Hi Thomas, I'm part of a large collaboration of European researchers who are now studying these birds.  A recent paper (see below) found ring-necked parakeets impact on the foraging of native birds.  Basically, as they are larger birds, they dominate birds feeders.  

Peck, H. L., Pringle, H. E., Marshall, H. H., Owens, I. P., & Lord, A. M. (2014). Experimental evidence of impacts of an invasive parakeet on foraging behavior of native birds. Behavioral Ecology, 25(3), 582-590.

Thank you Hazel. I shall give the paper a read.

Dear Thomas, greetings.

For what it's worth, I have no empirical data on these aliens but I can share some of my observations in the field.

Assuming you are aware of the history of the introduction and subsequent re introductions by unscrupulous aviary enthusiasts,I will press on.                                    I have been watching a fairly steady and inexorable advance southwards from Herts to London and Sussex via Surrey of Psittacula krameri and other Parakeet species. That now number in the thousands

There may well be a correlation with milder winters and a greater diversity of introduced alien plants from the countries of origin of these animals, which as it happens makes these birds feel right at home.

It goes without saying that the Parakeets are only one more nail in the coffin in the general decline of native song birds in some parts of this country.                                                                                                                                                                                      As yet there are no natural predators of adults but some predation of eggs and fledglings must occur. I have seen Sparrow Hawks (Accipiter nisus) attempting to give chase and Corvids mobbing individual parakeets but generally large flocks of fifty to a a hundred or more is more than adequate defense.

Domestic cats, the Grey Squirrel (Scurius caroliensis) and pesticides in suburban areas seem to be the only threat but there is little effect in number reduction.

My field observations have not included many bird feeding stations, however, the aggressive and territorial behaviour as well as flocking habits, together with a catholic taste in food and a misplaced enthusiasm by Metropolitans to harbour all birds, usually insures that a visit to a garden, deprives resident birds of a much needed food resource particularly in the Winter months. There are some areas, like Richmond Park for example where these birds seem to be more noticeable by virtue of the dearth of bird diversity, other than Corvids.

 In the field, competition for food and nest sites is fierce and more noticeable, as a large flock of these birds can strip hedgerow plants and trees of fruit,nuts,flowers or buds in minutes and a whole area in days, as they are not specialist feeders and wide ranging in habit, time and distance are less of an issue.

It is unfortunate that the rarer hole nesting native birds are in decline, partly from the felling or tidying up of veteran or derelict and damaged tree but also competition for suitable nest sites by alien species of birds and other animals, not least of which in some areas are Mandarin Ducks and these blessed parrots.

The spread of this species is also being aided by a change in choice of agricultural crops, that hitherto had not been possible to grow in our climate and advances in plant breeding.                                                                                                                           The expansion of countryside devoted to game bird husbandry, where a lot of land is being planted up with game crops for cover and seed production. Particularly, Maize, Chicory, Brassicas, Sorghum, Millets, Legumes,Artichokes, Sunflowers,  Quinoa and wild bird conservation mixes to name but a Parrot friendly few.

Psittacula krameri is no longer on the protected species list and is now considered a pest and fair game and in rural areas, where land owners and managers are less squeamish, they are now being shot and considered by some to be a good sporting bird. The Parakeets' habit of flocking and communal roosting and roost faithfulness makes them vulnerable to game shooters, trapping and crop protection interests. This alien is good on a Kebab stick or in a pot and sought after by the Fly Tying fraternity.

A succession of hard winters,whilst not recognizing friend or foe might set back their spread, however such events may prove unlikely at this time.

Anecdotal information is no substitute for good quality empirical data but I hope this may be of some interest. 

Kind regards, Jim.

Thank you for your response Jim.

In Bakersfield, California, USA, where I casually study a growing flock now numbering in the low thousands, I have noticed competition for nest cavities. In one instance, I witnessed a Rose-ringed Parakeet decapitate a European Starling (a highly invasive species in the United States) when the starling attempted to occupy the parakeet cavity. In the suburban setting, the parakeet has the privilege of human altered habitats where it finds a year round food supply. But it does compete with Acorn Woodpeckers and American Kestrel for nesting strata. I have not watched them closely enough to witness food resource competition.

Alison's observation clearly demonstrates a negative fitness effect for the starling (i.e., death and no future reproduction). Hazel, is it your sense that the dominance by these parakeets over native birds at feeders actually results in a decrease in fitness for the native birds? Or in other words, are the native birds able to get plenty to eat at the feeders when the parakeets aren't there or at non-feeder locations such that the parakeets are just a nuisance without causing any real fitness consequences for the native birds?