Biology
Olfactory Foraging and Chemical Ecology in Procellariiform Seabirds
Quick fact
Procellariiform seabirds—such as albatrosses, petrels, and shearwaters—have some of the largest olfactory bulbs (relative to body size) of any bird, and they can detect the scent of dimethyl sulfide (DMS), a gas produced by phytoplankton when grazed, guiding them to productive feeding areas.
Why this is interesting
Imagine flying for hours over a vast, featureless ocean with no visual landmarks—how would you find food? Some seabirds don't rely on eyesight; they follow their nose across thousands of kilometers.
Read the full explanation
Understanding Olfactory Foraging and Chemical Ecology in Procellariiform Seabirds
Procellariiform seabirds are masters of the open ocean, but their food—fish, squid, krill—is patchy and often hidden beneath the surface. To find it, they use a surprising tool: smell. Unlike most birds, which rely heavily on sight, these seabirds have evolved an exceptional sense of smell, thanks in part to their unusually large olfactory bulbs. Their foraging strategy works like this: as ocean currents and winds move across the water, they pick up chemical signals from food sources. For example, tiny marine plants called phytoplankton produce a gas called dimethyl sulfide (DMS) when they are being eaten by zooplankton. DMS escapes into the air and, carried by the wind, forms an odor plume. When a foraging petrel or shearwater detects this smell, it flies upwind, following the concentration gradient toward the source—areas where zooplankton are feeding, which often also coincide with fish and squid. Procellariiforms also home in on the scent of prey carcasses and on the smell of other seabirds that are feeding, which signals a potential meal. This is not just a simple sniff-and-go process. The birds need to translate chemical information from a turbulent atmosphere into a spatial map. They do this by flying in characteristic zigzag patterns across the wind, sampling the air to determine the direction of the strongest odor. This behavior, called 'windward foraging', allows them to zero in on a localized patch of food.
A deeper explanation
The effectiveness of olfactory foraging in procellariiforms hinges on several anatomical and ecological adaptations. First, their olfactory bulbs—the brain region that processes smell—are significantly enlarged. In some species, they occupy up to 50% of the forebrain, a proportion far exceeding that of other birds. This allows for a greater number of olfactory receptor neurons and more processing power, enabling the birds to detect incredibly low concentrations of airborne chemicals. Second, their nasal passages are specialized. Procellariiforms possess a 'tube-nose' structure, where the nostrils are enclosed in a tubular sheath on top of the beak. This design may enhance the flow of air over the olfactory epithelium, allowing them to sample scents more efficiently during flight. It also helps prevent water from entering the nose during plunging dives. The key chemical cue, DMS, is a product of the marine food web. When zooplankton graze on phytoplankton, the algae release DMS as a metabolic byproduct. DMS is volatile, so it escapes into the atmosphere. Its presence signals a concentration of zooplankton, which in turn attract larger predators like fish and squid—a feast for seabirds. Therefore, DMS acts as a reliable chemical indicator of potential prey availability. The birds' ability to follow odor plumes also depends on the physics of wind. Odor molecules are carried downwind, forming a meandering plume that expands and contracts with turbulence. Procellariiforms have evolved flight behaviors to exploit this: they fly crosswind, casting back and forth to detect where the plume is strongest, then turn upwind to follow it. This 'anemotactic' orientation uses wind direction as a reference. Beyond foraging, olfaction plays a role in navigation. Many procellariiforms return to their nests on remote islands after months at sea, and they can locate their specific burrow even at night or in conditions of poor visibility. They likely use a combination of wind-borne home scents and local odor landmarks to home in on their breeding site. In essence, olfactory foraging in procellariiforms is a sophisticated mechanism where behavior, anatomy, and chemistry converge to solve an ecological problem: finding sparse and ephemeral food in a vast and featureless environment.