Tap Start camera
Identification
Smallest diver. Slender uptilted bill. In summer: grey head, dark red throat patch (appears black at distance). In winter: white flecking on dark back. Flies with faster wingbeat than Black-throated.
Calls
· Wail — haunting rising note, contact / distress
· Quacking rattle — flight call, distinctive to species
· Growling — territorial, near nest
Diving behaviour
Typically 30–45s dives. Forages in shallower water than Black-throated. Will commute from nesting lake to sea to feed — can fly with crop full. Pursues fish using feet, wings rarely used underwater.
In Kvarken
Uses sheltered bays and river mouths. The brackish-freshwater transition suits its prey range. Often seen alone or in loose groups offshore during migration.
Identification
Medium-large diver. Straight bill, carried horizontal. Summer: black throat, grey head, black-and-white chequered back. Winter: clean white flanks visible at waterline (key field mark vs. Red-throated).
Calls
· Wail — lower, more melancholic than Red-throated
· Yodelling chorus — territorial advertising, two birds
· Cackling — alarm, aggressive
· Moan — soft contact call
Diving behaviour
Dives 45–75s, regularly to 6m, occasionally 25m+. More pelagic forager. Uses feet primarily; wings help in direction changes at depth. Tends to resurface facing same direction as entry — useful tracking cue.
In Kvarken
More common offshore in the deeper channels of the Archipelago Sea side. Favours clear water — water clarity affects where it hunts. Presence/absence signals salinity conditions.
Dive rhythm tells you about prey
Short dives (under 30s) with quick resurfacing = small shallow prey, likely sand eels or small perch. Long dives (60s+) with slow resurfacing = deeper pursuit, possibly herring or vendace. Repeated dives to same spot = school located.
Resurfacing direction
Consistent resurfacing in one compass direction = following a prey school or current edge. Scattered resurfacing = prospecting, no school located.
Surface displays
Splash-dive (rolling forward) = disturbance response, not foraging. Wing-spreading on surface = thermoregulation or display. Upright neck-stretch = alarm or mate contact. Bill-dipping = testing water (salinity, temperature sensing).
Weather reading
Loons move inshore before storms — dropping pressure shifts fish to shallower water. Increased calling before rain fronts documented. Flat-water foraging preference means choppy conditions push birds to sheltered bays.
GoPro / underwater cues
Wing use = fast direction change, likely active pursuit. Foot-only propulsion = cruising or slow search. Upward glance repeatedly = orienting to surface light (tracking fish silhouettes). Spiral descent = scouting before committing to depth.
Salinity gradient
The Kvarken marks the sharpest salinity transition in the Baltic: from ~5–6 PSU south of the narrows to ~2–3 PSU in the Bothnian Bay. This controls which fish species are present — vendace, whitefish and perch dominate the north; herring and sprat thin out. Loon species composition shifts with this gradient.
Isostatic rebound
The coast rises ~8–9mm per year — fastest in the world. New skerries and shallows appear within decades. These shallow new grounds are exactly the habitat loons (and their prey) colonise first. A loon foraging on a skerry that wasn't there in your parents' lifetime.
Stora Fjäderägg
The island's long-running ringing station provides population trend data. In a declining Baltic context, the Bothnian Bay loon population has been relatively stable — likely because the fresher water has avoided some marine pollution loads. The labyrinths and compass roses left by fishers are also navigational technology, like the observatory. Same problem, different centuries.
The loon is not the subject. The loon is the instrument.
This tool uses diver behaviour — dive timing, resurfacing direction, calls, movement, presence, absence — as a way of reading the Kvarken. What the bird does reveals what is in the water, what the weather is doing, where the fishing pressure is, where new land is forming, what the sea's legal and political condition is. The bird has been navigating this threshold longer than any human system for measuring it. Its behaviour is a record of the place.
The Kvarken sits at the sharpest salinity gradient in the Baltic. The coast rises nearly 9mm a year — the fastest in the world. Both diver species have lost over 80% of their Baltic wintering population since the 1980s, mostly to gillnets. A bird foraging here is moving through a sea that is simultaneously making new land and losing the species that live on it. What it chooses to do, and where, and how, reads all of that.
The approach draws on Kari Weil's animal studies work — that thinking with another species changes what you can see — and on Thomas Nagel's honest limit: we can know what the bird does without knowing what it is like to be the bird. The tool stays on the right side of that line. The output is always about the world around the bird, not inside it.
Built for the Godzilla expedition, Holmsund–Luleå, June 2026. Darren Banks · darrenbanks.uk · Don't Forget the Birds