Are parrots competing with native bird species for essential resources or are they living in unexploited ecological spaces?

Adult male rose-ringed parakeet (Psittacula krameri) resting on a tree branch. (Credit: Siegfried Poepperl, via Pexels)

Siegfried Poepperl via a Creative Commons license

Some of you may have read my recent piece where I shared a study that reports that native trees better support urban native birds than exotic tree species do (read it here). That study may have made you wonder whether introduced exotic species are always a threat to native biodiversity?

No, they’re not. According to a recent study, when “empty” ecological niches exist that are unexploited by native local species, exotic species can colonize and exploit those niches without directly competing with native species. This revelation came from studying introduced parakeets living in Italian cities and finding that their success is not the result of chance nor is it through direct competition with native avifauna, such as sparrows, blackbirds or robins. On the contrary, the success of introduced parrots is the result of their ability to occupy and exploit an empty niche space.

To most peoples’ eyes, all parrots are the same, but in fact, parrots are like all other animals: each species has their own special traits and abilities. For example, the Rose-ringed (or Ring-Necked) Parakeet, Psittacula krameri, has a large, disjunct native range in tropical northern Africa and the Indian subcontinent. This slender parrot nests in natural tree cavities and thus, may compete with woodpeckers for nestholes — sometimes the very cavities that woodpeckers are in the process of constructing.

There also is the Monk Parakeet, Myiopsitta monachus, which has a large range across South America. The Monk Parakeet, which lives in family groups, is an expert at constructing nests out of sticks and twigs, which it gathers and weaves into an apartment that houses multiple parrot families. As these structures grow, they attain considerable size and can collapse entire trees under their weight.

These two parakeet species were introduced into Italy’s capital, where they have lived for more than 30 years. Their invasion started slowly, but then their populations suddenly exploded, so that today there are thousands of individuals thriving in these urban settings, which offer exotic trees and ideal nesting opportunities.

“Introduced parrots are among the most visible, beautiful, and controversial non-native birds in Europe. They are conspicuous, charismatic, noisy, and very easy for people to notice, so they often trigger strong opinions. Scientifically, that makes them especially interesting: many people assume they must be strongly competing with native species, but that assumption is not always tested directly,” explained the study’s lead author, conservation ecologist Fabio Marcolin, a Research Fellow in Mountain Ecology at the Università di Torino.

There are two opposing hypotheses that aim to explain how introduced species can successfully establish populations outside their native range. The first hypothesis proposes that non-native species are closely related to local native species and thus, their environment selects for species with similar traits (but see: ref), whereas the second hypothesis proposes that non-native species are only distantly related to local native species and because of their general lack of similarity, competition between exotic and native species is limited.

“I was interested in understanding the ecological mechanism behind their success. Are they succeeding because they are similar to native birds, or because they are exploiting opportunities that native species use less?” Dr Marcolin said in email.

Dr Marcolin and an international team of collaborators sought to distinguish between these two hypotheses with regards to these parakeets’ success by characterizing the functional niche space occupied by local bird communities, specifically asking whether the parrots share similar ecological characteristics with native species, or if they occupy parts of the ecological niche space that are otherwise “empty” in urban areas.

A ecological niche space used to the identify the specific roles and spaces that organisms occupy within an ecosystem, encompassing both their physical environment and the various interactions they engage in with other species and their surroundings.

“It includes things like what it eats, where it finds food, and how it uses the habitat,” Dr Marcolin explained in email. “In our study, we asked whether parakeets are doing roughly the same ecological job as native birds (therefore competing with them), or whether they are taking advantage of opportunities that native birds use less.”

Urban environments are ideal systems in which to explore this. Human activities profoundly modify habitats, alter resource availability and reshape species interactions. Most birds cannot live in cities. Thus, urban bird communities are often simplified, and dominated by a limited group of adaptable species. This simplification can reduce competition for some resources, but it may also leave parts of the ecological niche space underused, creating opportunities for species with different ecological strategies (ref).

