Herring and Sprat Abundance Indices Predict Chick Growth and Reproductive Performance of Common Terns Breeding in the Wadden Sea

Herring (Clupea harengus) and sprat (Sprattus sprattus) are the key prey resources of common terns (Sterna hirundo) breeding in the Wadden Sea. Breeding success of the terns has been below average since 2002, coinciding with exceptionally low herring recruitment and sprat abundance. Time series of herring and sprat abundance in the North Sea and in the Wadden Sea were analyzed to explain long-term breeding success and chick development at two common tern breeding colonies. North Sea herring recruitment and sprat abundance in the Wadden Sea explained the largest part of common tern breeding success, both as single variables and in a multiple regression approach. Breeding success showed stronger correlations with herring recruitment indices derived from the North Sea region compared to the Wadden Sea. Also, herring and sprat abundance data explained more variability in breeding success than of more directly responding measures such as growth rate and maximum weight of chicks. Despite spatial and temporal incoherences between fish surveys and the common tern breeding season, breeding success of common terns reflected the abundance of their key prey fish beyond their foraging range and breeding season. We argue that the ecological connectivity between large- and small-scale herring abundance and the responsiveness of common tern breeding success is strong enough to establish a fish–seabird indicator system to be potentially valuable in monitoring and conservation.     

Invasive predators and the conservation of island birds: the case of American Mink Mustela vison and terns Sterna spp. in the Western Isles,Scotland

Capsule Colonies were larger and breeding success lower in mink-inhabited areas.

Aims To examine the impact of mink on dense aggregations of ground-breeding seabirds on islands previously isolated from mammalian predators.

Methods We compare 1990–93 tern breeding data with records before and after mink arrival.

Results In the early 1990s, terns showed no preference for mink-free islands. The breeding behaviour and success of tern colonies in mink-inhabited areas did not significantly differ from that in mink-free areas. However, colonies were larger and breeding success lower in mink-inhabited areas compared to mink-free areas, trends which might reflect longer term mink impact.

Conclusions As mink spread south there seems to be a gradual increase in tern colony size affecting Lewis, then Harris and, recently, the Sound of Harris.     

Aggressive neighbors and dense nesting: nest site choice and success in high-Arctic common eiders

Minimizing the risk of nest predation has led some bird species to exploit the nest defense behavior of other species. At Nasaruvaalik Island, Nunavut, Canada, some common eiders (Somateria mollissima borealis) nest within the boundaries of Arctic tern (Sterna paradisaea) colonies, while others nest elsewhere on the island, away from the terns. We tested the effects of location (within vs. outside the tern colonies), density of common eider nests, and annual variation on the nesting parameters of common eiders. Our results suggest that nesting in association with Arctic terns does not confer an obvious benefit to eiders. Such associative nesting of eiders and terns may be the result of overlapping habitat preferences between the two species, or a general scarcity of suitable nesting habitat for ground-nesting species in the high Arctic. However, eiders nesting in higher densities with other eiders had greater nest success and lower total clutch predation, indicating a positive correlation between nest density and success.     

Black tern (<Emphasis Type="Italic">Chlidonias niger</Emphasis>) breeding site abandonment in U.S. Great Lakes coastal wetlands is predicted by historical abundance and patterns of emergent vegetation

Breeding colonies of black terns (Chlidonias niger) have become increasingly rare in U.S. Great Lakes coastal wetlands since the mid-twentieth century, with an almost 90% decline in the number of active colony sites since 1991. Although the specific causes of this wetland species’ decline are unknown, habitat loss and degradation are thought to be a major barrier to conservation. Using data from the Great Lakes Colonial Waterbird Survey, we took a unique regional and historical approach to investigate the relationship between black tern colony site abandonment and a suite of local and landscape-scale habitat features in U.S. Great Lakes coastal wetlands. We employed logistic regression models and a combination of stepwise selection procedures to identify the best predictive model for black tern colony abandonment. According to the selected model, breeding colonies with fewer nests were more likely to be abandoned over the following decadal observation period than breeding colonies with more nests. Colony sites were also more likely to be abandoned when vegetation within the wetland shifted towards larger, denser clusters. We performed a simulation study that showed that failing to account for association between observations from the same site affected model selection results, but that cross-validation error for the selected model remained low unless site effects were very strong. Results of this study suggest that focus on protection of sites harboring large numbers of black terns and vegetation management will help limit further colony abandonments.     

