In his recent discussion of the Grarock et al. Myna paper Philip Veerman states “It is abundantly obvious that the Common Starling decreased as the Common Myna increased, that is described in the GBS Report as the most likely relevant interaction to any other species by the Myna.”
I wonder about the closeness of that interaction.
There is no doubt that the Myna would win in any direct confrontation with a Starling, and competition would be strongest during the breeding season for nesting sites. However, the strong decline of Starlings occurred just as much in Myna-free areas as in those with Mynas present. For example, the more than 80 pair breeding population at the ANU (from my earlier surveys) largely vanished without a Myna being seen, nor were the Starling breeding sites (mainly in buildings) occupied by other species. In 2008 I revisited the breeding birds of Ainslie, and found Mynas and Starlings sharing the same building for nesting, albeit using different entry holes to get under the roof space or in the wall cavities, but both species would often sit fairly close to each other on the roofs, and singing.
As Philip also points out in the GBS report, the Starling starts to nest earlier than the Myna (a fact that would give it some advantage). The more slender-built Starling would also be able to use hollows with a smaller entrance, Mynas could not access. Outside the breeding season, one can sometimes see both species in mixed flocks, but overall the Starling tends more frequently to visit pasture, sports fields and the like.
To me, all this points to many factors impacting on the Starling, with the Myna being one, but not necessarily the key factor.
And this brings me back to the Grarock et al. paper.
Factors other than the Myna with a negative impact on populations of other species are not properly explored (see also Geoffrey Dabb’s and Mike Braysher’s earlier comments). And it would have to be explained in which way Mynas could impact non-cavity nesting birds such as Superb Fairy-wren, Grey Fantail and Silvereye. The biology of those 3 species would largely exclude direct interaction with the Myna. Yet predation (e.g. Pied Currawong, cats etc.) may be a far bigger issue with those or the increasing footprints of buildings at the expense of green space, drought etc.
In this context, the House Sparrow is of interest. It has declined worldwide (with some significant exceptions, e.g. the city of Berlin with a stable House Sparrow population). Many factors have been identified that could explain its decline. Grarock et al. add the Myna for Canberra to that list of negative factors. Yet there is agreement in the wide literature on House Sparrow decline (none quoted by Gararock et al.) that no single factor satisfactorily explains the world-wide decline.