Humans can be a terrible influence on birds. Crows that live near us end up with high cholesterol, sparrows screech to be heard over oil pumps, and instead of migrating, some storks now just eat trash.

At times, our habits and behaviors are contagious enough to change whole ecosystems. According to a study published in Royal Society Open Science on Wednesday, members of a particular New Zealand bird species, weka, that spend time around humans become worse at a crucial task: seed dispersal.

The study suggests that human interaction with weka “could be changing the spatial structure of new plants to a noticeable extent,” said Kim McConkey, an adjunct assistant professor at the University of Nottingham, Malaysia, who was not involved in the study.

Weka, ground-dwelling birds found in New Zealand, have long bills, big stomachs and gizzards full of grit. They have no problem swallowing seeds, even big ones — like those in the fruit that falls from New Zealand’s hinau and miro trees, which can be about a centimeter in diameter. In fact, weka are critical seed dispersers for large-seeded plants like these, bringing them into new areas by eating their fruits, moving around and then excreting the seeds. Being spread out in this way helps the plants to avoid pathogens, and allows them to grow without competing with each other for space, nutrients and sunlight.