A ‘fish finder’ for birds? Route-dependent foraging behavior of the brown booby ( <i>Sula leucogaster</i> ) following a passenger ferry in the Ogasawara Islands, Japan

While seabird associations with fishing vessels are well-documented, interactions with non-fishing vessels remain poorly understood. This study investigated the foraging behavior of the brown booby ( Sula leucogaster ) associated with a passenger ferry in the Ogasawara Islands, Japan, home to one of the largest breeding colonies in the archipelago. During four daytime, two-hour trips aboard the inter-island ferry Hahajima-Maru in 2017, the number of accompanying boobies and their plunge-diving events were recorded once per minute. A consistent directional asymmetry was observed in this behavior. Trips originating from the main colony island (Hahajima) showed substantially higher numbers of accompanying birds and dives compared to southbound trips. Birds followed the ferry immediately upon departure from Hahajima but only appeared mid-route when traveling towards the colony. This activity was highest in the August observation, coinciding with the chick-rearing period. These observations suggest that the ferry may function as a ‘fish finder,’ disturbing surface-associated prey ( e.g. , flying fish) that boobies exploit by diving. The spatially structured pattern, influenced by colony proximity, indicates potential cognitive abilities such as spatial memory and associative learning. This study demonstrates that simple, low-cost observational methods from public transport can yield valuable behavioral insights and offer a scalable approach for citizen science.