Albatrosses orient toward infrasound while foraging

Many large marine vertebrates show impressive movement abilities, traveling thousands of kilometers during foraging trips or migrations and returning repeatedly to a specific site using directional travel (1). These species leave their natal site as juveniles, often traveling across ocean basins and returning years later to the same site to breed (2). In undertaking these extensive movements, pelagic marine vertebrates must overcome the formidable challenge of navigating in a highly dynamic and comparatively featureless environment, without fixed visual landmarks. Effective navigation is paramount to economical movement, not just during extended movements or migrations but also day-to-day, and allows animals to move efficiently in order to forage, avoid predators, and search for mates (3, 4). Birds have played a central role in research examining animal navigation (5), but a detailed understanding of the navigational abilities of pelagic seabirds such as albatrosses has remained elusive. Gillies et al. (2023) use data from tagged albatrosses to provide new insight into potential navigational cues for pelagic seabirds (6).

Animals undertaking long movements across oceans are exposed to environmental cues that differ considerably from those in terrestrial habitats (7). In contrast to studies on terrestrial bird species (8), there is little evidence that pelagic seabirds rely heavily on the use of a sun or celestial compass, or geomagnetic cues, to navigate over long distances. In multiple studies, disrupting the perception of the Earth’s geomagnetic field did not influence the navigational abilities of pelagic seabirds (9–12). Further, reliable geomagnetic cues for bicoordinate navigation are not available in some regions of the ocean (13). While recent research shows that seabirds can incorporate information from a sun compass during navigation (14), other studies have noted that seabird navigation did not seem to be influenced by cloudy conditions as would be expected when relying primarily on a sun compass (9). Olfactory cues play an important role in navigation in procellariiform seabirds, which have highly developed olfactory systems. Odors are used during nest and mate recognition (15), and gradients of odors may form an olfactory landscape that can be used as navigational cues (16, 17). Indeed, while procellariiforms can home directly to their colonies when displaced hundreds of kilometers away, birds whose olfactory abilities were manipulated were impaired in their ability to navigate to their colony (11, 12). However, olfactory cues can be influenced by the speed and direction of wind (18), which may limit their utility, and odors may primarily be useful for providing directional information. Further, seabirds whose olfactory abilities were impaired were eventually able to navigate to their goal, suggesting the use of an alternate navigational cue (12).Thus, current knowledge of the sensory cues used by seabirds cannot fully explain their navigational abilities across all conditions (19).

Gillies et al. shed light on additional cues that pelagic seabirds may use while navigating over large spatial scales. Flying seabirds differ from other marine vertebrates in that they primarily travel above, rather than in, the water, and wind patterns can strongly influence their movements and flight paths. Seabirds are able to adjust their course relative to wind and optimize their routes so as to exploit favorable wind patterns, suggesting the ability to anticipate wind conditions in other locations using environmental cues (20). Albatrosses, long recognized for their exceptional flight performance, use dynamic soaring to extract energy from the wind in order to fly with minimal energetic cost. Albatrosses can also use waves to minimize energetic expenditure, taking advantage of updrafts associated with waves, though dynamic soaring is thought to be their dominant flight mode. These behaviors allow albatrosses to efficiently traverse vast expanses of ocean and to exploit prey resources at long distances (hundreds of kilometers) from their nest site. However, dynamic soaring and wave slope soaring require sufficient wind speeds or wave heights, and thus these factors constrain movement. As such, the ability to sense regions with strong winds and large waves from a distance could be particularly advantageous for albatrosses. Gillies et al. suggest that they may do so using acoustic cues.

Infrasound, or low-frequency sound, has been proposed as a means of providing directional information to migrating birds (19, 21, 22), but this theory was untested in free-ranging seabirds until now. Low-frequency sounds attenuate less quickly than high-frequency sounds, and infrasound can travel for thousands of kilometers if uninterrupted. Gillies et al. suggest that microbarom infrasound—very low–frequency sound (0.1 to 0.6 Hz) produced by strong storms, regions of large interacting waves traveling in opposing directions, or ocean swells breaking along a coastline—could allow albatrosses to navigate toward regions with favorable conditions for efficient flight (Fig. 1). The intensity of microbaroms decreases with distance to the source of the sound and could therefore provide a gradient indicative of proximity to distant storms and regions of large waves. By assessing the microbarom sound pressure at the locations of albatrosses tracked with Global Positioning System (GPS), Gillies et al. assess whether this may have influenced movement decisions within a foraging trip. They conclude that birds are oriented toward areas of higher microbarom sound pressure.

Fig. 1.

Microbaroms, a form of infrasound produced by the collision of large ocean waves of similar frequencies traveling in opposite directions, can be detected over large distances (thousands of kilometers). Gillies et al. suggest that microbaroms could provide a mechanism through which seabirds can detect stormy regions associated with large waves. The authors show that tagged wandering albatrosses orient toward regions of high microbarom sound pressure levels.

Gillies et al. use data from tagged albatrosses to provide new insight into potential navigational cues for pelagic seabirds.

Long-distance animal navigation is complex and requires the detection and integration of information from multiple sensory cues, often at different scales (23). Different cues may be used during particular movements or time periods to guide species to their goal. For example, sea turtle hatchlings use visual cues to locate the ocean after emerging from their nests, then rely on wave direction to orient toward offshore waters, and then use magnetic cues to navigate in pelagic habitats (24). The findings of Gillies et al. contribute to an improved understanding of seabird navigation over the ocean across spatiotemporal scales. Infrasound may guide seabird movement over large scales, and seabirds likely also rely on visual, olfactory, and potentially geomagnetic cues depending on their goal, the scale of movement, and the stage of navigation. As Gillies et al. note, “Having garnered correlative evidence that albatrosses may be behaviorally responding to infrasound, a next step is to establish the mechanisms by which albatrosses detect direction or heading from it.” Knowledge of avian hearing abilities at low frequencies, and of directional hearing mechanisms across frequencies, is limited and is needed to better understand the role of infrasound in seabird navigation and the scale over which seabirds could respond to infrasound. In addition, a better understanding of the extent to which microbaroms reflect regions of advantageous winds for seabirds, as opposed to stormy regions specifically, is needed to better understand the role of these low-frequency sounds in seabird navigation. Large waves alone are not sufficient to generate significant microbarom signals (25). Stormy regions are thought to produce strong microbarom signals because storms can generate high-amplitude waves traveling in opposite directions (26). Microbaroms could thus allow seabirds to detect storms in distant waters, but assessing the relevance of microbaroms for detecting wind and wave conditions for efficient flight in seabirds more broadly will require further research. Ultimately, Gillies et al. illustrate that infrasound presents a novel direction for addressing gaps in our knowledge of seabird navigation and the mechanisms through which wind-obligate seabirds can sense or anticipate environmental conditions in distant regions of the ocean.