Imagine two patches of forest next to each other. Same rainfall, same hill, same part of the map. Walk through one and birdsong follows you all the way. Walk through the other, and it's noticeably quieter, even though the trees look just as thick. That’s no accident and, as a 2025 study published in the journal Forest Ecology and Management finds, that’s exactly what’s going on all over Galicia, in northwest Spain.
Researchers from the University of Santiago de Compostela and the Biological Mission of Galicia sampled 240 locations in and around the Fragas do Eume Natural Park, comparing native woodland to eucalyptus plantations. That research found that native forests had 1,806 birds across 35 species, while eucalyptus stands had only 754 birds across a similar 36 species. The species list hardly changed at all. The actual number of birds living there was reduced by nearly half.
Same number of species, way fewer birds
It is this detail that makes the study interesting rather than merely sad. Researchers found that a typical plot in native forest held around 9 species and 14 birds, compared with about 5 species and 7 birds in eucalyptus plantations. So the eucalyptus forests are not empty. They can only support a fraction of the population density of a native forest, almost like comparing a full neighborhood to one where half its people have moved out.
Why one fast-growing tree changes the math
Eucalyptus was brought to Spain in the 1800s and has expanded ever since paper and pulp companies discovered how fast it grows. The same study found that it now accounts for around 30 percent of the forested land in Galicia, and the percentage of eucalyptus in a given plot was the single best predictor of bird decline, more so than any other factor measured.
