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Reproductive Success of Red-Billed Tropicbirds (Phaethon aethereus) on St. Eustatius, Caribbean Netherlands
Abstract
The daily nest-survival rates of Red-billed Tropicbirds (Phaethon aethereus) were estimated over six breeding seasons on St. Eustatius in the Caribbean. We analyzed 338 nesting attempts between 2013 and 2020. The daily survival rate (DSR) of tropicbird nests was modeled as a function of nest initiation date, sea surface temperature (SST), elevation, vegetation in front of the nest, and year. Yearly nest survival rates (± SE) of the best fitting models ranged from 0.21 ± 0.06–0.74 ± 0.13 (n = 338 nests). DSR of the most parsimonious models averaged 0.39 ± 0.04 during the incubation period, 0.83 ± 0.05 during the chick-rearing period, and 0.30 ± 0.04 during the nesting period (incubation through fledging) when data were pooled across all years. Models with linear and quadratic trends of nest initiation date combined with SST and elevation received strong support in the incubation and nesting periods. Nests initiated in peak nesting season, when SSTs were lower, had higher DSR estimates than nests initiated early or late in the season. Compared to studies of the same species from Saba and the Gulf of California, survival probability on St. Eustatius was lower during the incubation stage but higher during the chick-rearing period. Similar to populations in the Gulf of California, tropicbird reproduction differed and laying date varied among years, and survival was influenced by SST. Our results are consistent with a study on White-tailed Tropicbirds (Phaethon lepturus) in Bermuda which found that survival was affected by temporal factors rather than physical site characteristics. Our study contributes to a better understanding of the factors that influence Red-billed Tropicbird survival on a small Caribbean island.
Biotic connectivity between ecosystems can provide major transport of organic matter and nutrients, influencing ecosystem structure and productivity1, yet the implications are poorly understood owing to human disruptions of natural flows2. When abundant, seabirds feeding in the open ocean transport large quantities of nutrients onto islands, enhancing the productivity of island fauna and flora3,4. Whether leaching of these nutrients back into the sea influences the productivity, structure and functioning of adjacent coral reef ecosystems is not known. Here we address this question using a rare natural experiment in the Chagos Archipelago, in which some islands are rat-infested and others are rat-free. We found that seabird densities and nitrogen deposition rates are 760 and 251 times higher, respectively, on islands where humans have not introduced rats. Consequently, rat-free islands had substantially higher nitrogen stable isotope (δ15N) values in soils and shrubs, reflecting pelagic nutrient sources. These higher values of δ15N were also apparent in macroalgae, filter-feeding sponges, turf algae and fish on adjacent coral reefs. Herbivorous damselfish on reefs adjacent to the rat-free islands grew faster, and fish communities had higher biomass across trophic feeding groups, with 48% greater overall biomass. Rates of two critical ecosystem functions, grazing and bioerosion, were 3.2 and 3.8 times higher, respectively, adjacent to rat-free islands. Collectively, these results reveal how rat introductions disrupt nutrient flows among pelagic, island and coral reef ecosystems. Thus, rat eradication on oceanic islands should be a high conservation priority as it is likely to benefit terrestrial ecosystems and enhance coral reef productivity and functioning by restoring seabird-derived nutrient subsidies from large areas of ocean.
Breeding cavity-nesting seabirds on remote, oceanic islands are threatened largely by destruction of nest cavities and introduced predators. In response, artificial cavities are often implemented as a conservation strategy. Effective artificial cavity programs require recognition of specific nest-site characteristics, which can be major determinants of survival and persistence of cavity-nesting seabirds. In Bermuda, we monitored 158 natural and 178 artificial cavities of the White-tailed Tropicbird (Phaethon lepturus catesbyi) from 2013-2015 to determine if specific nest-site characteristics could explain cavity selection, nest survival, and predation by introduced rats (Rattus spp.) and the American Crow (Corvus brachyrhynchos). We found that tropicbirds preferred natural cavities lined with sand, and which had smaller entrances and were located on steeper cliffs. Occupancy of artificial cavities on the mainland varied across years and increased with cavity depth, whereas neither variable had any effect on artificial cavity occupancy on satellite islands. Clutch survival declined in cavities with rubble in nesting sand, and those with shallower cavity depths and larger entrance heights. Nestling survival varied by year, declined as the nesting season progressed, and increased with nestling age. Rat predation was significantly higher on mainland sites compared with satellite islands. Crow predation occurred only on offshore island sites, where natural cavities and shallower cavities with larger entrance heights were vulnerable. Rat and crow predation rates varied across years. Our results suggest multiple nest-site characteristics and temporal factors are important predictors of productivity in White-tailed Tropicbirds. Conservationists should simultaneously consider nest-cavity siting and dimensions, while they conduct biosecurity and predator control to improve the nest success of cavity-nesting seabirds.
The Seabird Breeding Atlas of the Lesser Antilles makes available data and narrative related to Environmental Protection in the Caribbean's (EPIC) census of breeding seabirds in the Lesser Antilles. The book provides information on methodology, species profiles and distribution maps and tables by species and country/territory, and site specific threats.
The Atlas reports key individual islands for breeding seabirds in the Lesser Antilles, nations or territories supporting the greatest number of globally and regionally important populations and clarifies the significance of the region in relation to Caribbean-wide populations. Species with restricted breeding ranges are also identified.
The study covered 3,162 nautical miles and over 200 islands between 2009 and 2010. Every island, islet, or cay above the high-tide level capable of supporting seabirds was surveyed by land and/or sea. Surveys were conducted over a two-year period to account for variations in breeding cycles. The only islands not surveyed by EPIC were locations for which colleagues contributed recent data.
