A comparative study of the breeding and feeding ecology of three sympatric tropical terns on the Houtman Abrolhos, Western Australia

A comparative study of the feeding and nesting ecology of three sympatric tropical terns was carried out on Pelsaert Island, Houtman Abrolhos, Western Australia (28°56’S, 113°58’E) between 1993 and 1996. Lesser Noddies (Anous tenuirostris), Brown Noddies (A. stolidus) and Sooty Terns (Sterna fuscata) bred annually and simultaneously, but started laying at different times in the years studied. The two noddies both had protracted laying periods with considerable overlap. Sooty Terns had a shorter laying period, that usually started later. All three species were more synchronised in 1995 and, particularly, during 1996. The timing of breeding of the Lesser and Brown Noddy was significantly related to the strength of the warm, tropical Leeuwin Current. The three species partitioned the nesting resources both horizontally and vertically. Sooty Terns invariably nested on the bare ground, usually below bushes, whereas the Brown Noddy nested in bushes and the Lesser Noddy in trees. Both noddies constructed nests of weed or sticks, differences in their design being attributable to the stability of their nesting habitat. Sooty Tern nestlings were highly mobile at an early age and did not recognise edges. In contrast, the young of both noddy species were immobile, with behavioural traits appropriate to tree-nesting. Other differences in nesting behaviour may reflect the influence of aerial and ground predators. Potentially, competition for food could become intense in an area where most similarly sized, surface-feeding terns are raising young at the same time. However, the food consumed by these terns around the Houtman Abrolhos appeared partitioned in time and space. The diets of the Lesser and Brown Noddy were dominated by small (< 80mm), diurnal fish larvae characteristic of continental shelf waters. In contrast, Sooty Terns consumed prey that were more readily available at night and typical of prey that inhabit oceanic waters. Some, but not all, Sooty Terns appeared to forage at night. The very slight overlap in the diets of Sooty Terns and the two noddy species appeared a consequence of differences in foraging area and time, rather than differences in foraging technique or adult size. In contrast, the diets of Brown and Lesser Noddies overlapped considerably, particularly during the early stages of breeding when both depended heavily on Beaked Salmon larvae. On average, the Brown Noddy took longer Beaked Salmon than the Lesser Noddy. In November of each year, the Lesser Noddy changed from almost exclusive dependence on Beaked Salmon to a diet with a large proportion of Black-spotted Goatfish, whereas the diet of the Brown Noddy remained unchanged. Possible reasons for these differences are explored. Direct observations at the foraging areas used by each tern confirmed the inferences from analysis of regurgitates. Sooty Terns fed 480-600 km to the north of the Houtman Abrolhos, off the shelf edge. In contrast, the two noddies foraged in areas to the southwest and north-west of the Pelsaert Group, within 180 km of the colony, most frequently in waters between the Houtman Abrolhos and the continental shelf edge. Records of activity patterns, flight paths and the times of nest changeovers were consistent with those at sea. Lesser Noddies appear to be strictly diurnal, offshore foragers during the breeding season, Brown Noddies also appear mainly diurnal, offshore foragers but Sooty Terns appear diurnal/nocturnal, pelagic foragers. Breeding by Sooty Terns, Brown Noddies and Lesser Noddies on Pelsaert Island coincided with the arrival of abundant larval fish, a lower incidence of storm fronts and rainfall, and regular southerly winds. Breeding was sometimes delayed in all species, apparently in relation to the strength of the flow of the Leeuwin Current during the previous austral winter. Differences in current flow may alter the supply of larval fish at the Houtman Abrolhos through dispersal, or the effects of warmer water on spawning fishes