White-tailed Tropicbirds are a common seabird species across all tropical oceans but their movement and foraging ecology has so far been poorly studied. A recent study tracked the feeding movements of White-tailed Tropicbirds breeding in Seychelles, which hosts the largest population of the species in the Indian Ocean. The study compared the feeding movements of two populations: one on Aride Island, in the inner granitic islands, and one on Aldabra Atoll, a coralline outer island approximately 1200km southwest of Aride. Both islands sit within marine protected areas.

 

A White-tailed Tropicbird in flight by Beth Clark

 

The results showed that White-tailed Tropicbirds undertake long-distance movements to reach their feeding grounds, both during incubation and chick-rearing, often feeding hundreds of kilometres away from their colony. On both colonies, birds fed well outside the boundaries of the protected areas around the islands, highlighting their limitations in protecting the birds’ feeding areas. The White-tailed Tropicbirds on Aride were found to forage mostly north of the island (the foreign fishing vessel prohibited area lies mostly south), while on Aldabra birds fed mostly south, and to a lesser extent, north-east of the island, with a substantial proportion of their foraging grounds even outside of the Seychelles Exclusive Economic Zone.

 

A White-tailed Tropicbird with its chick in the nest on Aride by Gérard Rocamora

 

A surprising result was the substantial difference in foraging range between the two populations. The White-tailed Tropicbirds on Aride fed at least twice as far as those on Aldabra, averaging a total distance per trip of ~860 km during incubation and ~300 km during chick-rearing (vs ~360 km and ~140 km on Aldabra). Using habitat models, the study shows that this difference is likely driven by the birds’ preference for deep waters, which are more easily accessible from Aldabra than from inner Seychelles islands. These habitat preferences are likely linked with the presence of the birds’ main prey, flying fish and squid.

 

Figure 1 from Ensanyar-Volle et al. 2023: White-tailed Tropicbird foraging trips (a) and distributions from Aldabra (b) and Aride (c) in the Western Indian Ocean region. Higher density areas are in darker shades and blue outlines represent the foreign vessel restricted area around the colonies.

 

Another potential driver of the larger foraging range of White-tailed Tropicbirds on Aride could be reduced foraging opportunities associated with tuna. Tropical seabirds, including White-tailed Tropicbirds, can associate with sub-surface predators, which bring school of small fish closer to the surface. Tuna stocks in the Indian Ocean have greatly declined due to longstanding overfishing, and there are higher levels of industrial fishing effort and human activity in the feeding range of White-tailed Tropicbirds from Aride than around Aldabra. With the White-tailed Tropicbird population on Aride declining, there is an urgent need to better understand the extent of their reliance on sub-surface predators to feed, and the potential links between their longer foraging trips, higher exposure to industrial fishing and human activity in their foraging grounds, and demographic traits.

 

 

Read the full paper here.

Tracking data from this work are available from the seabird tracking database 2122 and 2158

This work was carried out in collaboration with, and with in-kind support from, the Seychelles Islands Foundation, the Island Conservation Society and the Island Biodiversity and Conservation Centre from the University of Seychelles.