How foraging tropical seabirds choose where to feed

Red footed booby

Variations in environmental conditions from climate change are causing changes in marine ecosystems, so understanding how marine predators behave becomes increasingly important to ensure appropriate conservation and management.

Dr Ruth Dunn from the Lancaster Environment Centre has investigated the foraging behaviour of red-footed boobies in the Chagos Archipelago. These seabirds face specific challenges in the tropical pelagic waters as their prey is less predictable and more sparsely distributed compared to oceans at higher latitudes.

The research team fitted adult birds with combined GPS and time-depth trackers, which provided detailed data on their foraging trips, including their routes, dive locations, and the environmental conditions encountered.

To the surprise of the researchers, the birds displayed a clear preference for diving in areas with higher sea surface temperatures. Originally it was thought cooler waters would be preferred as they form upwellings of colder water typically rich in nutrients and therefore more likely to attract marine life. However, warmer waters may allow prey species to swim faster and therefore bring them closer to the surface, but this remains to be confirmed.

Whatever the temperature of the water, red-footed boobies were seen to dive more frequently in areas where food sources were greatest. These areas were often found a long way from the birds’ home colony, suggesting sources of food closer to nesting areas had been depleted. This is unsurprising, as many seabird species call the Chagos Islands home, resulting in strong competition for close sources of food. Similar long-distance travel to food sources have been seen in non-tropical seabirds, suggesting it is a common strategy to help balance the energy costs of long-distance travel with the benefits of accessing richer foraging grounds.

The depth the birds dived to catch prey also varied throughout the day. The shallowest dives occurred at dawn and dusk and the deepest around midday, suggesting as sunlight penetrates further into the waters deeper swimming prey become easier to spot. Additionally, large predator fish tend to be more active at the start and end of the day, possibly pushing the prey fish closer to the surface.

Overall, the study added to the understanding of how different factors responsible for red-footed boobies’ foraging and feeding behaviour interact with each other. Dr Dunn explained, “We currently know relatively little about how seabirds thrive in unpredictable tropical oceans. Discovering more about how seabirds within different areas of the world live is important at a time where environmental conditions are changing rapidly.”

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