Tropical Terns Research Articles

Diet of tropical Roseate Tern chicks on Aride Island and the role of local oceanographic conditions and age of chicks on food provisioning

Small seabirds are very sensitive to changes in the availability of prey and thus may be good indicators of changes in species composition and availability of prey. However, long-term data on dietary variation is scarce for tropical tern species. This paper assembles 9 years of data on deliveries of fish prey to Roseate Tern chicks of known-age on Aride Island, the Seychelles, in the western Indian Ocean, to examine: (1) the role of intrinsic (age of chicks) and extrinsic (local oceanographic conditions) factors on chick food provisioning, and (2) the temporal variation in intake rate of chicks. Our results show low annual variability in dietary composition: 75–97% of all ingested items were goatfish (Mullidae), which agrees with previous studies showing that these form the main prey fish for the Seychelles seabird community. Intake rate of chicks varied considerably among years, from 0.86 g fish ingested chick−1 h−1 in 1997 to 3.12 g chick−1 h−1 in 2005, with poor years showing a much higher coefficient of variation than good years. Intake rate of chicks was significantly correlated with local chlorophyll-fl concentration and sea-surface temperature of the preceding 2 weeks. This suggests that food provisioning of tropical Roseate Tern chicks is sensitive to fluctuations in local oceanographic conditions that affect the availability of prey fish at a short temporal scale, and that Roseate Terns are suitable sentinel organisms to indicate local changes in environmental conditions.

Nestling escape behaviour in tree, bush and ground nesting tropical terns

Nestling escape behaviour in tree, bush and ground nesting tropical terns

Annual variation in laying date and breeding success of Brown Noddies on Aride Island, Seychelles

We examined annual variation in timing of breeding, productivity, growth-rates of chicks and adult body condition of Brown Noddies (Anous stolidus) on Aride Island, Seychelles, from 1995 to 2002 (8 years), and assessed whether poor breeding success was related to El Niño events. Our results were compared with similar studies on tern species that feed more in inshore waters and with faster chick growth, the Lesser Noddy (A. tenuirostris) and the Roseate Tern (Sterna dougallii), to evaluate the extent of variation in laying date and breeding performance of tropical terns in the western equatorial Indian Ocean. Most of the Brown Noddy population laid eggs between late May and late June in most years. Breeding success varied substantially between years. Productivity of < 0.15 chicks per breeding pair occurred in three out of the eight study years. The years with poorest breeding success were El Niño (1997 and 2002) and La Niña (1999) years. Chick mass at hatching was significantly correlated with an egg-volume index, but there were no correlations between size of eggs and either hatching date or linear growth-rate. Both adult mass and body condition were significantly lower in an El Niño year (1997), when birds laid later and were less successful, than in non-El Niño years (1995 and 1996). Overall, our data suggest strong variations in food availability among years. The productivity of Brown Noddies was significantly correlated with that of Lesser Noddies, but not with that of Roseate Terns, which suggests that similar ecological forces may influence the breeding of the two noddy species.

Comparative foraging ecology of five sympatric terns at a sub‐tropical island in the eastern Indian Ocean

AbstractOver one million pairs of seabirds breed annually on the Houtman Abrolhos island group, off the mid‐western coast of Australia, the largest seabird breeding station in the eastern Indian Ocean. Eight of the 13 species that breed annually on Pelsaert Island are terns. Dietary samples (regurgitates) were collected from the five most numerous tern species from 1993 to 1999. The prey items identified were related to observations of foraging seabirds around the island. The largest species studied, the crested tern Sterna bergii, foraged mainly for reef fish over shallow reef flats near the breeding islands, as well as for schooling clupeid fish over coastal shelf waters. The smallest species, the roseate tern S. dougallii, also foraged within sight of its colonies, but over deeper waters than crested terns. Three larval fish species characteristic of continental shelf waters (a gonorhynchid, a goatfish and a bellowfish) figured prominently in the diets of roseate terns and three other tern species studied. Of these, only the sooty tern S. fuscata also ate squid and lanternfishes, foraging farthest (480–600 km) from its breeding island, off the shelf edge. The lesser noddy Anous tenuirostris and brown noddy A. stolidus took very similar taxa of prey and also overlapped considerably in their foraging ranges, about 180 km from their colonies, between the islands and the continental shelf edge. However, the lesser noddy took smaller fish than did the larger brown noddy. Although these five abundant tropical tern species, that bred sympatrically, showed some segregation in their diets and, more clearly, in their foraging ranges, considerable overlap of both aspects remained.

An experimental investigation of the effects of human intrusion into breeding colonies of Bridled Terns Sterna anaethetus in the Great Barrier Reef

Many studies have demonstrated that recreational visitors impede the breeding success of nesting seabirds, although recently some authors challenged the view that human visitation is always detrimental to the breeding birds. This emerging lack of consensus, as well as the fact that almost all disturbance studies to date were conducted in habitats and on species of high latitudes, presents a challenge to managers of Australia's Great Barrier Reef World Heritage Area. The present study used an experimental approach to examine the impact of recreational visitors on aspects of the reproductive biology of a tropical tern, the Bridlled Tern, Sterna anaethetus, at Rocky Islets in the Far Northern Section of the Great Barrier Reef World Heritage Area. Two levels of human disturbance schedules were implemented over five experimental sites and over three breeding seasons. The dependent variables were hatching success, body mass and bill length of chicks at both 12-13 and 21-22 days of age. Chicks at 12-13 days of age were significantly heavier and hatching success was higher at sites exposed to higher level disturbance than at sites exposed to lower level disturbance. The results suggest that the early development of Bridled Tern chicks may be adversely affected by non-lethal human disturbance and that such adverse effects may be ameliorated by habituation to human intrusion into nesting areas.