Evidence for genetic differentiation among Caspian tern (Hydroprogne caspia) populations in North America

The Caspian tern (Hydroprogne caspia) is a globally distributed seabird that breeds throughout North America, generally in low numbers. Many colonies are threatened by habitat loss and pollution. Additionally, adult terns compete directly with salmonid stocking programs on the west coast, where a large proportion of the fish they feed their young are stocked salmon smolts. North America colonies have been classified into five ‘breeding groups’ based on banding data and geography. To help delineate effective management units, we characterized variation in mitochondrial DNA (488 base pair fragment of cytochrome b) and five microsatellite loci among 111 terns from six sites representing three of the North American breeding areas. We found significant range-wide population differentiation (cytochrome b: global Φ_ST = 0.12, P < 0.01; microsatellites: global F_ST = 0.094, P < 0.001). Pacific Coast sites differed genetically from sites east of the Rocky Mountains, and sites in Central Canada differed from those in the Great Lakes region. Gene flow among these three regions appears to be restricted. Thus, our results indicate that at least three of the breeding regions delineated using banding data and geography should be treated as separate management units.     

Breeding biology of Saunders’s tern (Sterna saundersi) in the Farasan Islands,Kingdom of Saudi Arabia

Saunders’s terns (Sternula saundersi) are a small, ground-nesting marine bird species that have a massive rearing range, including the shores and islands of Asia and Africa adjacent to the north Indian Ocean. Despite occupying a large breeding range, little is known about the breeding ecology of this species. This research explored aspects of Saunders’s terns’ breeding ecology and predation rate in 2013 on the Farasan Islands of Saudi Arabia. The outcomes confirm that the mean clutch size of a Saunders’s tern was 1.77 ± 0.08 (n = 31) eggs per clutch and the mean egg size was 31.05 × 23.15 mm. The results of this study show a remarkable relationship between clutch size and egg volume and length (p = .002, p = .004, respectively). Predation was the major reason for nest damage (62.5%). Evidence from cameras at nests showed that the predators of Saunders’s tern nests on the Farasan Islands were white-tailed mongoose (Ichneumia albicauda) and Egyptian vultures (Neophron percnopterus). This is the first study on the breeding ecology of Saunders’s terns, and it shows that predator control is essential to the existence of the species. The results of this study suggest that fencing some breeding sites may help to minimize human disturbance and decrease the risk of nest predation from mammalian predators. Further research is needed to compare the predation rates on the mainland and islands and to develop efficient strategies to conserve this ground-nesting species.     

Use of sandbanks by terns in Queensland,Australia: a priority for conservation in a popular recreational waterway

Many terns (Laridae) from the northern hemisphere overwinter in Australia, but they are rarely covered comprehensively in bird counts in Australian coastal waterways. We conducted censuses over 3 years on terns in an increasingly popular recreational waterway at the northern end of Moreton Bay, known as Caloundra sandbanks, to provide the first overview of the relative abundance of non-breeding tern species on estuary-type sandbanks in Australia. The maximum number of birds counted was > 43,000, of which over 90% belonged to six species of terns. The most abundant tern species was the common tern Sterna hirundo longipennis which reached > 38,000 in one Austral summer. Other northern-breeding migrant terns were the white-winged black tern Chlidonias leucopterus ( > 17,000) and the little tern S. albifrons sinensis ( >11,000), which also breeds in Australia. These migrant terns arrived at Caloundra sandbanks at around late-October and a vast majority had departed by mid-April. Numbers were generally highest from late-December through to early-April. An additional three Australian-breeding terns of smaller population sizes frequented the sandbanks. Density of terns using the habitats could reach a peak of 600 ha⁻¹ during the warmer months of the year, coinciding with peak human recreational activities. In light of the results, Caloundra sandbanks should be regarded as an Area of International Importance based on criteria contained in the Ramsar Convention. The situation at Caloundra sandbanks presents challenges to recreational and tourism issues, requiring review of traditional government priorities in the conservation of wetlands based primarily on waders.     