A full seabird population survey for this region had never previously been undertaken. Existing records varied greatly from areas of intense research, to sites where data were based on anecdotal notes, sometimes dating back to the early 19th century. This ground-breaking study sheds light on numerous undocumented seabird breeding colonies, as well as colonies previously thought extirpated. The standardised methods and results will provide a baseline for the future monitoring of seabirds within the region and guide conservation priorities.
Significance
Plastic pollution in the ocean is a rapidly emerging global environmental concern, with high concentrations (up to 580,000 pieces per km ² ) and a global distribution, driven by exponentially increasing production. Seabirds are particularly vulnerable to this type of pollution and are widely observed to ingest floating plastic. We used a mixture of literature surveys, oceanographic modeling, and ecological models to predict the risk of plastic ingestion to 186 seabird species globally. Impacts are greatest at the southern boundary of the Indian, Pacific, and Atlantic Oceans, a region thought to be relatively pristine. Although evidence of population level impacts from plastic pollution is still emerging, our results suggest that this threat is geographically widespread, pervasive, and rapidly increasing.
Seabird population changes are good indicators of long-term and large-scale change in marine ecosystems, and important because of their many impacts on marine ecosystems. We assessed the population trend of the world's monitored seabirds (1950-2010) by compiling a global database of seabird population size records and applying multivariate autoregressive state-space (MARSS) modeling to estimate the overall population trend of the portion of the population with sufficient data (i.e., at least five records). This monitored population represented approximately 19% of the global seabird population. We found the monitored portion of the global seabird population to have declined overall by 69.7% between 1950 and 2010. This declining trend may reflect the global seabird population trend, given the large and apparently representative sample. Furthermore, the largest declines were observed in families containing wide-ranging pelagic species, suggesting that pan-global populations may be more at risk than shorter-ranging coastal populations. © 2015 Paleczny et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Destruction of nests and predation by introduced species are among the main factors responsible for seabird declines. The red-billed tropicbird (Phaethon aethereus) is a tropical, colonially nesting seabird whose distribution in Brazil is restricted to a small, isolated breeding colony located within Abrolhos National Park. This represents the southernmost population of the species in the western Atlantic, and is among the most southerly in its global distribution. Despite its isolation, the population on Abrolhos is threatened by egg predation by two invasive rat species: the house rat (Rattus rattus) and brown rat (Rattus norvegicus). In this study we conduct a population viability analysis of P. aethereus in Abrolhos to estimate the potential long term impacts of the rats. Our results indicate that egg and chick predation by rats has the potential to quickly drive the Abrolhos tropicbird population into serious decline. Reducing this threat may require the urgent implementation of a rat control program.
Many animals reproduce in large aggregations, which can vary in size from dozens to millions of individuals across species, time and space. The size of breeding colonies is a complex trade-off between multiple costs and benefits to an individual’s fitness, but the mechanisms by which colony size affects fitness are still poorly understood. One important cost of breeding in a large colony is the spatial constraint in resource use due to the need to regularly return to a central location. Large aggregations, like seabird breeding colonies, may therefore deplete food resources near the colony, forcing individuals to travel farther to find food, which may ultimately limit their reproductive output and population size. This hypothesis, proposed in 1963 by Ashmole for tropical oceanic islands, has so far not been tested at tropical seabird colonies, where food availability is less predictable than in colder waters. We compare the foraging distribution of a common tropical seabird, the masked booby Sula dactylatra, breeding on two islands in the South Atlantic that differ in the size of the breeding seabird community by 2 orders ofmagnitude, but are surrounded by similar oligotrophic waters. Foraging trips from the island with the smaller colony were on average 221 km (61 %) and 18.0 h (75 %) shorter because birds from the smaller colony rarely spent the night at sea and foraged on average 64 km (46%) closer to the colony. Energy expenditure was significantly lower, and nest survival higher (47 vs. 37 %, n=371) on the island with the smaller colony. These results are fully consistent with the predictions from Ashmole’s hypothesis and indicate that competition for food around tropical oceanic seabird colonies may indeed be a limiting factor for populations. Identifying important feeding areas for seabirds based on their foraging range may need to account for colony size of both the target and potential competitor species.
The destructiveness of major (Category 3 to 5) hurricanes along the United States Atlantic Ocean seaboard has been recognized for centuries. While the effects of hurricanes on coastal ecosystems are well known, the influence of hurricanes on pelagic seabirds is difficult to assess. During the annual Atlantic hurricane season (similar to 1 June to 30 November), the endangered black-capped petrel Pterodroma hasitata aggregates in Gulf Stream habitats from Florida to North Carolina. On at least 8 occasions over the past century, hurricanes have driven petrels far inland (sometimes as far as the Great Lakes), suggesting the demise of 10s to 100s of individuals. This paper models >100 yr of data to characterize and compare key aspects of hurricanes that did and did not drive petrels inland. Our model suggests that the predicted increase in the frequency of Category 3 to 5 hurricanes in the region due to climate change could nearly double the expected number of wrecked petrels over the next century and place an endangered species at greater risk of extinction.
Climate variability can affect population dynamics via adult survival or via offspring production and recruitment. The relative importance of both processes is still an unresolved matter, especially in long-lived species, where the time lags between the climate signal and the population response differ greatly depending on the process involved. We address the issue using 378 time series from 29 seabird species from 187 breeding colonies throughout the North Atlantic. The effect of climate on population growth rate is estimated as the slope of the North Atlantic Oscillation (NAO) index at different time lags when used as a covariate in population models. Using nonlinear mixed effects models, we can demonstrate that climate affects the population dynamics of seabirds, both through adult survival and through the recruitment of offspring produced. The latter effect is stronger, and the long time lags involved make it likely that its magnitude is still underestimated. Because different processes are involved, the sign of the relationship with the NAO differs between time lags. The relationship between the NAO and the population growth rate is also highly variable, both within and across species. In a second analytical step, we address the factors that may cause this interspecific and inter-colony variation, considering the ecological, demographic and geographical characteristics of the populations. Among comparatively 'fast-lived' seabirds, i.e. species with large clutches, the relationship with the NAO reverses its sign depending on latitude, while no such trend is apparent among 'slow' species.
