About the Myna in the GBS results

["Philip Veerman" <>]

Following on the talk by Bill last night and Martin's comments below. The line in bold (here) of my text from the GBS Report sort of refers to a possible bias of observers and what Martin picks up on Bill's comments. That suggests something related that when there are a lot of mynas, that people don't bother doing the GBS. Of course people should record introduced birds as well as natives on their charts. I'm sure most are (as I have described below, on page 47).

The GBS graph shown by Bill indicated the initial increasing rate of population increase, this has recently changed to a declining rate of increase (in other words the rate of increase is declining) of the Common Myna population. This leads to a levelling off of the population, at a high abundance level that had not occurred previously. This kind of (sigmoid) pattern is quite normal for invasive species. Indeed it is almost inevitable. It would be nonsensical to assume that the increase would continue at the same rate indefinitely. Although it is great that we are doing something to mitigate against it.

In terms of GBS methods, I would make the comment that it is not well supported to suggest that the steadying of the population is because people are not recording the species now, UNLESS it can be reasonably shown that people are recording the species less now than they did before, for reasons associated with people's recording bias and not because there are fewer (or not more) birds. Such assumptions are hard to prove and risky in interpretation. The data is the data and should not be worked around. Any explanations of biases need to be considered as a separate exercise and if there is strong grounds for believing the biases, then that is fine. However the great thing about the GBS is that mostly the data were collected in ignorance of what it should show, or without any expectations as to what the results would show. Therefore the data mostly are not suffering the impacts of intentional recording or non recording biases.

Here are some extracts from the GBS report about the Myna:  

Page 17

Year 1 of the GBS was not the start of bird populations in Canberra and Year 21 was not the end. Many species show trends in abundance in Canberra and the end point of these trends has not yet been reached. Of the species increasing or decreasing, when will these trends even off? An obvious question relates to the increase in the Common Myna population and decrease in the Common Starling population. How long will that continue and will the two species reach equilibrium?

Page 47

There are no extensive areas of absence of native vegetation and most GBS sites comprise Australian and exotic vegetation. Pryor & Banks (2001) have described the vegetation. All 10 exotic bird species with ACT populations (Mallard, Rock Dove, Spotted Turtle-Dove, Skylark, House Sparrow, European Greenfinch, European Goldfinch, Common Blackbird, Common Starling and Common Myna) were recorded in the GBS. However as a group, exotic species do not dominate the bird community. Canberra does not have any introduced species that are not also well represented in other major cities of eastern Australia. Canberra is also lacking in large feral populations of: Spotted Turtle-Dove, Eurasian Tree Sparrow, Nutmeg Mannikin, European Greenfinch, Red-whiskered Bulbul and Song Thrush that are well established in some other major cities of eastern Australia (Chapman 1969). The only dominant species, more than 5% of the bird population (Huhtula & Jarvinen 1977) in Canberra are the Common Starling, House Sparrow and recently the Common Myna. Of the rest, two more are ranked within the top 32 most often recorded species (with the next one at 113).

The 21 years results show that the Common Starling and House Sparrow have been the two most abundant species, though both are in decline. The Common Myna has shown a dramatic increase in the same time. The increase in this species is of concern because of its nest competition with other birds.

Page 49

There are only a few species where the data suggest a lack of seasonal pattern. Many species (e.g. Galah, Common Starling, Common Myna and Australian Magpie) are so omnipresent that you would not have thought there is a seasonal pattern but there is usually a very clear one. This has rarely been shown before.

It would be wrong to interpret too much cause and effect in these data, though there are suggestions of linkages. Most obvious is that the increase in the Common Myna matches a decline in the Common Starling. Both species are likely to reach a steady state in the top ten common species (unless some action is taken). Over that time, most of the larger parrots have increased drastically, even though the Common Myna has been strongly implicated as a nest competitor. The larger parrots are long-lived, so any reduction in breeding success may not show for several years. They still are prolific within the city. Of concern though, is that small parrots are not doing so well.

Page 60, Red-rumped Parrot Psephotus haematonotus

........  It tends to feed in small flocks among rough grass and fly higher than the larger parrots, making it less likely to be noticed. However, these attributes do not account for its change in status. .........The species appears to be in strong decline, so there may be a basis for concern on its long-term trends but the evidence is confusing at this stage. ...........  Likely causes may be increased competition for nesting sites by the Common Myna, which has not reduced numbers of the larger parrots and increased competition for food by the Crested Pigeon.

Page 89 (about Starling)

Available GBS and other data imply a link between the increase in the Common Myna population and the reduction in the Common Starling population. These data were not available ten years ago and Crawford (1990) (in her report about the Starling) did not mention them.

Common Myna Acridotheres tristis

This species has shown a dramatic rise in numbers, probably the biggest increase in population of any species. This has already been described (Gregory-Smith 1985, Davey 1991, Pell & Tidemann 1997b). It is not surprising that the monthly pattern is very similar to that of the Common Starling. Numbers are at their minimum in November, then rise rapidly to a peak in March then decline again from June to November. The fact that June abundance is higher than that of July (eleven months earlier), is most likely due to the steady population increase of the species. There was a 20 fold increase from Year 1 (A=0.23) to Year 21 (A=4.56). In Year 1 this bird was 37th in the list of abundance, it ranked fourth in Year 21. So for the first time it was in front of the Starling. This appears likely to be the major impact of the Common Myna on other species. The increase was initially quite slow and even, until the species became well established. It became dramatic from Year 6 onwards. The values of R and F (not shown here) have increased steadily. There is a geographical aspect to this spread. There were more records for the first nine years in southern Canberra (1.75% of all records in this zone were of this species), then central (0.9%), then northern suburbs (0.2%). This trend has reduced during the last 12 years. There is also an unfortunate bias operating, as has occurred in the past when observers failed to properly count Common Starlings, some observers are now failing to count this species. Undoubtedly both are under counted even though counts of this species of 30 or thereabouts are common. The Common Myna can have such a domineering manner that other species become harder to find and some observers even pull out of the GBS because they say, (stretching the truth) they "only get mynas now".

Breeding activity occurs later than with the Common Starling. Inspecting hollows and nest building commences from August, and peaks in September. Nest with eggs or young peaks in November and December and are finished by March. Most dependent young are noted from November to February and up to the end of March. The percentage breeding of records of the species has fluctuated over the first several years but has been much higher over the last four years than in any preceding year. However the percentage breeding of breeding charts and percentage breeding of all charts has increased markedly. This was from no breeding records in Year 1, to recorded breeding on 37% of breeding charts in Year 19.
Graphs on page: 105, Rank: 27, Breeding Rank: 13, Breeding graph on page: 107, A = 2.59934, F = 65.07%, W = 51.9, R = 43.067%, G = 6.04.