Penguins are a unique family of birds. They are the most fully adapted for an aquatic marine lifestyle, as they are flightless and fairly ungainly on land but swift, graceful swimmers underwater. All species of penguin are predators, basic penguin fare generally being crustaceans such as krill as well as small fish and squid. Penguins seem to hunt mainly by sight and use various methods to catch their quarry, from passively swimming through clouds of krill with a snapping beak to chasing down larger fish.

Read more about the life cycle of penguins.

Many penguin species hunt in pelagic (open-ocean) environments, targeting both surface waters as well as mid-level depths of several hundred to more than a thousand feet, in the case of the large king and emperor species. Numerous types of penguins, including emperor, king, gentoo, rockhopper and yellow-eyed penguins, will also forage in the benthic (seafloor) environment in coastal waters around their colonies.

Penguins specifically target prey that offer the biggest bang for the buck: in other words, the most nutritional gain for the least amount of effort. They also opportunistically hunt other organisms. For example, a study on the hunting behavior of the little penguin – accurately named, as it’s the smallest kind – in Australia showed that the birds would sometimes catch jellyfish when ascending to the surface after unsuccessful hunts for more preferred fish and krill.

Read more about how penguins protect themselves from enemies.

Group foraging is commonplace among some pelagic-hunting penguins, including the banded penguins of the genus Spheniscus and little penguins. The advantage of group foraging when pursuing schooling fish may be partly due to the better ability of a many-eyed group to find schools, less to any specific prey-capture strategy. Group foraging may also be an anti-predator behavior.

Penguins hunting alongside one another may compete for prey. Biologists have recorded at least one instance of a penguin (a gentoo, specifically) trying to actively steal another's catch.

However, groups of banded penguins such as the African penguin may be better able to bunch schools or pin them against the surface, allowing individual penguins to sweep through the resulting “bait-ball” and snatch fish, or grab panicked fish escaping from the tight-packed cluster. It's thought that the strikingly contrasting black-and-white patterns of banded penguins may be an adaptation to confusing schooling baitfish.

While the above-mentioned study on Australian little penguins showed them quite capable of grabbing fish from above or from the side, penguins in general often catch prey from below. Emperor penguins foraging under Antarctic ice, for example, dive to a modest depth and then rise up to catch fish against the underside of the sea ice.

While a tendency to grab prey from below may partly be simply a function of its greater visibility from that orientation, there may be other factors involved. A study on gentoo penguins in the Falkland Islands showed that one prey item, the lobster krill, engaged in active defense with its pincers. Rushing krill from below, therefore, might be a way to ambush the crustacean before it has a chance to fight back.

Another study, incidentally, showed Magellanic penguins swam through masses of lobster krill not to munch the krill themselves but instead anchovies and other fish feeding on them.

Large schools of small pelagic “food fish” often attract the attention of seabirds such as gannets, fulmars, shearwaters and gulls. Some evidence suggests that penguins may clue into these gatherings to find prey. The study on little penguins in Australia, which assessed their foraging strategies by attaching video cameras to the birds themselves, suggested the possibility that the penguins spotted and followed short-tailed shearwaters on the wing to locate fish schools.