Introduced species to oceanic islands can cause tremendous declines and extinctions of native avifauna. On Langara Island, British Columbia, Canada, the burrow-nesting ancient murrelet (Synthliboramphus antiquus) has declined from an estimated original population of 200,000 pairs to 14,600 pairs in 1993. Previously, causes of this decline were unknown, but the introduction of Norway rats (Rattus norvegicus) has been implicated as a major factor and has resulted in a recent rat eradication program. A major obstacle in the investigation of the effects of rats on seabirds, here and elsewhere, has been the inability to accurately assess the importance of seabirds to the diets of rats that also consume a variety of plants and invertebrates. We used stable-carbon (δ 13C), nitrogen (δ 15N), and sulphur $(\delta {}^{34}\text{S})$ isotope analyses of muscle and liver tissues of rats and prey organisms from 3 regions of Langara Island to evaluate evidence for marine foods, including seabirds, in the diets of rats. Rats were segregated into 3 isotopic groups corresponding to upland, littoral, and seabird nesting areas on the island. We interpret these groups to represent individuals consuming predominantly C-3 terrestrial foods (x̄ δ 15N = 5.4‰; x̄ δ 13C = -24.9‰; n = 24), intertidal invertebrates (x̄ δ 15N = 8.9‰; x̄ δ 13C = -14.3‰; n = 21), and ancient murrelet adults, chicks, or eggs (x̄ δ 15N = 13.2‰; x̄ δ 13C = -17.6‰; n = 8). We found strong correlations between liver and muscle isotope values for both 13C and 15N, suggesting that dietary preferences within individuals remained relatively constant. Stable-sulphur isotope values of rat liver were less useful in segregating marine and terrestrial dietary inputs, possibly because sources of sulphur to the terrestrial food web were of marine origin (pooled x̄ δ 34S value = 17.8‰; n = 20). Our measurement of liver and muscle tissues gave dietary estimates based on relatively short- and long-term integrations and revealed the 3 groups of rats remained isotopically segregated at least over the 2-month period of ancient murrelet breeding on Langara Island. Our results have important ramifications for dietary investigations of introduced fauna and their effects on native seabirds on oceanic islands.
In tropical waters resources are usually scarce and patchy, and predatory species generally show specific adaptations for foraging. Tropical seabirds often forage in association with sub-surface predators that create feeding opportunities by bringing prey close to the surface, and the birds often aggregate in large multispecific flocks. Here we hypothesize that frigatebirds, a tropical seabird adapted to foraging with low energetic costs, could be a good predictor of the distribution of their associated predatory species, including other seabirds (e.g. boobies, terns) and subsurface predators (e.g., dolphins, tunas). To test this hypothesis, we compared distribution patterns of marine predators in the Mozambique Channel based on a long-term dataset of both vessel- and aerial surveys, as well as tracking data of frigatebirds. By developing species distribution models (SDMs), we identified key marine areas for tropical predators in relation to contemporaneous oceanographic features to investigate multi-species spatial overlap areas and identify predator hotspots in the Mozambique Channel. SDMs reasonably matched observed patterns and both static (e.g. bathymetry) and dynamic (e.g. Chlorophyll a concentration and sea surface temperature) factors were important explaining predator distribution patterns. We found that the distribution of frigatebirds included the distributions of the associated species. The central part of the channel appeared to be the best habitat for the four groups of species considered in this study (frigatebirds, brown terns, boobies and sub-surface predators).
Laake, J. L. 2013. RMark: An R Interface for analysis of capture-recapture data
with MARK. AFSC Processed Rep. 2013-01, 25 p. Alaska Fish. Sci. Cent.,
NOAA, Natl. Mar. Fish. Serv., 7600 Sand Point Way NE, Seattle WA 98115.
RMark provides a formula-based interface for analysis of capture-recapture data with
the program MARK. RMark reduces the development time of models and removes the
potential for errors introduced from manual creation of design matrices in MARK. By
providing an interface to MARK in R, all of the tools in R can be accessed to extract
and manipulate data, plot or simulate. It also provides the potential to include MARK
analysis in a reproducible document with Sweave and LATEX. I demonstrate RMark
using the commonly analyzed European dipper data.
Tropicbirds (order Phaethontiformes) are a well-studied seabird taxon, but there have been no
published studies on sexual size dimorphism. Here, we investigated sexual size dimorphism in the Red-billed Tropicbird (Phaethon aethereus) and the White-tailed Tropicbird (P. lepturus), and evaluated the applicability of using morphometrics and discriminant functions to sex birds in the field. We sampled Red-billed Tropicbirds (n = 160) and White-tailed Tropicbirds (n = 57) in the Atlantic Ocean off the coast of Brazil. Each individual had seven morphometric traits measured and blood sampled for molecular sex determination. Discriminant functions were generated using generalized linear models (GLM). Red-billed Tropicbirds had significant male-biased intersexual differences in bill length, wing chord, nostril-to-bill-tip, and head-plus-bill, and the best GLM included bill length and wing chord. White-tailed Tropicbirds were characterized by significant intersexual differences for wing chord, with females larger than males, and the best GLM for this species included wing chord, bill width, and bill depth. Discriminant functions had accuracy similar to those used for other seabird species with similar body sizes
Both intrinsic and extrinsic factors recorded at individual nests can predict offspring fitness and survival but few studies have examined these effects in the tropics. We recorded nestling survival, post‐fledging survival and age at first return of Roseate Terns breeding at Aride Island, Seychelles, over a 12‐year period (1998–2009). Nest data recorded at the egg, nestling and fledging stages were collected during six breeding seasons (1998, 2001–2005) and a capture‐mark‐recapture dataset of six cohorts of fledglings was obtained from 2001–2009. Logistic regression models were used to assess the predictive effect of reproductive variables on fledging success, while multistate capture‐mark‐recapture models were used to estimate post‐fledging survival and return–recruitment probabilities to the natal site. Nestling survival probability increased with earliness of laying and was negatively affected by tick infestation during the growth period (0–23 days). Fledging probability was also positively related to chick body condition, whereas other pre‐fledging reproductive parameters such as clutch size and egg size were not influential. A multistate modelling of age‐specific survival and return–recruitment (transition) rates found that first‐year survival differed between cohorts and was also negatively affected by tick infestation. Annual survival stabilized from age 2 onwards at 0.83 ± 0.02. Transition rates were positively related to body condition at fledging, with heavier individuals returning for the first time to the natal colony at a younger age compared with lighter individuals. These results highlight the importance of local conditions encountered by tropical seabirds during the breeding season in shaping demographic parameters.