The role of saltwort (Batis maritima L.) in regeneration of degraded mangrove forests

While saltwort (Batis maritima L.) is common in the fringe mangrove forests of southwest Florida, its role in regeneration of degraded mangrove communities is not known. Given the potential encroachment and subsequent degradation of mangrove communities by sea-level rise, it is important to quantify the effect of early-colonizing vegetation to early mangrove seedling survival. A greater number of mangrove seedlings were observed in existing B. maritima patches compared to surrounding mudflats. A planting experiment was designed to determine whether B. maritima was responsible for the observed pattern. Black mangrove (Avicennia germinans L.) seedlings, raised in a nursery, were planted in previously established B. maritima patches and on mudflats with and without nursery-raised B. maritima. There was significantly lower mortality of A. germinans seedlings when planted in existing B. maritima patches (69%), compared to seedlings planted on the mudflats (93%), demonstrating that existing B. maritima improved A. germinans seedling survival. Nursery-raised B. maritima had lower mortality on open mudflats (28%), suggesting that it can tolerate conditions, which make it an early colonizer of newly available habitats. The primary mechanism proposed for improving seedling success is a slight increase in elevation provided by the dense root network of established B. maritima. These findings have implications for scientists and managers anticipating the response of mangroves to sea-level rise.     

Populations and trends of Canadian Arctic seabirds

Canada’s eastern Arctic (Nunavut and Arctic Quebec—Nunavik, N of 60°) supports large numbers of seabirds in summer. Seabird breeding habitat in this region includes steep, rocky coasts and low-lying coasts backed by lowland sedge-meadow tundra. The former areas support colonial cliff- and scree-nesting seabirds, such as murres and fulmars; the latter inland or coastal seabirds, such as terns, gulls and jaegers. The region supports some 4 million breeding seabirds, of which the most numerous are thick-billed murres (Uria lomvia; 75%), black guillemots (Cepphus grylle; 9%), northern fulmars (Fulmarus glacialis; 8%) and black-legged kittiwakes (Rissa tridactyla; 6%). The majority of Arctic seabirds breed in a small number of very large colonies (>10,000 birds), but there are also substantial numbers of non-colonial or small-colony breeding populations that are scattered more widely (e.g. terns, guillemots). Population trends among Canadian Arctic seabirds over the past few decades have been variable, with no strongly negative trends except for the rare ivory gull (Pagophila eburnea): this contrasts with nearby Greenland, where several species have shown steep declines. Although current seabird trends raise only small cause for concern, climate amelioration may enable increased development activities in the north, potentially posing threats to some seabirds on their breeding grounds.     

Population structure in the South American tern Sterna hirundinacea in the South Atlantic: two populations with distinct breeding phenologies

The South American tern Sterna hirundinacea is a migratory species for which dispersal, site fidelity and migratory routes are largely unknown. Here, we used five microsatellite loci and 799 bp partial mitochondrial DNA sequences (Cytochrome b and ND2) to investigate the genetic structure of South American terns from the South Atlantic Ocean (Brazilian and Patagonian colonies). Brazilian and Patagonian colonies have two distinct breeding phenologies (austral winter and austral summer, respectively) and are under the influence of different oceanographic features (e.g. Brazil and Falklands/Malvinas ocean currents, respectively), that may promote genetic isolation between populations. Results show that the Atlantic populations are not completely panmictic, nevertheless, contrary to our expectations, low levels of genetic structure were detected between Brazilian and Patagonian colonies. Such low differentiation (despite temporal isolation of the colonies) could be explained by demographic history of these populations coupled with ongoing levels of gene flow. Interestingly, estimations of gene flow through Maximum likelihood and Bayesian approaches has indicated asymmetrical long term and contemporary gene flow from Brazilian to Patagonian colonies, approaching a source–sink metapopulation dynamic. Genetic analysis of other South American tern populations (especially those from the Pacific coast and Falklands–Malvinas Islands) and other seabird species showing similar geographical distribution (e.g. royal tern Thalasseus maximus), are fundamental in gaining a better understanding of the main processes involved in the diversification of seabirds in the southern hemisphere.     