The Breeding Biology of the Lesser Noddy on Pelsaert Island, Western Australia

SummaryThe Lesser Noddy Anous tenuirostris (melanops) is a small, dark-plumaged, tropical tern which breeds only on three islands in the Houtman Abrolhos, off Western Australia, where it nests in mangrove trees. On Pelsaert Island, in 1991, the single egg clutches were laid over a 102day period from late August to early December, peaking in September. The incubation period averaged 34 days and the fledging period 40 days. Overall, 60% of eggs hatched and 47% gave rise to free-flying young. Storms caused extensive egg loss from the more exposed nests; some pairs laid replacement eggs. Birds laying earlier in the season tended to use more sheltered sites and, consequently, had a higher reproductive success than those laying later and in more exposed areas.

Lead, mercury, and cadmium in feathers of tropical terns in Puerto Rico and Australia

Levels of lead, cadmium, and mercury were examined in breast feathers of terns nesting on offshore islets near Culebra, Puerto Rico and on Michaelmas Cay and Heron Island on the Great Barrier Reef, Australia. Levels of all metals in these tropical terns were predicted to be lower than those of terns nesting in temperate regions, because the tropical species feed offshore of non-industrial areas where contamination should be less than for temperature-nesting species that feed in inshore estuaries near industrialized areas. This prediction was not supported by the evidence. In Puerto Rico, lead and cadmium levels were highest in bridled tern (Sterna anaethetus), and mercury levels were highest in sooty (S. fuscata) and roseate tern (S. dougallii). In Australia, levels of lead and mercury were higher in black noddy (A. minutus) and lower for sooty tern; and cadmium levels were highest for brown noddy (A. stolidus) and sooty tern, and lowest for black noddy. Metal levels for the tropical terns nesting in Puerto Rico and Australia generally were not lower than levels reported for temperate-nesting or mainland nesting birds (except for mercury in Australia).

Body Size, Prey Size, and Ecological Segregation in Five Sympatric Tropical Terns (Aves: Laridae)

Body Size, Prey Size, and Ecological Segregation in Five Sympatric Tropical Terns (Aves: Laridae)

Body Size, Prey Size, and Ecological Segregation in Five Sympatric Tropical Terns (Aves: Laridae)

Collection and analysis of food samples permitted comparison of the diets of five species of terns that occur together on Christmas Island, Pacific Ocean. All the terns feed mainly while flying, catching fish and squid chased to the surface by predatory fish. Sterna fuscata and Anous stolidus, which are of equal size, catch prey of similar kinds and sizes, but S. fuscata can exploit feeding areas further from land, and is more numerous. Gygis alba and especially Anous tenuirostris are smaller and eat smaller prey. Gygis eats many fish and squid of kinds not important to A. tenuirostris, and may catch them at different times. Procelsterna cerulea, the smallest species, eats mainly fish larvae and tiny inveltebrates. The three most closely related species, A. stolidus, A. tenuirostris, and Procelsterna, differ mainly in size. The ratios of their weights are 3.8: 2.0: 1, but the ratios of the mean volumes of the largest 2 per cent of their fish prey are 30: ca. 7 : 1. The reasons for the discrepancy are discussed. The value of data on bill length, bill thickness, and body size in assessing the extent of competition for food among bird species is considered. It is argued that the phenomenon of ecological segregation need not indicate the operation of competitive exclusion, and it is suggested that many natural habitats cannot be assumed to contain the greatest number of related species that could coexist there indefinitely if the opportunity to establish themselves arose.

Experiments on Homing 1

PROF. J. B. WATSON and Dr. K. S. Lashley have made some important experiments at Bird Key, in the Tortugas, on the homing capacity of the noddy tern (Anous stolidus) and the sooty tern (Sterna fuliginosa), which breed there in large numbers. The island is peculiarly suitable for the purpose, since it marks the northern limit of the migration of these two tropical terns (so that, if the birds are experimentally transported further north they find themselves in regions which they have not previously visited); moreover, on the westward side there is only the open water of the Gulf of Mexico until the shore-line of Texas is reached, Galveston being 855 statute miles distant. “This strip of open water proves a magnificent route for homing experiments.” The authors caught terns at their nesting-places, put individual marks of paint on their head and neck, tied a small tag recording the date, locality, and marking round the neck, fixed a larger duplicate tag beside the nest, transported the birds in large cages to a distance, liberated them, and watched for their return. The general result is of great interest:—“The noddy and sooty terns can return from distances up to 1000 miles in the absence of all landmarks, at least so far as the term landmark is understood at present.”