During the 2002 austral summer abnormally high sea-surface temperatures (SST) occurred in the southern Great Barrier Reef (GBR), Australia. This phenomenon was accompanied by reduced provisioning, decreased growth rates and reproductive failure of wedge-tailed shearwaters in the region. In 2002, adults were unable to compensate for changes in either the availability and/or accessibility of forage-fish by increasing food loads or foraging rates. This is one of few studies to explicitly correlate decreases in chick provisioning with above-average annual variation in SST and is the first to do so for a tropical seabird species in the western Pacific. It adds to an increasing number of data sets identifying the potential negative impacts of increasing SST at upper-trophic levels. As SST continue to rise with global climate change, our results predict substantial detrimental effects on seabird populations of the GBR. This finding has important implications for both seabird and coral reef ecosystem management in the region. Our results also suggest that wedge-tailed shearwaters are sensitive indicators to changes in forage-fish availability/accessibility associated with SST variation that can be used to develop models of, and monitor for, these potential impacts.
Breeding periodicity allows organisms to synchronise breeding attempts with the most favourable ecological conditions under which to raise offspring. For most animal species, ecological conditions vary seasonally and usually impose an annual breeding schedule on their populations; sub-annual breeding schedules will be rare. We use a 16-year dataset of breeding attempts by a tropical seabird, the sooty tern (Onychoprion fuscatus), on Ascension Island to provide new insights about this classical example of a population of sub-annually breeding birds that was first documented in studies 60 years previously on the same island. We confirm that the breeding interval of this population has remained consistently sub-annual. By ringing >17000 birds and re-capturing a large sample of them at equivalent breeding stages in subsequent seasons, we reveal for the first time that many individual birds also consistently breed sub-annually (i.e. that sub-annual breeding is an individual as well as a population breeding strategy). Ascension Island sooty terns appear to reduce their courtship phase markedly compared with conspecifics breeding elsewhere. Our results provide rare insights into the ecological and physiological drivers of breeding periodicity, indicating that reduction of the annual cycle to just two life-history stages, breeding and moult, is a viable life-history strategy and that moult may determine the minimum time between breeding attempts.
We review the conservation status of, and threats to, all 346 species of seabirds, based on BirdLife International’s data and assessments for the 2010 IUCN Red List. We show that overall, seabirds are more threatened than other comparable groups of birds and that their status has deteriorated faster over recent decades. The principal current threats at sea are posed by commercial fisheries (through competition and mortality on fishing gear) and pollution, whereas on land, alien invasive predators, habitat degradation and human disturbance are the main threats. Direct exploitation remains a problem for some species both at sea and ashore. The priority actions needed involve: a) formal and effective site protection, especially for Important Bird Area (IBA) breeding sites and for marine IBA feeding and aggregation sites, as part of national, regional and global networks of Marine Protected Areas; b) removal of invasive, especially predatory, alien species (a list of priority sites is provided), as part of habitat and species recovery initiatives; and c) reduction of bycatch to negligible levels, as part of comprehensive implementation of ecosystem approaches to fisheries. The main knowledge gaps and research priorities relate to the three topics above but new work is needed on impacts of aquaculture, energy generation operations and climate change (especially effects on the distribution of prey species and rise in sea level). We summarise the relevant national and international jurisdictional responsibilities, especially in relation to endemic and globally threatened species.
The most common invasive mammals—mice, rats, and cats—have been introduced to islands around the world, where they continue to negatively affect native biodiversity. The eradication of those invasive mammals has had positive effects on many species of seabirds. However, the removal of one invasive mammal species may result in abundance changes of other species due to trophic and competitive interactions among species. Understanding the overall impact of several invasive species is a key challenge when evaluating the possible effects of eradication programmes. Here we assess the influence of the three most common invasive mammals on nest survival of Cory’s shearwater (Calonectris diomedea). We monitored six breeding colonies over 3 years and measured the activity of mice, rats and cats to examine the influence of invasive mammals on nest survival. We found that nest survival showed a similar temporal trend in all years, with lowest weekly survival probabilities shortly after chicks hatched. Cats were identified as major predators of chicks, but no measure of colony-specific cat activity was able to adequately explain variation in shearwater nest survival. Nest survival was on average 0.38 (95 % confidence interval 0.20–0.53) and varied among colonies as well as over time. We found a small positive influence of rats on nest survival, which may indicate that the presence of small rodents as alternative prey may reduce cat predation of chicks. Our findings suggest that the eradication of rodents alone may exacerbate the adverse effects of cats on shearwater nest survival.
