42%: The Bird Gap Between Tropical Cities and the Wild

Build a city near the equator and, on average, a square kilometre of it holds 42% fewer bird species than a comparable square kilometre of natural land. Build one at 65 degrees latitude and the shortfall is 12%. ^[1][3]

That gap is why I think urban planning has to count as conservation work in the tropics, a partner to forest reserves and no longer a footnote to them. A city is a design, and a design can be changed.

What the study counted

The paper comes from Jory Griffith, Jennifer M. Sunday and Anna L. Hargreaves and was published in 2026 in Proceedings of the National Academy of Sciences. ^[1][2] The team took checklists filed by birdwatchers through eBird and used them to estimate how many species occur in each one-by-one-kilometre grid cell. They then matched the cells to land-use data and sorted them into natural, suburban or urban. ^[2][3]

The scale is large: 10,163 bird species, 143 countries, five continents. ^[2] Across all of it, urban cells averaged 95 species per square kilometre and natural cells 141, which is 32% fewer in the city. ^[1][2]

The tropics are where that average gets ugly. For centuries naturalists have known that bird species pile up toward the equator, a pattern called the latitudinal diversity gradient. The authors find that in cities this gradient is three times weaker than in natural areas. ^[1][2] The city flattens the most distinctive feature of the planet's bird map.

Why the tropics lose more

The researchers offer a mechanism, and it is a plain one. Bird species at lower latitudes tend to have narrower habitat and diet niches. ^[2] Think of a bird that needs one kind of place and one kind of food, set beside a bird that will eat what is on offer wherever it lands. The first is a specialist, the second a generalist.

In the data, the species missing from cities tend to be more specialised than the ones that stay, and the difference is greatest in the tropics. ^[2] The tropics hold more specialists, and cities filter specialists out. The arithmetic follows.

The authors also report that the pattern holds in both hemispheres and across seasons, and that it survives when they switch to five-kilometre cells or use stricter definitions of natural areas and sampling effort. ^[2] That is a lot of ways to try to make a result go away, and this one did not.

The usual view, and its limit

The usual view of tropical conservation is a map of forests. Protect the big green blocks, fight the loggers and the ranchers, and treat the city as somewhere else, a grey smudge that people live in while the real biology goes on elsewhere.

That view is right about one thing. Forests are where most of these species live, and nothing in the study says a city park can stand in for a protected forest. The researchers do not claim it. A tidy lawn is not a rainforest, and I say that as someone who loves her garden and would never pretend it was a wood.

But the same data shows why the forest-only view is incomplete. Cities are where the tropics' richness is being erased most steeply, per square kilometre, in the comparison the study makes.

If the plan is to guard the forests and then build the cities in whatever way is cheapest, the plan quietly writes off a large part of the bird map. It protects the inventory and designs the exit.

So why not simply leave the cities to the generalists and spend the money on forests? Because cities keep spreading, and what a city does to the species around it is a design choice, not a law of nature. The study gives a reason to think design matters.

What planners can take from it

The authors report that cities with more green space had higher bird diversity than less-green cities at the same latitude. ^[1][3] It is an association, and the paper is observational. It compares places rather than changing a city and watching what returns. It does not show that adding green space alone restores lost species, and it does not say which kinds of green space work. ^[1][2]

I will concede all of that, and the consequence is smaller than it sounds. We do not need proof that a given tree-planting scheme brings back a given bird before we change how the question is asked. Right now, in most cities, green space is a line in a budget justified by shade, flood control or the look of a boulevard. The study is a reason to ask what it is habitat for.

That means treating green space as habitat rather than decoration. A planner who has to answer for the species a district can hold, and not only for its hectares of park, will make different choices about size, connection and what gets planted.

Which choices are best is an open question. The researchers still have to establish which sizes and arrangements help tropical specialists, and whether specific changes to a city actually cause diversity to rise. ^[3]

That gap in knowledge is an argument for acting carefully and measuring, not for waiting. The tool for measuring already exists. The same eBird checklists that revealed the pattern can be filed, neighbourhood by neighbourhood, before and after a city rebuilds a district. Cities that build habitat on purpose and count what turns up would answer the open questions faster than any paper.

What is at stake beyond the birds

Two cautions belong here, once. The study compares richness across places and does not follow cities over time, so it does not show that bird populations are shrinking in any particular city as it grows. ^[2]

The claim that cities have fewer species than natural land is established. The claim that a given city is losing birds this year needs another kind of study.

The larger stake is what birds do. The authors say their next work will examine how changing urban bird communities affect interactions such as predation and pollination. ^[1][3] A city that loses its specialists loses more than a list of names. It loses the work those birds do in the system around it, and the people who live there inherit what is left.

There is something to be said, too, for what a species list is: a place's memory of itself. A tropical city that has kept only the birds that tolerate pavement has forgotten most of what its ground once held, and forgetting is much easier than recalling.

What to do with this

If you live in or near a tropical city, or vote on how one grows, ask your planning office a simple question. What does your plan say about the species the new district can hold? If the answer is a count of trees and a square-metre target, you have learned something about how seriously the city takes the 42%.

Conservation groups should stop filing cities under somebody else's problem. And planners should stop being flattered by the idea that they are only decorating. In the tropics they are deciding, one district at a time, whether the richest bird communities on earth have a place in the places where people live.

The forests will not save the tropics' birds alone. The map says so, and the map was drawn by birdwatchers with phones.