A new study of 117 young birds in Madrid has found no evidence that invasive monk parakeets increase the transmission of the pathogens investigated to neighboring native birds. The discovery challenges an important assumption about biological invasions.

FLI Institute | Biology, Wildlife & Urban Ecology | October 27, 2026

Across Europe’s cities, bright green parrots have become an increasingly familiar sight.

Originally introduced through the international pet trade, several parrot species have established breeding populations far beyond their native habitats.

Among them is the monk parakeet (Myiopsitta monachus), a South American species famous for constructing enormous communal nests.

In Madrid, these birds have become a prominent part of the urban landscape.

Their presence has also raised concerns about the diseases they might transmit to native wildlife.

But new research published on October 27, 2026, in Animal Microbiome suggests that one of these concerns may have been overstated.

Scientists studying invasive monk parakeets and neighboring native birds found no evidence of increased transmission of the pathogens they investigated.

The discovery challenges the assumption that close physical contact between invasive and native birds necessarily results in greater disease transmission.

The Parrots That Build Apartment Buildings

Most parrots nest in natural cavities such as holes in trees.

Monk parakeets are different.

They construct elaborate nests from branches and twigs, often creating large communal structures containing several separate nesting chambers.

These nests can become remarkably complex.

They also attract other bird species.

In Madrid, native stock doves sometimes occupy spaces within the same nesting structures.

Other native birds, including Eurasian tree sparrows, share feeding areas with the invasive parakeets.

Such close contact creates opportunities for microorganisms to move between species.

Scientists refer to the transmission of pathogens from one host species to another as pathogen spillover.

The concern is particularly important when introduced animals encounter native species that may lack previous exposure to unfamiliar pathogens.

But does physical proximity automatically translate into disease transmission?

Examining 117 Young Birds

Researchers led by Jon Blanco-González, working with Spanish scientific institutions including Universidad Rey Juan Carlos and Universidad Complutense de Madrid, investigated this question.

They collected samples from 117 nestlings in Madrid.

The birds represented three species: invasive monk parakeets, native stock doves and native Eurasian tree sparrows.

The researchers compared birds that shared nesting or feeding environments with parakeets and birds from sites without the same exposure.

They used DNA sequencing to analyze intestinal bacterial communities.

They also conducted targeted molecular tests for selected viral pathogens.

The goal was to determine whether living alongside invasive parakeets changed the microorganisms found in native birds.

A Surprising Result

The scientists discovered that the intestinal bacterial communities of monk parakeets were markedly different from those of native birds.

The parakeets had a relatively simplified bacterial community dominated by Lactobacillus.

Native birds possessed different microbial communities.

Most importantly, the researchers found no increased overlap of the investigated potentially pathogenic bacteria between parakeets and native species.

Stock doves sharing communal nesting structures with parakeets did not exhibit the expected increase in pathogen similarity.

Tree sparrows using the same feeding environments also showed no evidence of the anticipated microbial changes.

All samples tested negative for the four targeted high-risk viral pathogens included in the investigation.

The results suggest that biological differences between bird species can limit the movement of microorganisms even when the animals live close together.

Why Microorganisms Do Not Always Cross Species Boundaries

A bird’s intestinal microbiome is shaped by numerous factors.

These include diet, physiology, immune defenses, genetics and environmental conditions.

A bacterium adapted to one species may struggle to survive or reproduce inside another.

The researchers found strong evidence that host identity helped determine the composition of intestinal bacterial communities.

In other words, sharing the same nest does not necessarily mean sharing the same microbiome.

This biological filtering may help explain why the researchers did not observe widespread pathogen spillover.

However, the findings do not establish that monk parakeets are incapable of transmitting diseases.

The investigation involved particular species, locations, age groups and pathogens.

Other pathogens or different ecological circumstances could produce different results.

The conclusions therefore apply to the transmission risks examined in this study.

Does This Mean Invasive Parrots Are Harmless?

No.

An invasive species can affect native ecosystems in several ways.

It may compete with native animals for food or nesting sites, damage vegetation, alter habitats or create conflicts with human infrastructure.

The new study does not eliminate those concerns.

Instead, it suggests that disease transmission should not automatically be treated as a major impact without supporting evidence.

This distinction is important for environmental management.

Policies should reflect demonstrated ecological risks rather than assumptions based solely on an animal’s non-native status.

A Lesson for Urban Biodiversity

Cities are becoming increasingly important environments for wildlife research.

Urban ecosystems bring together species that might rarely encounter one another in natural habitats.

They also create new opportunities for studying how animals adapt to human-modified environments.

Madrid’s monk parakeets provide an example of how introduced species can become integrated into complex ecological communities.

Their large communal nests create physical connections between different bird species.

But the new research suggests that those connections do not necessarily erase the biological boundaries separating their microbial communities.

Better Science, Better Environmental Decisions

For conservation authorities, the findings underline the value of targeted monitoring.

Instead of assuming that an invasive species is a major disease reservoir, researchers can test which pathogens are present and whether transmission is actually occurring.

Modern genetic sequencing makes this increasingly practical.

Environmental agencies can use molecular tools to identify microorganisms, compare host species and evaluate disease risks.

The result may be more precise and proportionate wildlife management.

The Leadership Lesson: Evidence Before Assumptions

The study also offers a broader lesson about decision-making.

When an unfamiliar species enters an established ecosystem, it is tempting to assume that every interaction creates additional risk.

But the Madrid researchers tested that assumption.

Their results were more complicated than expected.

For leaders in science, business and public policy, the principle is valuable.

Risk assessment should begin with evidence.

An assumption may be plausible, widely repeated and still require testing.

Rethinking the Urban Invader

The monk parakeet remains an invasive species in many European environments.

Its ecological effects deserve continued investigation.

But the new study demonstrates that the relationship between invasive and native wildlife cannot be reduced to a single narrative.

Different biological mechanisms can produce different outcomes.

Sometimes close contact increases risk.

Sometimes natural barriers prevent the expected transmission.

The presence of an invasive species may be obvious. Understanding its actual impact requires considerably more than simply observing that it has arrived.

Scientific source

Blanco-González, J., Cayuela, L., Rodríguez-Solís, D. R., et al. (2026).

Gut microbiota divergence and pathogen transmission risk between invasive monk parakeets and native sympatric hosts.

Animal Microbiome. Published October 27, 2026.