We analysed the effects of local- (sea-surface temperature [SST] and windspeed) and large- (multivariate El Nino index) scale weather conditions and timing of breeding on reproductive parameters of tropical roseate terns Sterna dougallii on Aride Island, Seychelles, using up to 17 years of data. The size of the breeding population and initiation of breeding were negatively and positively correlated, respectively, with both SST and the multivariate El Nino index for the laying season (May-June). It is the first time that an El Nino index obtained for the Pacific Ocean is shown to be correlated with reproductive parameters of seabirds in the Indian Ocean. Hatching success decreased significantly with later initiation of breeding. Virtually no chicks fledged when breeding started in June (40 % of the years monitored). We suggest that oceanographic conditions over a relatively large scale have an influence on tern arrival date to the breeding grounds and that SST abound the breeding colony influences the number of birds that attempt to breed. Despite the influence of factors such as predatory fish on food availability, this influence appears to be overridden by the importance of weather events and oceanographic conditions, which are likely to determine marine productivity. This study suggests that ecosystem-level phenomena appear to be important in shaping the population dynamics of tropical roseate terns.
We studied seasonal and inter-annual variations in the diet, amount of food delivered to chicks, and body condition of chicks and adult red-tailed tropicbird at Europa Island (Mozambique Channel, Indian Ocean), during 3 consecutive seasons. The diet consisted of epipelagic fishes (13 families) and squid (3 families) totalling 32 taxa. Three families, the squid Ommastrephidae (mainly Sthenoteuthis oualaniensis) and the fishes Exocoetidae (flying-fish) and Coryphaenidae (dolphin-fish), accounted for 68% of the number of prey and 83% of the ingested biomass. There were considerable differences in size and mass of the prey items, dolphin fish being the largest and squid the smallest. There were important seasonal variations in diet. During chick-rearing, 41% of the mass of food delivered to chicks consisted of dolphin fish, whereas these fish represented only 2% of the mass of food during incubation. The squid were caught in similar proportion throughout the breeding season but were larger during chick rearing. We did not find any inter-annual variation in the diet of chicks, nor in the body condition of either fledglings or adults, suggesting that no major inter-annual changes in food availability occurred during the course of our study. However, there was some inter-annual variation in feeding frequency and body condition of chicks of less than 60 d. There were also significant day-to-day variations in the feeding frequency in one of the 3 seasons studied. This suggests that day-to-day variation in the marine environment or in foraging conditions may have short-term consequences in the food provisioning and body condition of growing chicks, but that these variations do not necessarily result in long-term (inter-annual) variations in diet and body condition of fledgling or adult red-tailed tropicbirds at Europa island. These results suggest that between 1995 and 1997, red-tailed tropicbirds of Europa relied on predictable and seasonal food resources when breeding.
The foraging behaviour of animals depends on the distribution, abundance and pre- dictability of their food resources. In the marine environment, top predators such as seabirds are known to concentrate their foraging effort in specific oceanic features where productivity is elevated or prey concentrated. When marine productivity is low and prey distribution unpredictable, such as in tropical waters, selection should favour the evolution of flexible foraging strategies. By using GPS, Argos transmitters and activity recorders, we studied the foraging strategy of red-footed boobies (RFBs) Sula sula breeding on Europa Island in the Mozambique Channel, to examine the way a cen- tral place forager searches for prey in tropical waters. RFBs only foraged during the day-time and never sat on the water at night; thus, they consequently had a limited maximum foraging range of 148 km. Primary production within this range was low overall (average 0.165 mg chlorophyll m -3 ), with a high spatial and temporal heterogeneity due to the presence of strong geostrophic currents and surface mixing by storms. RFBs appear to adjust their foraging strategy to cope with this situa- tion. Individual birds do not return to the same oceanic sites from one trip to the next, but generally head in the direction of zones where productivity is enhanced. Active foraging, i.e. periods when birds spent time on the sea surface or dived, occurred at the most distant part of the foraging trip. Despite their limited foraging range and the low productivity of surrounding waters, red-footed boo- bies are able to track zones of prey availability located in areas of enhanced primary production. Searching is probably favoured by the presence of sub-surface predators that chase prey to the sur- face. The foraging strategy of this tropical sulid appears to differ extensively from that of temperate or polar seabirds whose prey distribution and availability are more predictable.
We assessed the influence of inter-annual changes in primary productivity and local, regional and large scale ocean indices on the breeding parameters of roseate terns S. dougallii on Aride Island, Seychelles, western Indian Ocean. Productivity (chicks/breeding pair), timing of breeding, clutch and egg sizes were monitored annually for 8 years and correlated with local ocean productivity (denoted by SeaWiFS estimates of chlorophyll concentration, CC), sea surface temperature and indices recording the status of the Indian Ocean Dipole and of El Niño. The rate of increase in CC (between mean laying date and CC peak value) was positively related to roseate tern productivity and mean clutch size over the 1998-2005 study period. Colony productivity seemed also to be influenced by the Multivariate El Niño Index. In most years, the breeding phenology of roseate terns corresponded to the local increase in CC around Aride, and failure to adjust timing of reproduction to the timing of the phytopankton bloom decreased the probability of breeding success. This is the first study showing that a tropical seabird species is sensitive to inter-annual variations in the intensity and timing of the phytoplankton bloom, which should be connected to annual variations in the availability of its main fish prey (juvenile goatfish). Overall, these patterns indicate that the reproduction of this top marine predator is dictated by the temporal variability in oceanographic conditions. We suggest that CC data available over the world's oceans may be a useful tool to develop models predicting the fate of colonies of inshore feeding seabirds when other, more conventional monitoring methods cannot be used.
