It was long believed that birds possessed little to no sense of smell, relying almost entirely on their highly developed vision. This misconception suggested the avian world was largely anosmatic, or without a functional nose. Modern research confirms that many bird species, including gulls, actively utilize a sophisticated sense of smell, known as olfaction, for important behaviors like foraging. For a bird navigating vast, featureless expanses of ocean, this chemical sense is valuable for locating food.
The Olfactory System in Avian Species
The anatomical structure responsible for a bird’s sense of smell is similar to that of other vertebrates. Air enters through the nares, or nostrils, and travels through nasal chambers lined with internal structures called conchae, or turbinates. These intricate, scroll-like bones are covered by the olfactory epithelium, which contains the scent-detecting receptor cells. The nasal cavity directs a portion of inhaled air toward this epithelium for chemical sampling.
Signals from the olfactory receptors travel directly to the olfactory bulb, a structure located in the forebrain. The size of this bulb relative to the rest of the brain classifies an animal’s olfactory ability; species heavily reliant on smell are considered macrosmatic. While gulls are not extremely macrosmatic like the kiwi or turkey vulture, their olfactory bulb is important for their ecology. The complexity of the olfactory anatomy correlates directly with the importance of odor cues in a bird’s life.
Chemical Signals That Guide Gulls to Food
The specific chemical compound that acts as a beacon for gulls and many other seabirds is Dimethyl Sulfide (DMS). This sulfurous gas is a byproduct of the marine food web, serving as an airborne indicator of a feeding frenzy below the water’s surface. DMS originates from dimethylsulfoniopropionate (DMSP), which is naturally produced by phytoplankton, the microscopic plant life at the base of the ocean food chain.
When zooplankton graze on phytoplankton, they release DMSP into the water. Bacteria and chemical reactions then convert this compound into the volatile, airborne DMS. This gas rises from the ocean, creating a plume of scent that signals concentrated prey, such as krill or small fish, in that area. Detecting this scent allows gulls to locate productive feeding grounds from considerable distances across the open ocean, where visual markers are absent.
The DMS signal is so strongly linked to food that it can mislead seabirds, which is a concern regarding ocean pollution. Marine plastic debris coated with decomposing algae can also emit DMS. This false signal tricks foraging gulls into investigating and sometimes ingesting plastic, mistaking the olfactory cue for a food source.
Integrating Sight and Smell in Foraging Behavior
Gulls use a dual-sensory approach for foraging, where smell and sight work in tandem to maximize feeding efficiency. Smell is primarily used for long-distance detection, enabling the bird to locate the general area of a potential food source. Flying high above the water, the DMS plume acts as a chemical map, pointing toward a concentrated biomass patch too small to be seen visually.
Once guided by scent, the bird’s superior eyesight takes over for the final, precise localization of the food item. This is especially true in coastal or urban environments, where visual cues often dominate their foraging strategy. Gulls use sight to spot discarded food or observe the behavior of other gulls to find a source.
In urban settings, gulls integrate learned visual cues, such as human behavior, with their foraging. They can associate the sight of a person handling food with a potential meal or learn to time their feeding activity to human schedules. However, when foraging over the vast, visually uniform surface of the open ocean, the initial, broad search depends almost entirely on olfaction.