Increase in Alphaproteobacteria in association with a polychaete,Capitella sp. I,in the organically enriched sediment

We conducted bioremediation experiments on the organically enriched sediment on the sea floor just below a fish farm, introducing artificially mass-cultured colonies of deposit-feeding polychaete, Capitella sp. I. To clarify the association between the Capitella and bacteria on the efficient decomposition of the organic matter in the sediment in the experiments, we tried to identify the bacteria that increased in the microbial community in the sediment with dense patches of the Capitella. The relationship between TOC and quinone content of the sediment as an indicator of the bacterial abundance was not clear, while a significant positive correlation was found between Capitella biomass and quinone content of the sediment. In particular, ubiquinone-10, which is present in members of the class Alphaproteobacteria, increased in the sediment with dense patches of the Capitella. We performed denaturing gradient gel electrophoresis (DGGE) analyses to identify the alphaproteobacterial species in the sediment with dense patches of the worm, using two DGGE fragments obtained from the sediment samples and one fragment from the worm body. The sequences of these DGGE fragments were closely related to the specific members of the Roseobacter clade. In the associated system with the Capitella and the bacteria in the organically enriched sediment, the decomposition of the organic matter may proceed rapidly. It is very likely that the Capitella works as a promoter of bacteria in the organically enriched sediment, and feeds the increased bacteria as one of the main foods, while the bacteria decompose the organic matter in the sediment with the assistance of the Capitella.     

Toxic coral gobies reduce the feeding rate of a corallivorous butterflyfish on Acropora corals

The obligate coral-dwelling gobiid genus Gobiodon inhabits Acropora corals and has developed various physiological, morphological and ethological adaptations towards this life habit. While the advantages of this coral-fish association are well documented for Gobiodon, possible fitness-increasing factors for the host coral are unknown. This study examines the influence of coral-dwelling gobies on the feeding behaviour of obligate corallivorous butterflyfishes. In an aquarium experiment using video observation, the corallivorous butterflyfish Chaetodon austriacus fed significantly less on corals inhabited by two Gobiodon species compared to unoccupied coral colonies of similar size. The more agonistic species G. histrio, which mostly displayed directed movements towards butterflyfishes, decreased butterflyfish bite rate by 62–98 % compared to uninhabited colonies. For Gobiodon sp. 3, which mostly displayed undirected movements in response to visits by C. austriacus, bite rate reduction was 64–68 %. The scale-less skin of Gobiodon spp. is covered by mucus that is toxic and multi-functional by reducing predation as well as affecting parasite attachment. A choice flume experiment suggests that the highly diluted skin mucus of Gobiodon spp. also functions as a corallivore repellent. This study demonstrates that Gobiodon spp. exhibit resource defence against coral-feeding butterflyfishes and also that coral colonies without resident Gobiodon suffer higher predation rates. Although the genus Gobiodon is probably a facultative corallivore, this study shows that by reducing predation on inhabited colonies by other fishes, these obligate coral-dwellers either compensate for their own fitness-decreasing impact on host colonies or live in a mutualistic association with them.     

Little tern breeding success in artificial and natural habitats: modelling population growth under uncertain vital rates

As a consequence of habitat loss, breeding in man-made habitats has become increasingly common for many coastal breeding bird species. While artificial sites provide valuable substitutes, they may also be more attractive, and importantly, differ in quality from natural sites. Therefore, information on habitat specific breeding success and their potential for supporting stable populations are needed. We compared little tern (Sternula albifrons) breeding success (nest and hatching success) between natural habitat (sandy beaches) and artificial port habitat at Bothnian Bay, Finland from 2006 to 2011. We further reviewed published estimates on pre-fledging and adult survival for little terns and least terns (Sternula antillarum), and used these ranges to estimate plausible parameter spaces for population growth rates given our estimates of breeding success. Nest success was among the highest reported for little terns in the artificial habitat (82 %) while being lower in the natural habitat (58 %). This difference may have resulted from differences in colony sizes and levels of disturbance. Hatching success did not differ significantly, but the percentage of successful nests containing unhatched eggs was twice as high in the natural habitat. The parameter spaces for population growth rates indicated that the artificial habitat has good potential to sustain stable populations (66 % positive growth rate) while for the natural habitat this potential was lower (37 % positive growth rate). While our results suggest that artificial habitats can be very productive breeding sites for habitat deprived tern populations, management should concentrate on improving both habitats with emphasis on natural sites.     

