds, Biological Conservation, Volume 121, Issue 4, February 2005, Pages 545-559

No competing interests declared.

RE: claims of this paper are unsupported

KateRock replied to andrewt on 30 Oct 2012 at 09:36 GMT

As part of the PLOS ONE review process, this manuscript passes through a rigorous peer-review evaluation process before receiving a favourable decision. A major focus of PLOS ONE publication is that experiments, statistics, and other analyses are performed to a high technical standard. As such, a prominent statistician with over 30 years experience reviewed our manuscript and recommended it for publication in PLOS ONE.

Our results are based on data from the Canberra Garden Bird Survey which is an observational study undertaken by volunteers. This means it is not possible to conclusively establish causal relations, since many aspects of the environment are changing concurrently. However we believe that by concentrating on changes in the trajectory of other species occurring at the time of Common Myna introduction we have estimated effects which can be reasonably associated with Myna introduction, and for which a plausible mechanism can be proposed.

We used the variable of years before and years after myna as we wanted to be able to undertake a ‘before and after’ comparison. Specifically we wanted to see if bird populations changed significantly after the arrival of the Common Myna.

We included rainfall and temperature in earlier analysis. During the editing process and due a requirement for a concise manuscript certain procedures were removed from the methods section. The lack of significance of rainfall may be due to the urban environment and watering of gardens and ovals, etc.

We agree that competition by other species may impact native bird populations. However, our analysis focused on bird populations before and after the specific time points of Common Myna establishment. No model will capture the complete complexities of the natural environment and we encourage further investigations into competition.

We did not incorporate Common Myna community trapping numbers in this model as it is unlikely that Common Myna community trapping is having a 'regional' impact on Common Myna abundance. Community trapping may have localised impacts but not on the scale of 'regions' that we analyse in this paper.

The species can lay 4 eggs per clutch and multiple clutches per season. We have observed clutches of up to 7 eggs. The removal of 6250 birds annually (calculated on your estimate of 25000 birds over 4 years) could easily be replaced by reproduction and reduced density dependence mortality. It would only take approximately 1600 females laying 4 eggs a year to produce 6400 young. A conservative estimate of Common Myna numbers in Canberra is 31 300 individuals. However, some estimate the population at 182,000 individuals. Assuming a 50/50 sex ratio (as the literature suggests) and that half of the females are at sexual maturity (1-2 years or older) we have 7825 individuals laying eggs each year, potentially 31,300 eggs (much higher than our earlier estimate of 1600 individuals required to replace trapped birds). Therefore, we did not include the trapping in our model.

We are currently investigating the impact of the Common Myna trapping program in more detail and preliminary results indicate that trapping has a localised effect, with reductions in abundance observed within 1 km of large cull efforts, however this reduction is not observed over the Region scale (as used in the analysis of this paper).

Competing interests declared: Author