Large trees can be keystone structures for birds in urban environments

In the bustling concrete canyons of modern cities, nature often feels like an afterthought, squeezed into small pockets of green. Yet, within these urban landscapes, a specific type of living structure plays a role far greater than its numbers might suggest. Ecologists call these "keystone structures": elements that hold an ecosystem together, providing essential resources that many other species depend on. For birds in cities, the most critical of these structures are often the largest, oldest trees. These giants offer more than just shade; they provide hollows for nesting, vast canopies for perching, and concentrated food sources like nectar and seeds that smaller trees simply cannot produce. Because these features take over a century to develop, they are rare and irreplaceable. As cities expand, these ancient trees are frequently removed, often because they stand alone or in small groups, leading to a common belief that isolated trees hold little value for wildlife. Understanding whether these solitary giants are truly vital or merely decorative is essential for deciding how to build cities that support life.

A team of researchers set out to test this idea directly in two distinct Australian cities: Canberra in the Australian Capital Territory and the Gold Coast in South East Queensland. They visited 83 different green spaces, ranging from public parks to remnant patches of forest tucked inside residential developments. Their goal was to see if large trees acted as keystone structures, meaning they supported a disproportionately high number of bird species and individuals compared to their sheer abundance. To do this, the scientists counted every bird they saw or heard in these spaces and measured the diameter of every tree at breast height, distinguishing between smaller trees and the massive ones with trunks at least 40 centimeters wide. They also mapped the surrounding landscape to see how many other trees existed nearby, testing whether a large tree in a lonely spot was more valuable than one standing in a dense grove.

The results revealed a story that changed depending on where the researchers looked. In Canberra, the data strongly supported the idea that large trees are indeed keystone structures. The researchers found that a very small number of the biggest trees could support the same proportion of bird species as a much larger number of smaller trees. For instance, just three trees with trunks wider than 100 centimeters were predicted to support the same share of the local bird community as fifteen trees that were only 40 centimeters wide. This suggests that in Canberra, the largest trees are doing the heavy lifting for bird biodiversity, providing resources that the smaller trees cannot. However, in South East Queensland, the story was different. There, the size of the trees did not seem to matter as much; having more large trees did not predict a higher proportion of bird species in the way it did in Canberra. The researchers suggest this difference might be because the Queensland sites generally had more tree cover overall, meaning the birds had more options and were not as reliant on a single giant tree.

The landscape context also played a subtle but important role. The study found that large trees tended to be most valuable for birds when they existed in relative isolation, surrounded by fewer other trees. In Canberra, where tree cover was generally low, the biggest trees attracted a greater share of bird species when they stood alone. In Queensland, a similar pattern emerged for the total number of birds: when large trees were scarce, they supported more birds if they were in areas with less overall tree cover. This challenges the old assumption that isolated habitats are useless for wildlife. Instead, it suggests that a single, massive tree in a built-up area can serve as a crucial hub for birds, perhaps because it is easier for them to find and because it offers a unique combination of nesting and feeding spots that the surrounding city lacks.

When the researchers looked at which specific birds were using these giants, a clear pattern emerged regarding the age of the trees. The birds most strongly linked to the largest trees were those that rely on hollows for nesting. In Canberra, 73% of the bird species that loved the big trees were hollow-dependent. Since Australia has no native woodpeckers to carve out nests, these hollows must form naturally in very old trees, a process that can take between 120 and 220 years. This makes the retention of these ancient trees a matter of urgency, as artificial nest boxes cannot fully replace the complex habitat a living giant provides. However, the study also noted that not all birds attracted to these trees are conservation priorities. Some of the species that flocked to the large trees were invasive species, like the common starling, or native birds that can be aggressive toward others, such as the noisy miner. This indicates that while keeping large trees is vital, it is not a complete solution on its own; managing the broader environment is also necessary to ensure a healthy mix of bird life.

Ultimately, the research confirms that large trees are not just aesthetic features but functional keystones for urban bird life, particularly in areas where they are rare. Their value is highest when they are preserved in relative isolation, proving that even a single giant tree in a sea of buildings can be a lifeline for wildlife. The findings suggest that city planners and conservationists should prioritize keeping these ancient giants, regardless of whether they stand alone or in a group, because their unique ability to support hollow-nesting birds and other wildlife is something no new planting can quickly replace.