Penguins exploit tidal currents for efficient navigation and opportunistic foraging

Abstract

Animals navigating in fluid environments often face forces from wind or water currents that challenge travel efficiency and route accuracy. We investigated how 27 Magellanic penguins (Spheniscus magellanicus) adapt their navigation strategies to return to their colony amid regional tidal ocean currents. Using GPS-enhanced dead-reckoning loggers and high-resolution ocean current data, we reconstructed penguin travel vectors during foraging trips to assess their responses to variable currents during their colony-bound movements. By integrating estimates of energy costs and prey pursuits, we found that birds balanced direct navigation with current-driven drift: in calm currents, they maintained precise line-of-sight headings to their colony. In stronger currents, they aligned their return with lateral flows, which increased travel distance, but at reduced energy costs, and provided them with increased foraging opportunities. Since the lateral tidal currents always reversed direction over the course of return paths, the penguins' return paths were consistently S-shaped but still resulted in the birds returning efficiently to their colonies. These findings suggest that Magellanic penguins can sense current drift and use it to enhance energy efficiency by maintaining overall directional accuracy while capitalizing on foraging opportunities.

Fig 1. Key vectors in penguin return journey analysis.

This diagram illustrates the primary vectors used in analyzing the penguin’s journey back to its colony. In panel (a), the ocean current vector (solid blue) combines with either the real penguin travel vector relative to the water (solid red) or the fully-compensated travel vector relative to the water (solid green) at position ‘n.’ These interactions result in the projected real penguin travel vector relative to the ground (dashed purple) or the projected fully-compensated travel vector relative to the ground (dashed cyan) at position ‘n + 1’. The dotted black arrow represents the direct line-of-sight path to the colony. The fully-compensated travel vector relative to the ground is aligned with this line-of-sight path unless the penguin’s speed is insufficient to fully counteract the current. Panel (b) shows a sample of these vectors recalculated every 5 min along a penguin’s southward return journey, based on its dead-reckoned track. The fully-compensated travel vector relative to the water (solid green) sometimes counteracts the ocean current at angles greater than 90° to help the penguin stay on the most efficient path back to the colony.

Fig 2. Overview of penguin tracks, regional currents, and heading distributions.

(a) The GPS-corrected dead-reckoned tracks of 27 penguins at sea, colored to distinguish between the outbound (red) and inbound (blue) phases of their foraging trips. The base map was constructed using OpenStreetMap (OSM) tiles—licensed under the Open Data Commons ODbL. (b) A ‘snapshot’ of the current conditions within the grid area, showing peak tidal strength during a single day as a function of the tidal cycle, including the outgoing (‘ebb’ – left panel) and incoming (‘flood’ – right panel) tides. Penguins are often subjected to strong cross-currents during these phases. In both (a and b), the colony location is marked by a small black filled circle with a white outline. (c) Distribution of heading differences between the penguin’s travel vector heading (relative to water) and the direct line-of-sight heading, shown for Slack Water (<0.3 m/s, blue) and Appreciable Current (≥0.3 m/s, red), and for outbound versus inbound phases. A heading difference of 0° indicates direct movement toward (inbound) or away from (outbound) the colony, whereas ±90° is perpendicular movement. In both ‘Slack Water’ (<0.3 m/s, blue) and ‘Appreciable Current’ (≥0.3 m/s, red) conditions, penguins align more closely with the line-of-sight heading during the return (inbound) phase, suggesting a more focused return to the colony compared to the departure (outbound) phase. The data underlying this figure can be found in https://doi.org/10.6084/m9.figshare.28517873.

Fig 3. Variation in penguin headings relative to line-of-sight and proportion of time spent swimming against ocean currents during the return journey.

(a) Mean (±1 SE) angular difference between real penguin travel vector headings and the line-of-sight direction to the colony. Headings are shown relative to the water (before accounting for ocean currents; red) and relative to the ground (after accounting for ocean currents; purple), with fitted loess smooth lines. For a complementary measure that factors in speed (the “Deviation Index ”), seeS7 Fig. . . (b) Mean (±1 SE) proportion of time penguins spent swimming against the ocean current. Blue represents periods with “slack water” (<0.3 m/s), while red indicates periods with “appreciable current” (≥0.3 m/s). Time spent swimming against the current is defined as instances where heading angles exceeded 90° in absolute terms. The dashed horizontal line at y = 0.5 marks the 50% threshold, indicating equal time spent swimming with and against the current. In both panels, the x-axis is divided into 0.05 increments of straight-line distance travelled. The data underlying this figure can be found in https://doi.org/10.6084/m9.figshare.28517873.

Fig 4. Penguin ease of transport for real vs. fully-compensated travel vectors relative to the ground during the return journey with respect to depth use and prey acquisition.

(a) Mean ease of e of transport (±1 SE, in m/J) is shown across the proportion of the return distance to the colony for the real penguin travel vector in any direction towards the colony (orange), the real penguin travel vector along the line-of-sight to the colony (purple), and the fully-compensated travel vector (cyan). Higher values indicate greater energy efficiency of movement. Significant pairwise differences (p < 0.05) between the real and fully-compensated vectors are marked with solid triangles at 0.05 intervals of the return journey, based on GAM-predicted ease of transport values, which account for individual variability and smooth trends over distance (see e S3 Text for details details). Upward-facing triangles indicate that the real travel vector (orange or purple) had significantly higher ease of transport than the fully-compensated vector (cyan), while downward-facing triangles indicate the opposite. The absence of a triangle indicates no significant difference at that interval. (b) Mean maximum dive depth (±1 SE, in m) and mean prey pursuit rate (±1 SESE, pursuits s⁻¹), plotted at 0.05 intervals of the return journey’s proportion of distance travelled. Note that the overall depth decreases over time showing that birds allocate increasing time to horizontal travel. Dive depths decrease to less than 10 m when ca. Eighty percent of the journey is complete (marked by the dashed grey vertical line). This pattern corresponds with substantive changes in the ease of transport and a corresponding reduction in prey pursuit. Importantly, this reduction in dive depth over time was not influenced by the surrounding seafloor depth (S8 Fig). The data underlying this figure can be found in https://doi.org/10.6084/m9.figshare.28517873.