1. What is going on under a flock of diving seabirds?
Many seabirds cannot dive deep or chase fish far. They need prey to come to them. Researchers who review these partnerships point out that some hunters under the water, such as dolphins, seals, and tunas, drive schools of prey toward the surface, and that this makes the prey much easier for birds to catch [1]. The best-known example is the bait ball: small "bait" fish packed into a dense ball and pushed up from below by diving predators [1].
Here is the usual order of events. It is a simple picture, and real events vary.
- Hunters such as tuna or dolphins find a school of small fish.
- The small fish bunch into a tight ball.
- The hunters keep attacking from below, and the ball is squeezed toward the surface. The surface acts like a ceiling that blocks escape upward, while the hunters below keep pressing the fish toward it.
- The surface churns with panicking fish. This is what anglers call nabura.
- Seabirds notice and join in from above.
That order explains why fishing guides look for flocks. The birds are a signpost for something that is hard to see: a hunt happening under the water. In the tropical Pacific, fishers have long used flocks of seabirds to find tuna and dolphins [1].
One more detail. For some tropical seabirds, such as sooty terns, the review suggests this help may not be a bonus but a necessity. These birds may depend on tuna schools to bring baitfish up to where they can reach them [1].
2. Why do small fish pack into a ball when a hunter attacks?
It sounds backwards. If something is chasing you, scattering sounds smarter. But for a fish in a school, there are several reasons that sticking together can help. Biologists who study group living describe a few related ideas [5].
Being one of many. In a simplified example, a predator catches only one fish per attack and every fish is equally likely to be picked. Then a fish in a school of a thousand is much less likely to be that one than a fish swimming alone. This is called the dilution effect. A simple way to picture it is a raffle: more tickets in the bowl means each ticket is less likely to be drawn.
Confusing the hunter. A swirl of similar-looking fish can make it hard for a predator to lock onto a single target. This is called the confusion effect. Scientists are still testing how strong it is. In one study, hawks hunting in a huge swarm of bats seemed to steer toward a fixed point in the swarm instead of tracking one bat, so a predator may sometimes get around the confusion [5].
More eyes. In a big group, many fish are watching for danger at once, so a threat is more likely to be noticed early.
A bait ball is an extreme way for small fish to bunch together when they are under attack. The cost is that a ball is easy to find. A dense, glittering clump of fish can attract more hunters, including seabirds, and which ones come varies. So the same behavior that lowers one fish's risk from one predator can make the whole group easier for many predators to spot. Packing together is a trade, not a perfect defense.
3. How do seabirds find a feeding spot in such a huge ocean?
Food in the ocean is patchy: a lot in a few places, almost none in between. Birds seem to use several clues together.
Sight. From high up, a bird can see a long way. Churning water, flashes of silver, and the dark shape of a school can all stand out from far away.
Other birds. A bird that sees a feeding flock may use the flock itself as a clue that food is there. Biologists call this local enhancement [1]. It works like a chain reaction: one bird dives, a second bird sees and flies in, and a third, farther away, sees the second. This is why a few birds can turn into a huge crowd so quickly.
Smell. Birds in the tubenose group (petrels, shearwaters, albatrosses and their relatives) have a good sense of smell. In 1995, researchers ran experiments at sea in the Southern Ocean. They found that some of these birds are attracted to dimethyl sulfide, or DMS, a gas that tiny sea algae release when small animals such as krill graze on them [2]. In the tests, Wilson's storm petrels flew into DMS-scented oil slicks about twice as often as into unscented ones. Some albatrosses and cape petrels showed no such preference [2]. So smell is a clue for some species, not all.
Different clues may work best together. A 2025 modeling study suggests that groups containing both "sniffers" and "watchers" find food better than groups of one type, because watchers can follow the sniffers [3]. That study is a computer model, so it shows what could happen, not a count from the real ocean.
4. Can a bird's sense of smell lead it to the wrong thing?
Yes, and the same smell is the reason. A 2016 study by Savoca and colleagues left plastic beads in the ocean for about a month. According to the study's abstract, the beads picked up a DMS smell, and across tubenose seabird species, the ones that responded more strongly to DMS also tended to swallow plastic more often [4]. The authors suggest plastic may act as an "olfactory trap" that smells like food. This is a link found in the data, and the authors present the trap as an idea the results point toward, not as a settled fact.
So a smell that can help some seabirds find food may also lead them toward plastic, which is a new kind of thing in the ocean.
5. How can you notice this yourself near the sea?
Few people get to see a full-scale bait ball. But you can practice reading the water from a pier, a ferry, or a beach, always with an adult.
What to look for: Do birds circle high up, or dive low? Are they all over one patch of sea, or spread out? Does the patch move over a few minutes? A patch that drifts and disappears is a clue that the event is short-lived.
For safety: Stay behind railings and on safe ground, watch the waves, and never go into the water to get closer. Do not throw food to wild birds.
Read the original sources: The review of birds, dolphins, and fish working in partnership [1] is a good place to start. It has many examples from around the world.