Myna inconvenience

The Common Myna (Acridotheres tristis) is hated so much that on ABC’s Wildwatch it was voted Australia’s number one pest or problem, above cane toads and rabbits. So much, that even when research shows it might not be worth trying to eradicate, newspapers report the exact opposite. In fact, their only redeeming feature seems to as an easy source of headline puns.

Common Mynas are also known as Indian Mynas, and as the name suggests they originally came from India. But, perhaps because they thrive in human habitats, they’ve since spread throughout the world, and are found on every continent except Antarctica.

So they’re an invasive species, but how damaging are they really? Well, it’s actually hard to tell. They lay their eggs in tree or wall cavities, and compete with native species that do the same. And they mate for life, forming a formidable pair that aggressively defends a 1-3 hectare territory from other birds.

This tendency to attack smaller birds is probably one of the reasons people hate them. Although, I should point out that many of these incidents are mistaken identification of the even more aggressive native species, the Noisy Miner (Manorina melanocephala). They both look fairly similar, with black heads and yellow beaks and eyeliner.

The big difference, apart from the spelling, is that Common Mynas are mostly brown and the native Noisy Miners are grey. And instead of attacking in pairs, Noisy Miners tend to gang up on other birds in larger numbers.

Comparison photos of a Common Myna, Noisy Miner and Bell Miner (click to embiggen)
From left to right, Common Myna (Acridotheres tristis), Noisy Miner (Manorina melanocephala) and Bell Miner (Manorina melanophrys). Photos by Dick Daniels, Quartl and Brett Donald respectively, via Wikimedia Commons

But whether they’re mynas or miners, the impact of competition is relatively difficult to measure compared to something like predation. If one species is actively killing another, it’s pretty easy to see the effect just by counting the victims.

But competition is more subtle. So what Kate Grarock and colleagues from the Australian National University and the University of Canberra have done is to use data from a birdwatcher club, the Canberra Ornithologists Group, to track how populations of various species across Canberra were changing after the arrival of mynas (Grarock K, Tidemann CR, Wood J & Lindenmayer DB 2012, “Is it benign or is it a pariah? Empirical evidence for the impact of the Common Myna (Acridotheres tristis) on Australian birds”, PLOS One, vol. 7, no. 7: e40622, doi:10.1371/journal.pone.0040622).

They tracked bird populations according to the number of years since mynas arrived – they were first introduced to Melbourne in 1862 to control insects in market gardens, but didn’t reach Canberra until 1968 – together with a lot of sophisticated statistical analysis that corrected for factors like urban development and type of vegetation.

The bird species they analysed were split into three groups: there were other cavity-nesters, such as cockatoos, parrots and kookaburras; then there were birds smaller than about 25 cm, like Willy Wagtails and Magpie-Larks, which you might expect to be intimidated by Common Mynas; and finally there were large birds about 30 cm or bigger, like magpies and currawongs, that should be harder to push around and so are sort of a control group.

What they found was a definite negative correlation between myna numbers and three of the cavity-nesting species: the Sulphur-crested Cockatoo, the Crimson Rosella and the Laughing Kookaburra. And seven of the small bird species were affected as well.

As expected, there was no correlation with large species numbers. But interestingly, there was also no significant effect on four other cavity-nesting species, namely the Galah, Australian King-Parrot, Eastern Rosella and Common Starling. And one of the small bird species seemed to be OK, although that was
only the House Sparrow.

But an important point is that the majority of the species actually increased in number over the 29-year study period. It’s just that their increase seemed to be at a slower rate after the arrival of mynas than it would otherwise have been.

There are two interesting aspects to this. The first is the point made earlier, that the level of hatred directed towards mynas in the community has led to some slightly inaccurate, hysteria-inducing science reporting.

The Age newspaper on 13 August 2012 reported on the Canberra study but got the numbers wrong. For instance, Crimson Rosellas increased in number by 5.9 birds per km2 every year. But The Age mistakenly reported that rate of increase as the total population density. So because the rate of increase was slower in the presence of mynas – 3.5 birds per km2 per year slower – they interpreted that as a decline in overall numbers to 2.4 birds per km2. Which of course it wasn’t (actual Crimson Rosella numbers were around 50-150 birds per km2).

These numbers may sound tricky, but they’re easy to verify as the entire research paper is available for free online (look  under ‘Results’). However, that didn’t prevent the publishing of another article with exactly the same mistake in The Age on 23 September 2012.

This time, they even quoted Kate Grarock, saying ”I think it’s great that the community is involved with environmental management. However I do fear that the passion for hating the myna is way too extreme. Australians appear to be more worried about mynas than cane toads, foxes, feral cats and rabbits.”

Unfortunately, the newspaper had already decided its conclusion that Common Mynas are a major pest that’s reducing species abundance. Which of course is the story that everyone expected to read, even though the actual research found an increase in the abundance of most of the species.

Which brings me to my other point, which is that the biggest factor affecting species numbers was change in habitat. Many of these birds, including both Common Mynas and Noisy Miners, do best in urban or lightly-forested areas, and are not found in dense forests.

The researchers concluded that attempting to eradicate Common Mynas would be very difficult and probably nowhere near as cost-effective as improving habitat to encourage native species.

It seems though that you won’t read that in The Age. Instead, you can read Kate Grarock’s own commentary on The Conversation website, http://theconversation.edu.au/we-love-to-hate-the-common-myna-but-what-should-we-do-about-it-8530

Advertisements

One thought on “Myna inconvenience

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s