Predator–prey interactions between the South Polar skua Catharacta maccormicki and Antarctic tern Sterna vittata

Antarctic terns have to co‐exist in a limited space with their major nest predator, the skuas. We conducted artificial nest experiments to evaluate the roles of parental activity, nest location and nest and egg crypsis in this simple predator–prey system. Predation on artificial (inactive) nests was higher in traditional nesting sites than in sites previously not occupied by terns, which suggests that skuas memorized past tern breeding sites. Predation on artificial nests in inactive colonies was higher than in active (defended) colonies. Parental defense reduced predation in colonies to the level observed in artificial nests placed away from colonies. This suggests that communal defense can balance the costs of attracting predators to active colonies. Within colonies, predation was marginally higher on experimental eggs put in real nests than on bare ground. Although it seems that the presence of a nest is costly in terms of increased predation, reductions in nest size might be constrained by the need for protective nest structures and/or balanced by opposing selection on nest size. Predation did not differ markedly between artificial (quail) and real tern eggs. A simultaneous prey choice experiment showed that the observed predation rates reflected egg/nest detectability, rather than discrimination of egg types. In summary, nesting terns probably cannot avoid being detected, and they cannot defend their nest by attending them. Yet, by temporarily leaving the nest, they can defend it through communal predator mobbing, and at the same time, they can benefit from crypsis of unattended nest and eggs.     

A control taste aversion experiment on predators of roseate tern (Sterna dougallii) eggs

European starlings (Sturnus vulgaris) are responsible for high rates of egg predation at one of the main colonies of the endangered roseate tern (Sterna dougallii) in the Azores archipelago. Control taste aversion has been effective in controlling raven predation in a colony of California least tern (Sterna antillarum browni), but there is little quantitative information about its efficacy on other species of predators taking eggs. We conducted a control taste aversion experiment on yellow-legged gulls (Larus michahellis) and European starlings eating eggs of terns in a mixed colony of common (Sterna hirundo) and roseate terns in the Azores. We treated quail (Coturnix coturnix) eggs with methiocarb and deployed them in artificial nests in the tern colony. On the first experiment, conducted before the terns laid eggs, predation rates on quail eggs by yellow-legged gulls showed significant and rapid decrease after deployment of treated eggs. During the second experiment, after the terns had started laying, results were mixed. Although predation rates by European starlings on treated quail eggs decreased, predation rates on tern eggs did not. We conclude that control taste aversion using methiocarb-treated eggs is likely to reduce depredation by gulls but not starlings because of the need to pre-train the birds and the tendency of starlings to be attracted by the movement of adults, not the presence of nests.     

Heterozygosity correlates with body size,nest site quality and productivity in a colonial waterbird,the whiskered tern (Chlidonias hybrida,Aves: Sternidae)

Positive effects of individual heterozygosity on naturally selected traits have been reported in wild populations of many animal taxa. The aim of this study was to test whether heterozygosity predicts the quality of acquired nest sites and productivity in a colonially breeding waterbird, the whiskered tern (Chlidonias hybrida). For this purpose, 40 adult terns from a small, recently established population in Central Poland were typed at eight microsatellite loci. We demonstrate that individual heterozygosity is positively related to hatching success. We hypothesize that this association could be mediated by direct effects of heterozygosity on the competitive abilities of individuals. We found that more heterozygous terns tended to breed in better protected central parts of the colony, suggesting that they had capabilities of outcompeting less heterozygous individuals and relegating them to the less attractive peripheries of the colony. It was also demonstrated that the link between heterozygosity and individual abilities to acquire more attractive nest site could be mediated by the larger size of heterozygous individuals. Although no correlations between heterozygosity and different components of condition were found, there was a positive association between female heterozygosity and both clutch size and egg size. We suggest that demonstrated heterozygosity‐fitness correlations could be primarily caused by inbreeding depression in the studied whiskered tern population.     

Modelling the occurrence and abundance of a colonial species,the arctic tern Sterna paradisaea in the archipelago of SW Finland

Knowledge of the habitat requirements and suitable breeding areas of sea birds is crucial for their management and conservation. However, there are still few studies that have modelled the breeding distribution and abundance of colonial sea birds. In this study, we created predictive distribution models for a colonial species, the arctic tern Sterna paradisaea, using 14 environmental variables calculated for 525 islands in the Archipelago Sea in SW Finland. We modelled the occurrence (presence‐absence) using generalised additive models (GAMs) and abundance (pair numbers/colony size) using hurdle models fitted with GAM. We tested for spatial autocorrelation in model residuals and evaluated the models on independent data. Critical factors influencing the occurrence of the arctic tern were the proportions of boulder or gravel and forest of island area, as well as island maximum elevation and area, such that the species seemed to prefer large and low islands with sparse vegetation. Abundance was influenced by the proportions of boulder or gravel and bare rock of island area, as well as exposure and island area. To some extent, different factors influenced the occurrence and the abundance. The evaluation results of the models were good, with an AUC value of 0.91 for the most accurate presence‐absence model and a Pearson's correlation coefficient of 0.60 for the most accurate hurdle model. The predictive ability of the models increased when we removed islands with single or few breeding pairs from the data set. Although the hurdle models did not produce accurate pair number estimates, they indicated which islands are suitable for larger colonies. Abundance is a crucial factor for colonial species. This modelling technique can therefore be of great value for the conservation and management of the arctic tern and similar colonial species.     

