Why counting ocean birds gives you the wrong map of seabird hotspots

A tale of two maps

Imagine flying over a city to protect its most important spots. If you only count heads, you will highlight a packed football stadium. But if you want to find a mix of people with different skills and backgrounds, you would pick a university campus instead. Both matter, but they are not in the same place.

Marine biologists face the exact same puzzle on the open water. When planners try to map seabird hotspots to protect ocean wildlife from human industry, they often ask a simple question: where do the birds gather? A team of researchers recently tracked ocean birds across 9,361 kilometres of ship surveys in the Northern California Current from 2022 to 2024. What they uncovered shows that simple headcounts can be remarkably misleading.

The offshore split

The team built statistical models for 22 species using visual strip transects, where observers log every bird seen within a fixed strip along the boat's path. They combined these counts with ocean data like sea temperatures and currents to build a density surface model, which predicts animal numbers across the entire seascape.

Next, the researchers evaluated bird density alongside two other vital measures: a Shannon index, which calculates species diversity by checking how evenly different species share the space, and the presence of nine vulnerable species of conservation concern. Using a spatial test called the Getis-Ord statistic, they mapped where high numbers clumped into true hotspots.

The results split the sea into two worlds. Hotspots of sheer bird density clustered near the coast along the shallow continental shelf, especially south of the Columbia River during spring and summer. But species diversity hotspots shifted further offshore into deeper waters over the continental slope. In fact, out of the entire survey area, not a single patch of ocean was a simultaneous hotspot across all three measures at once.

Why the wrong metric causes clashes

This geographic split changes how we manage industrial projects. Consider proposed floating wind energy zones spanning 2,299 square kilometres. If you look only at bird density, these zones seem entirely safe: they had 0% overlap with density hotspots and up to 29% overlap with coldspots.

Yet, look through the lens of species diversity, and between 18% and 49% of that same footprint sat directly inside diversity hotspots. Meanwhile, core commercial fishing areas—covering over 5,000 square kilometres in summer—collided heavily with nature, overlapping with density hotspots by up to 51% and vulnerable species hotspots by up to 61%.

What we still don't know

The models explained between 20% and 83% of deviance, meaning wild oceans still hold plenty of surprises. Observers had to exclude rarer birds seen on fewer than 25 survey segments, narrowing 76 spotted species down to 22. The surveys also only ran through spring, summer, and autumn, leaving winter movements unmapped. How these habitats will shift as marine heatwaves warm the Pacific remains unknown.