Ibis (2009), 151, 265–273 © 2009 The Authors Journal compilation © 2009 British Ornithologists’ Union Blackwell Publishing Ltd Does interference competition explain why White Terns of Aride Island, Seychelles, breed predominantly when marine productivity is lower? TERESA CATRY, 1 * JAIME A. RAMOS, 1 EMMA SAMPSON 2 & MATTHIEU LE CORRE 3 1 Institute of Marine Research (IMAR), Department of Zoology, University of Coimbra, 3004-517 Coimbra, Portugal 2 Aride Island Nature Reserve/Island Conservation Society (ICS), Box 775, Victoria, Seychelles 3 Laboratoire ECOMAR, Université de la Réunion, 97715 Saint Denis message cedex 9, France White Terns Gygis alba breed throughout the year on Aride Island but show a marked preference to nest during the northwest monsoon (November–March), when interspecific competition and also marine productivity are considered to be lower. In this study we investigated and compared breeding and foraging parameters of White Terns between the northwest and southeast (May–September) monsoons to assess whether interspecific competition affected foraging activity and explained the timing of breeding of White Terns. Goatfish (Mullidae) dominated the diet of White Terns and no significant differences were found in diet composition between the northwest and southeast monsoons. Similar diets and patterns of food provisioning of White Terns between the two monsoons do not support the hypothesis of strong exploitation competition during the southeast monsoon. On the other hand, foraging behaviour and frequency of capture attempts of White Terns differed between the two monsoons. The percentages of multi-species flocks with White Terns and of large flocks with more than 25 noddies Anous and terns Onychoprion were significantly higher during the southeast monsoon. Moreover, capture attempt frequencies of White Terns foraging in multi-species flocks were lower than those of solitary individuals, and both solitary and flocking White Terns had significantly lower capture attempt frequencies during the southeast monsoon in comparison with the northwest monsoon. These results suggest that interference competition depresses foraging activity of flock-foraging White Terns, particularly during the southeast monsoon, which could potentially explain why White Terns breed preferentially during the northwest monsoon. However, and despite avoiding interference competition, White Terns nesting during the northwest monsoon did not show higher breeding success than those nesting during the southeast monsoon. Other hypotheses that might explain the preference of White Terns for breeding during the northwest monsoon, such as avoidance of agonistic interactions with Lesser Noddies Anous tenuirostris at nest sites, should also be investigated. Keywords: breeding phenology, breeding success, competition, diet, foraging behaviour, tropical seabird. Of all the factors that may contribute to explain timing of breeding by birds, the abundance of food resources for egg formation and chick rearing is the best known (Lack 1967, Perrins 1970). For the majority of seabird species, breeding is a seasonal event and the phase of chick growth and fledging should coincide with the period of highest food availability within the foraging range of the breeding colony (Lack 1967, Hamer et al. 2001). When compared with temperate and polar areas, food resources in tropical marine environments have been described as scarcer, with a more patchy distribution and with little seasonal variation (Ashmole 1971, Ainley & Boekelheide 1983, Weimerskirch 2007). Nevertheless, food availability plays the main role in explaining the breeding phenology for most tropical seabirds (Hamer et al. 2001, Jaquemet et al. 2007, Monticelli et al. 2007), although some *Corresponding author. Email: teresa_catry@yahoo.com