To better understand how these birds live alongside each other in urban areas, Dr Marcolin and collaborators conducted their study by surveying 220 points in breeding and winter periods along an urbanization gradient in six Italian cities (Figure 1).

Rather than focusing solely on species identity, Dr Marcolin and collaborators instead mapped avian communities into multidimensional ecological niche spaces based on traits like diet, foraging behavior, habitat use and life history. They then modelled these metrics in relation to the presence or absence of the two non-native parakeet species along the urbanization gradient.

“Our results suggest that they are exploiting ecological opportunities that already exist in urban environments, especially opportunities that native species appear to use less,” Dr Marcolin told me in email. “For example, I have observed Rose-ringed Parakeet (Psittacula krameri) several times feeding on resources that seem to be little used, or not used at all, by native urban birds, such as unripe apples, fruits of Cupressus sp., and oranges in Lisbon area.”

Could introduced parrots expand out of cites and into the nearby countryside?

“Urban areas can act as centres of establishment and potentially as stepping stones for further spread, so this is an important concern. Our study did not directly test range expansion away from cities,” Dr Marcolin replied in email.

“That said, there is evidence that some naturalized parrot populations are spreading beyond core urban areas into suburban and even agricultural landscapes, especially in the case of Monk Parakeets. For Rose-ringed Parakeets, the evidence I am aware of is more mixed: they are still strongly associated with urban and suburban habitats, but there are also records of use of suburban orchards and observations outside core city areas. More generally, I think prevention deserves far more attention and funding, because preventing establishment is usually much more realistic than trying to eradicate non-native bird populations once they are well established.”

Dr Marcolin and collaborators found that non-native parakeet species probably established their communities in the vacant niche space in urban bird communities. Their results support the hypothesis that limiting trait similarity allows the establishment of non-native parakeets at the local scale by reducing competition with native species due to trait dissimilarity.

It’s interesting to note that communities where parrots are established consistently exhibited a wider ecological niche space than cities without them. Importantly, this expansion was not driven by parrots overlapping heavily with existing native bird roles, but by parrots occupying empty corners of the ecological niche space in urban environments.

Further, both Ring-necked and Monk Parakeets consistently appeared at the boundaries of their communities’ niche spaces — indicating that they contribute novel ecological functions rather than displacing native bird species. It’s also important to point out that when the parrot species were hypothetically removed from invaded communities, the structure of the native species niche space remained largely unchanged, supporting the idea that parrots add rather than reshape existing roles.

This pattern was found in both breeding and winter seasons but was particularly pronounced in winter, when bird communities tend to be functionally simpler and resources more limited. In cities without established parrots — including Turin and Campobasso — niche space was narrower, especially outside the breeding season.

“What surprised me most was how consistently the pattern pointed in the same direction,” Dr Marcolin told me in email. “In both breeding and winter, parakeets tended to expand the ecological trait space of the communities where they occurred rather than simply overlap strongly with native species. I also found it striking that this pattern became more evident along the urbanization gradient. I was expecting something similar but not as strong as the results we had.”

This insight into niche space processes in urban areas, which can act as centers for expansion of non-native birds into other environments, can be used to implement management strategies that can reduce the chances of further establishment of non-native species.

“The most important message is that urban environments can create ecological opportunities for non-native species. In our case, parakeets seem to be succeeding not simply by becoming copies of native urban birds, but by exploiting parts of urban ecological space that are relatively less used by native species,” Dr Marcolin summarized in email. “That matters because it means invasion risk in cities is not only about direct competition, but also about the opportunities that cities themselves create.”

Source:

Fabio Marcolin, Riccardo Alba, Stefano Mammola, Giacomo Assandri, Luca Ilahiane, Diego Rubolini, Luís Reino, and Dan Chamberlain (2026). Non-native parrot species expand the trait space of avian communities by filling empty niches in urban areas, IBIS | doi:10.1111/ibi.70032

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