Seabirds breeding in tropical environments experience high energetic 21 demands, when foraging in an oligotrophic environment. The globally threatened 22 Trindade petrel Pterodroma arminjoniana has its largest colony in Trindade Island 23 (20°30'S-29°19'W) inside the oligotrophic South Atlantic Subtropical Gyre. Diet 24 sampling methods, geolocator tracking and stable isotope analysis were used to describe 25 its diet, compare foraging trips and distributions, and assess temporal variations in the 26 trophic niche throughout the breeding period. Diet consisted mainly of squid and fish. 27 The high species diversity and wide range of prey sizes consumed suggests the use of 28 multiple foraging techniques. Stable isotope mixing models confirm that Trindade 29 petrels rely mainly on squid throughout the breeding period. Its broad isotopic niche 30 seems to reflect both a diverse diet and foraging range, since birds can reach up to 3335 31 km from the colony. Isotopic niche showed limited variation even in an eight-year 32 interval, apparently due to oceanographic stability, although changes in the isotopic 33 niche have demonstrated an adjustment to different conditions in different seasons. 34 Petrels change foraging areas and prey during the breeding period: pre-incubating birds 35 use more productive areas west of Trindade Island and obtain low-trophic-position prey; 36 incubating petrels perform longer trips southward to consume prey of high trophic 37 position; and chick-rearing petrels use areas around the island. These results 38 demonstrate that to deal with high demand breeding in a colony surrounded by 39 oligotrophic waters, Trindade petrels need to explore wide foraging areas and utilize a 40 diverse diet, besides adjusting trophic niche according to breeding stage. 41 42 43 44 90 tool to assess trophic ecology of organisms (Newsome et al. 2007, Mancini et al. 2014).
91Combining these two methods in dietary studies gives the advantages of both 92 approaches, thus allowing more robust inferences on trophic ecology of consumers. 93 The Trindade petrel, Pterodroma arminjoniana, breeds in Trindade Island in the 94 South Atlantic Ocean, and at least since 1950s in Round Island, Indian Ocean (Brown et 95 al. 2010). The species is listed as 'vulnerable' by IUCN (2016), and 'critically 96 endangered' by the Brazilian Red List (MMA 2014) because of its susceptibility to 97 human impacts and stochastic events, due to very small breeding range and population 98 size, besides hybridizing with Kermadec petrel P. negleca in Round Island (Brown et al. 99 2010). In Trindade Island ca. 1130 pairs breed all year round, with laying peaks in Luigi et al. 2009). The 101 period from laying to fledging is about 150 days-long; petrels incubate the single egg 102 for ~52 days and feed the chick for ~97 days (Luigi et al. 2009). Thus, a portion of the 103 population breeds during the austral spring-summer and another during the austral fall-104 winter seasons. 105 The gadfly petrels, genus Pterodroma, contain the largest number o...