We studied the foraging ecology of a tropical seabird community in 2 islands of the Seychelles from 2005 to 2007. Chick dietary samples were used to compare feeding habits among species and assess inter-annual and seasonal variations in diet. Fish prey dominated the diet of the community (68 to 100% of prey consumed), although cephalopods were present in 61.3, 40.0, 27.1 and 32.9% of the food samples from white-tailed tropicbirds, sooty terns, brown noddies and wedge- tailed shearwaters, respectively. We found high diet overlap between species (Mullidae fish being the first prey consumed (30 to 90%) for all species except for the white-tailed tropicbird (
Although a species of conservation concern, little is known about the reproductive success of Least Terns (Sternula antillarum) throughout the southeastern USA where availability of natural beaches for nesting is limited. Daily survival rate (DSR) of nests and chicks was examined at four natural nesting sites in Cape Romain National Wildlife Refuge (CRNWR), South Carolina, 2009-2010. Measures of nest success (N = 257 nests) ranged from 0– 93% among colony sites. The DSR of nests was primarily related to colony site, but year and estimates of predation risk also were related to DSR. Predation was the principal cause of identifiable nest loss, accounting for 47% of nest failures when the two years of data were pooled. The probability of a chick surviving from hatching to fledging = 0.449 ± 0.01 (N = 92 chicks). DSR of chicks was negatively related to tide height and rainfall. Therefore, productivity of Least Terns is being lost during both the nesting and chick stage through a combination of biotic and abiotic factors that may prove difficult to fully mitigate or manage. Although natural nesting sites within CRNWR intermittently produce successful nests, the consistency of productivity over the long term is still unknown. Given that the long term availability of anthropogenic nest sites (e.g. rooftops, dredge-spoil islands) for Least Terns is questionable, further research is required both locally and throughout the region to assess the extent to which natural sites act as population sources or sinks.
This study investigates the breeding success of two tropical seabirds that exploit dissimilar nesting habitats on Cousine Island in the Seychelles archipelago, the White Tern Gygis alba and the White-tailed Tropicbird Phaethon lepturus, which nest in trees and in crevices on the ground, respectively. Both species have a clutch of one egg. Over a 23-month study period, the outcomes of 134 nesting attempts by White Terns and 285 by White-tailed Tropicbirds were followed. White Terns produced an average of 0.4 chicks per attempt, significantly more than that of White-tailed Tropicbirds (0.25). Hatching success did not differ between the two species, but fledging success of White Terns (62%) was significantly higher than that of White-tailed Tropicbirds (43%). Nesting success of White-tailed Tropicbirds may be less successful than White Terns because their ground nests are at risk to purely terrestrial predators, in addition to predators that are both terrestrial and arboreal. Many nests of White-tailed Tropicbirds failed during the first two weeks of the incubation and nestling periods, but the reasons for this are unclear. Food availability may influence the reproductive success of both species.
We studied the breeding ecology of Red-billed Tropicbirds Phaethon aethereus at Farallón de San Ignacio, Sinaloa, in the south-central part of the Gulf of California, during two years with different oceanographic conditions: 2004, a neutral year, and 2007, a moderate El Niño year. We characterized oceanic changes by sea surface temperature and chlorophyll a concentration (150 km around the colony) from monthly satellite images. During two 5-day visits every month from January to May of each year we recorded timing of breeding, parental attendance, chick diet, and maximum diving depth of adults. We analyzed hatching and fledging success, chick growth and body condition. In agreement with oceanographic differences, we expected differences in timing of breeding, diet and foraging behaviour and lower breeding success and chick growth in the El Niño year. Sea surface temperature was higher, and chlorophyll concentration lower during early 2007 than in 2004. Average laying date and peak of laying occurred later in 2004 than in 2007. In 2007 adults dived deeper (2.09 ± 0.96 vs. 0.96 ± 0.66 m), had a different and more varied diet, and spent more time at sea at the expense of nest attendance than in 2004, suggesting that less food was available for this species in 2007. In agreement, hatching success was lower (35 vs. 75%), and chicks were lighter (641 vs. 739 g) and in worse body condition in 2007. Overall, despite the lower body condition of the chicks, the species seemed to exhibit some capacity to cope with the different conditions derived from warmer waters during the mild El Niño of 2006–2007.
Mate choice and nest cavity selection are directly linked to reproductive perform- ance in seabirds, and disruption of these can result in breeding failure. The Red- billed Tropicbird Phaethon aethereus mesonauta breeds on inaccessible and remote islands, nesting primarily in natural rock cavities. I examined the relation- ship between reproductive performance and mate and nest cavity fidelity on St. Eustatius from 2012 to 2016, corresponding to four successive breeding seasons. The majority of nest failures occurred during the incubation stage. Pairs that failed in their first breeding attempt and laid a replacement clutch were more likely to be faithful to their mate and nest cavity within and between seasons than pairs that bred successfully. Only two replacement clutches (3.8%) were laid after the loss of a chick in the first breeding attempt, whereas the remaining 50 replacement clutches (96.2%) were laid after the loss of an egg. The stage at which pairs split had no influence on mate or nest cavity retention the following season. There were no significant differences in the weight or morphological measurements of chicks from pairs that retained their nest mate and/or cavity and those that switched. The results of this study suggest that switching nest mate or nest cavity does not necessarily improve subsequent breeding success among Red-billed Tropicbirds on St. Eustatius.
In 2011 and 2012 the breeding success of Red-billed Tropicbirds Phaethon aethereus mesonauta was monitored on Saba, a 13-km² island in the Caribbean Netherlands. The two colonies on the south and south-west side of the island both contain approximately 100-300 nests; breeding success was zero at these sites, due to predation of newly hatched chicks by Feral House Cats. In the single large colony of Old Booby Hill (estimated to contain approximately 1000 nests) on the east side of the island, rates of predation were lower, resulting in a breeding success (survival from laying until fledging, as far as observed) of 65%. Calculations of daily survival of eggs and chicks following Mayfield, resulted in a nesting success of 48% per pair. I conservatively estimated the size of the Saba breeding population to be at least 1300-1500 pairs, which is larger than the previous estimate of 750-1000 pairs. Potentially, the number of breeding pairs may be as high as 1850 pairs.