Plant-water regime management in a wetland: consequences for a floating vegetation-nesting bird, whiskered tern Chlidonias hybridus

In this study we investigated the interplay between water level management, floating macrophytic vegetation and nesting whiskered tern (Chlidonias hybridus) during 8 years (1995–2002) at a shallow macrophyte-dominated lake in western France. The specific question was to see if slight increases in the water regime of the lake (three scenarios), as part of a restoration programme, affect the timing of nesting and occupation of colonies by whiskered terns by way of changes in biomass of waterlily (Nymphaea alba) beds where colonies always establish. During the study period, egg-laying dates were progressively delayed up to 40–50 days (mid-May to early July) in relation to late appearance of suitable waterlily biomass (R ² = 0.67, p = 0.01) associated with high spring water levels (1.10–1.25 m vs. 0.95 m). Simultaneously some nesting attempts were observed in sub-optimal habitats (adjacent wet grasslands) in high flooding conditions leading either to poor breeding success or colony desertion. We concluded that there is a need for compromise between the experimental water levels used to control waterlily biomass and the nesting requirements of the whiskered tern population in this lake of international importance (260–510 pairs in 1995–2002, ≥ 1% Ramsar level).     

Spatial-concentration effects and the importance of local enhancement in the evolution of colonial breeding in seabirds

The information center hypothesis (ICH) suggests that birds breed in colonies because this behavior favors information exchange at the colony about the location of food patches. However, the complex suite of behaviors the ICH requires implies that information center following is more likely to evolve after colonial breeding has become established than to promote its initial development. A simpler hypothesis to explain the evolution of colonial breeding is that coloniality concentrates foragers in space, which leads to more rapid discovery of food patches and, by means of local enhancement, more efficient transfer of information about the location of patches than if foragers bred in a dispersed fashion. To assess the effects of breeding dispersion on foraging success, I simulated the foraging behavior of cliff-breeding seabirds (nesting either solitarily or colonially) searching for patchily distributed prey. Results show that colonial breeding is favored when food patches are sufficiently large or short-lived that competition for food is ameliorated. Conversely, dispersed nesting is favored when patches are small or long-lived. Individuals playing a colonial breeding strategy can invade a population of solitarily breeding birds, and once a colonial breeding strategy becomes established, it generally is resistant to invasion. These findings suggest that the spatial-concentration model is a plausible mechanism for the initial development of coloniality.     

Feral cat diet and impact on sooty terns at Juan de Nova Island, Mozambique Channel

Feral cat Felis catus predation on seabirds has been well documented; however, details regarding shifts in feral cat diet in relation to seabird availability, seabird predation rate and impact on seabird population dynamics are scarce. Here, we present data documenting a seasonal shift in feral cat diet at Juan de Nova Island, Mozambique Channel. We also quantify sooty tern Sterna fuscata predation by feral cats and examine the impact on sooty terns over both the short term (by removing individual cats from sub-colonies) and over the longer term by highlighting their influence on population growth rate ( λ ) using a deterministic matrix model. Cat diet shifted dramatically from insects, rats and mice outside the tern breeding season to primarily terns when terns were breeding. The predation rate of sooty terns at Juan de Nova was estimated at 5.94 terns cat⁻¹ day⁻¹, with a proportion of these (22%) being killed without being consumed ('surplus kills'). When only one cat was removed from each sub-colony, tern predation declined tenfold in the short term. From our matrix model, the annual growth rate for sooty terns was 1.01 in the absence of cat predation. It remained above one until a predation impact equivalent to approximately three times the estimated cat density (12.04 per km²) was incorporated. Our results demonstrate that cats preferentially predate and have an impact on breeding sooty terns at Juan de Nova, and that an increase in cat density could lead to negative effects on population growth, despite the large breeding tern population.