Comparative foraging ecology of a tropical seabird community of the Seychelles, western Indian Ocean

Abstract

We studied the foraging ecology of a tropical seabird community in 2 islands of the Seychelles from 2005 to 2007. Chick dietary samples were used to compare feeding habits among species and assess inter-annual and seasonal variations in diet. Fish prey dominated the diet of the community (68 to 100% of prey consumed), although cephalopods were present in 61.3, 40.0, 27.1 and 32.9% of the food samples from white-tailed tropicbirds, sooty terns, brown noddies and wedgetailed shearwaters, respectively. We found high diet overlap between species (Mullidae fish being the first prey consumed [30 to 90%] for all species except for the white-tailed tropicbird [< 2%]) but some segregation in prey length. Other important prey were Exocoetidae, Carangidae, Scombridae and Clupeidae mainly for white-tailed tropicbirds, sooty terns and brown noddies, Hemiramphidae and Coryphaenidae for white-tailed tropicbirds, and Engraulidae and fish larvae for lesser noddies and Audubon's shearwaters. Despite some inter-annual and seasonal variations in diet, these were not consistent within the community. Responses of the seabird community to an environmental perturbation that negatively affected chick growth and breeding success of lesser noddies emphasised the higher vulnerability of species with smaller foraging ranges and/or with lower ability to switch diet (lesser noddy, roseate tern) compared to less range-restricted and/or more opportunistic ones (white-tailed tropicbird, brown noddy, sooty tern, white tern). Although situated in a tropical region, the food availability in the Seychelles seems to be predictable at a large (annual) temporal scale, but highly unpredictable at a small (intra-seasonal or daily) temporal scale.

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“…Mean prey lengths were almost the same; 70.0 mm and 70.7 mm for Common Noddy and Sooty Tern, respectively. The Morisita-Horn overlap index of dietary composition indicated a significant overlap of 0.851 between the two species (Catry et al, 2009). Another study on regurgitated prey items of Sooty Tern chicks from four different islands in the Mozambique Channel and Seychelles by the same team found that they were fed significantly different proportions of types of food, reflecting changes in prey availability and seasonality of breeding (Jaquemet et al, 2008).…”

“…The Sooty Tern is also the most abundant tropical marine species, with 6 million pairs estimated to breed in the southwest Indian Ocean (Feare et al, 2007;Jaquemet et al, 2008). Its chicks regurgitated very much the same prey items as the Common Noddy; 82% of the fish items were made up of Mullidae (35%), Carangidae (10%), Engraulidae (9%), Exocoetidae (19%), and fish larvae (1%) (Catry et al, 2009). Cephalopods contributed 18% of prey items.…”

“…The Sooty Tern and Common Noddy are of comparable size and mass and share many types of food (Catry et al, 2009), but the eggs differ in terms of eggshell thickness and %lipid (Table 1, Fig. 2A).…”

“…During fledging, adult foraging patterns change (Young et al, 2010), but changes in prey abundance might also be in effect (Catry et al, 2009). Mcleay et al (2009) found that Crested Tern chicks were fed with smaller prey items than normally consumed by the adults, and Morrissey et al (2010) found that female passerines utilised higher trophic level food prior to egglaying.…”

“…The Common Noddy, also known as the Brown Noddy (Anous stolidus), weighs about 185 g and is 40 cm long (Gochfeld and Burger, 1996). Based on a study from the Seychelles (Catry et al, 2009), Common Noddies forage at intermediate distances from shore (50-300 km), and feed its chicks mainly on fish items (90% of induced regurgitated prey items) consisting mainly of Mullidae (40%), Carangidae (10%), Engraulidae (15%), Exocoetidae (3%), and fish larvae (17%). Cephalopods made up 11% of the items.…”