Tropical cyclones are renowned for their destructive nature and are an important feature of marine and coastal tropical ecosystems. Over the last 40 years, their intensity, frequency and tracks have changed, partly in response to ocean warming, and future predictions indicate that these trends are likely to continue with potential consequences for human populations and coastal ecosystems. However, our understanding of how tropical cyclones currently affect marine biodiversity, and pelagic species in particular, is limited. For seabirds, the impacts of cyclones are known to be detrimental at breeding colonies, but impacts on the annual survival of pelagic adults and juveniles remain largely unexplored and no study has simultaneously explored the direct impacts of cyclones on different life-history stages across the annual life cycle. We used a 20-year data set on tropical cyclones in the Indian Ocean, tracking data from 122 Round Island petrels and long-term capture-mark-recapture data to explore the impacts of tropical cyclones on the survival of adult and juvenile (first year) petrels during both the breeding and migration periods. The tracking data showed that juvenile and adult Round Island petrels utilize the three cyclone regions of the Indian Ocean and were potentially exposed to cyclones for a substantial part of their annual cycle. However, only juvenile petrel survival was affected by cyclone activity; negatively by a strong cyclone in the vicinity of the breeding colony and positively by increasing cyclone activity in the Northern Indian Ocean where they spend the majority of their first year at sea. These contrasting effects raise the intriguing prospect that the projected changes in cyclones under current climate change scenarios may have positive as well as the more commonly perceived negative impacts on marine biodiversity.
Understanding the factors influencing nesting success is a primary goal of many studies. To do this effectively, more advanced tools than Mayfield's ad hoc estimator are needed. The recent development of a nest-survival model in program MARK provides a powerful and flexible tool for the study of avian nest survival that can incorporate seasonal variation in survival and nest-specific covariates. We briefly review the model and its development, illustrate how to include the effects of daily nest age and observer visits to nests, and conclude with an example analysis of Red-winged Blackbird (Agelaius phoeniceus) nest survival in Iowa. In this example, we found evidence for stage-specific differences in nest survival, seasonal patterns in nest survival that were best explained by a quadratic-time trend, and that survival differed between years. An exploration of several nest-specific covariates revealed that blackbird nest survival was positively affected by nest height, weakly affected by nest placement (nests placed in living vegetation may have experienced slightly higher survival), and unaffected by clutch size and within- and between-cell nest placement.
Demography is the study of the size and structure of populations and of the process of replacing individuals constituting the population. The study of demography was developed to forecast population growth. The rate at which a population increases or decreases depends basically on the fecundity (number of eggs laid) and survivorship of the individuals that belong to the population (Figure 5.1, bottom), but also to a lesser extent (especially for seabirds) on migration. Because many organisms, and especially seabirds, breed several times in their lives, a population consists of cohorts of individuals of different ages, born in different years. Moreover, mortality and fecundity rates are generally age-specific; life tables represent these birth and death probabilities. The relationship between the rate of increase or decrease and demographic parameters can be translated into more or less complex equations. The basic equation is the Euler-Lotka equation (Euler 1760, Lotka 1907) that specifies the relationships of age at maturity, age at last reproduction, probability of survival to age classes, and number of offspring produced for each age class, to the rate of growth of the population (r).
Many animals reproduce in large aggregations, which can vary in size from dozens to millions of individuals across species, time and space. The size of breeding colonies is a complex trade-off between multiple costs and benefits to an individual’s fitness, but the mechanisms by which colony size affects fitness are still poorly understood. One important cost of breeding in a large colony is the spatial constraint in resource use due to the need to regularly return to a central location. Large aggregations, like seabird breeding colonies, may therefore deplete food resources near the colony, forcing individuals to travel farther to find food, which may ultimately limit their reproductive output and population size. This hypothesis, proposed in 1963 by Ashmole for tropical oceanic islands, has so far not been tested at tropical seabird colonies, where food availability is less predictable than in colder waters. We compare the foraging distribution of a common tropical seabird, the masked booby Sula dactylatra, breeding on two islands in the South Atlantic that differ in the size of the breeding seabird community by 2 orders of magnitude, but are surrounded by similar oligotrophic waters. Foraging trips from the island with the smaller colony were on average 221 km (61 %) and 18.0 h (75 %) shorter because birds from the smaller colony rarely spent the night at sea and foraged on average 64 km (46 %) closer to the colony. Energy expenditure was significantly lower, and nest survival higher (47 vs. 37 %, n = 371) on the island with the smaller colony. These results are fully consistent with the predictions from Ashmole’s hypothesis and indicate that competition for food around tropical oceanic seabird colonies may indeed be a limiting factor for populations. Identifying important feeding areas for seabirds based on their foraging range may need to account for colony size of both the target and potential competitor species.
Presence of one or both members of a pair at the nest site during the incubation and early chick stage reduces reproductive losses due to predation and weather stresses. We monitored the presence of pair members by the temporary removal of one member of several pairs of Roseate (Sterna dougallii) and Common (Sterna hirundo) Terns at nests at Cedar Beach, New York, to determine if vulnerability varies by reproductive stage, to compare species differences that might partially account for declines in Roseate Tern populations, and to examine their response to trapping. There were significant differences between species in the time to return to the nest following an initial disturbance, and Roseate Terns that were trapped and released took longer to return to the nest and resume incubating than did Common Terns. The nests of Roseate Terns were vulnerable (neither adult in attendance) for longer time than were the nests of Common Terns.
