Monday, 27 July 2015

Some comments on two invasive species

When we lived in Dar es Salaam (2001-03) there was considerable concern about the spread of Indian House Crows, and the impact on indigenous species.  Similar concerns have been expressed about the Common Myna in Canberra, and as I have better data about that species, I will start off with some comments on them.

Common Myna Acridotheres tristis

This species is native to Asia.  The species has an alternative name of Indian Myna which is not that sensible as there are 8 species of myna (in various genera) found in the sub-Continent.  It also has a popular name of Rat-with-wings, which is possibly rude to rats.

The beast in question was introduced to Australia in the 1860s.   For reasons that escape me they were popular cage birds, which had the almost inevitable outcome that they escaped and (less usually) became established in the wild.  They are widely regarded as pests, out-competing native species, as noted by my friend Denis in his comment below especially through competing for nest hollows.  Interestingly it has been reported that in Singapore it is itself being out-competed by the White-vented Myna (A. cinereus).

In the ACT the mynas were deliberately introduced by a misguided individual.  The tale as I have heard it is that:
the person was recovering from an operation in a Sydney Hospital and the fiirst sound he heard was a Myna walking on the window ledge outside his room. To him this represented a return to life and he wished to share this with others in Canberra. So some birds were trapped and then released near the Kingston shops.
When we came to Canberra in 1983 Kingston was still the epicentre of the infection with a sub-population in Scullin.  Shortly after this they exploded in numbers and became common throughout the ACT.  It is very difficult to estimate the number of Common Mynas in Canberra: a Minister did refer, in a Press Release, to an estimate of up to 150,000 at peak and I did some analysis which concluded this was a possible number, although my inclination would be to take a somewhat smaller one - 100,000 might be better.

CIMAG was established in 2006 to take action on the Myna, (I presume the obsolete name is used as it gives the group a more easily pronounced acronym.)  Their core business is assisting with trapping and euthanasing the birds.  The explosion of the population of Mynas in the ACT and the success of CIMAG in reducing their numbers is illustrated by this chart of the abundance (average number of birds per site-week in the COG Garden Bird Survey) of the Mynas in the ACT (the star represents the year of foundation of CIMAG).
The uptick in the year ended 2/7/2014 is not a great surprise.  Some years ago an experienced birder commented to the effect that the Myna will never be eradicated from the ACT unless "something is done about the population at the Queanbeyan racecourse."  Whether the up-tick represents a re-invigoration of the species will be decided in the future, but mechanistic projections of the trend line give very variable results depending upon the length of the data series input to the projection.

House Crow (Corvus splendens)

Image from Avibase sourced to
I am surprised that I didn't have an image of this species from our time in Dar es Salaam as they were very common there.  They are sometimes described as a 'jackdaw' since they resemble the mainly European species Eurasian Jackdaw- Corvus monedula.  As an aside I did report seeing a Jackdaw in Dubai, in 2005, which would have been a first for the country - but I rapidly changed the observation to House Crow, explaining that I had recently been in Europe and had forgotten the House Crows of Dar es Salaam.  

The principal concerns about House Crows in Tanzania were that they predate the nests of smaller native species and were out-competing the native Pied Crow C. albus.

An attempt had been made to control the species by shotgunning the nests when there were young or eggs in the nests.  This took out two generations in one blast.  After some 20,000 cartridges the programme ran out of funds and the House Crow population boomed again.

This post was stimulated by reading a mention of House Crows in a post by Ian Fraser about Borneo which led me, via Uncle Google to an article in the Tanzanian Daily News about the demise of another control program.  This one had involved poisoning the Crows with DRC 1339 and other techniques including trapping and stoning the crows!  The article estimates that 900,000 Crows were killed in this programme.  This seems a huge number of birds, but is broadly consistent with an estimate of 700,000 Crows killed a couple of years earlier and an estimate of 1.5 million Crows in Dar es Salaam in 2010, before the programme started.

The article also states that the Crows were originally introduced to Zanzibar, then part of the Sultanate of Oman (not far from Dubai!) to control rubbish.

Unfortunately it seems that at the time the article was written funding had run out again and the crows were rapidly rebuilding their population.

A couple of conclusions

In both cases the birds were deliberately introduced by man.  Naughty, naughty, man.

In the case of the House Crow man, in the form of the Tanzanian Government, appears to have dropped the ball and allowed what seems to have been a very successful programme to finish too soon.  This should be a big message for the folk of Canberra to sustain their support for CIMAG and not to say that because the Mynas have dropped from 3rd most common species in the GBS to the 20th that the war has been won.


Ian Fraser said...

I totally concur - thanks for a thoughtful and timely article that should be more widely read. I wasn't able to ask any locals about their perspective on the House Crow mob in KK, but my impression is that the crows have been there for a while and are still in very low numbers, oddly. However we saw how the mynas here took some time before the population exploded, a classic model of feral population establishment and growth. I'll be working with a local birder in Sabah next year, and will ask him.

Denis Wilson said...

I agree wth your comments re the Common. Myna. Definitely a pest species in Australia.
I would add a note that they aggressively compete for nest hollows, which many native birds
use for their nests.

Flabmeister said...

Thanks for comments Ian and Denis.

To try to enlarge the readership I have noted the post to the COG Chatline and I understand it is to be mentioned in the next edition of Myna Matters.

I have also referred to the issue of competition for hollows on the main post. WRT that I recall with much amusement seeing a pair of mynas, punching well above their weight, and themselves getting ejected from a hollow by the Sulphur-crested Cockatoo that was also interested in it.