We tested hypotheses that prey population fluctuations limit reproduction in Common (Sterna hirundo) and Roseate (S. dougallii) terns. In a 2-year study, both species laid earlier, delivered more fish/hour to nests, grew faster, and survived better in the year when prey populations were higher. Common, but not Roseate, terns had larger clutches and broods in the better food year.
Two species of tropicbirds (Phaethontidae) live in the Western North Atlantic. The White- tailed Tropicbird of the region is an endemic race, Phaethon lepturus catesbyi, with approximately 5000 pairs. This is half the estimate made less than two decades ago. The Red-billed Tropicbird, P. aethereus, has approximately 2000 pairs regionally and fewer than 8000 pairs globally. Neither species has been considered to be of conservation concern. We suggest that the Atlantic populations of Phaethon lepturus and the global population of Phaethon aethereus be considered for appropriate conservation status listings reflecting their rare and declining populations.
It is generally assumed that the extreme life history traits of pelagic seabirds, such as low fecundity or slow growth of chicks, result from the difficulties obtaining energy at sea from unpredictable and patchily distributed resources. However, little information on seabird prey distribution and availability exists to sustain this widely accepted hypothesis. Using tracking studies of 68 sub-populations of flying seabirds, I examine whether it is possible to gain information on the predictability of their marine resources. Because prey are clustered from fine to large scale in nested unities, from swarms to patches and concentrations of patches, it is important to take into account spatial scale. In temperate and polar regions, at large and meso-scales, seabirds appear to have a good knowledge of the location and concentrations of patches and generally use a commuting type of trip to reach foraging zones. Predictability appears to be high at large and meso-scales, with individuals from each sub-population heading in a particular direction from the colony to reach favoured habitats of known enhanced productivity such as shelf edges, frontal zones, upwellings. Within these mesoscale features, the animals use an area-restricted search behaviour to search for patches and swarms at finer scales. Using information on foraging site fidelity of individual birds, I show that differences in predictability at coarse scales are related to the distance and time spent foraging, and in particular to the specific types of foraging habitat. Some habitats appear to be more predictable than others: birds return consistently to the same coarse-scale sectors on shelf edges, whereas predictability is low in oceanic waters, even in frontal zones. Preliminary results on tropical species suggest that the environment here is less predictable in tropic than in temperate or polar zones. This review highlights that patchiness and predictability of marine resources are complex notions: predictability is dependent on the spatial and temporal scale considered, and especially on the marine habitat of foraging interest. I discuss the potential consequences of these results for the breeding success and life history of seabirds.
The purpose of this study was to examine whether variation in the extent to which marine birds track prey over small spatial scales can be attributed, in part, to fluctuations in regional prey abundance. The distributions of 4 marine bird species were compared to the distribution of acoustically determined prey biomass on days with contrasting prey abundances measured over a 10 x 20 km (regional) spatial scale. Spatial associations were measured at smaller (local) spatial scales, ranging from 0.2 to 7.6 km. Spatial concordance (i.e. overlap) between the distribution of acoustic biomass and the rhinoceros auklet Cerorhinca monocerata and between acoustic biomass and the Pacific loon Gavia pacifica at the smallest spatial scale (0.2 km) was greater on days when regional prey abundance was relatively low than on days when regional prey abundance was relatively high. This pattern was not evident in Brandt's cormorant Phalacrocorax penicillatus or the common murre Uria aalge. The densities of all 4 marine bird species examined were, however, only significantly correlated with densities of acoustic biomass on days when regional prey abundance was relatively low. I propose that short-term fluctuations in regional prey abundance account for some of the wide-spread variation previously observed in the strength of spatial associations between marine birds and their prey.
The annual record of hurricane activity in the North Atlantic basin for the period 1886-1996 is examined from the perspective of time series analysis. Singular spectrum analysis combined with the maximum entropy method is used on the time series of annual hurricane occurrences over the entire basin to extract the dominant modes of oscillation. The annual frequency of hurricanes is modulated on the biennial, semidecadal, and near- decadal timescales. The biennial and semidecadal oscillations correspond to two well-known physical forcings in the local and global climate. These include a shift in tropical stratospheric winds between an east and west phase (quasi-biennial oscillation (QBO)) and a shift in equatorial Pacific Ocean temperatures between a warm and cold phase (El Nino-Southern Oscillation (ENSO)). These climate signals have previously been implicated in modulating interannual hurricane activity in the North Atlantic and elsewhere. The near-decadal oscillation is a new finding. Separate analyses on tropical-only (TO) and baroclinically enhanced (BE) hurricane frequencies show that the two components are largely complementary with respect to their frequency spectra. The spectrum of TO hurricanes is dominated by timescales associated with ENSO and the QBO, while the near-decadal timescale dominates the spectrum of BE hurricanes. Speculations as to the cause of the near-decadal oscillation of BE hurricanes center on changes in Atlantic SSTs possibly through changes in evaporation rates. Specifically, cross- correlation analysis points to solar activity as a possible explanation. ''The hurricanes . . . used to come every seven years, or every five years, but they have become more frequent following the settlement of the Antilles.''
There are three species of breeding seabirds in Bermuda. All are migrants: the Bermuda Petrel or Cahow (Pterodroma cahow); the White-tailed Tropicbird (Phaethon lepturus catsbyii); and the Common Tern (Sterna hirundo). These seabirds face a number of common threats to their breeding success. The problems include hurricanes and tropical storms, rising sea level, invasive animal spe- cies, and loss of habitat. Conservation measures have been implemented in an effort to conserve these species. Artifi cial nest sites have been created for both the Cahow and White-tailed Tropicbird. A translocation project has been carried out for the Cahow. The Cahow breeding grounds and the most important breeding areas for the White-tailed Tropicbird have been incorporated into a national park and the area has been designated an Important Bird Area.
The variability of zooplankton biomass on a scale of thousands, hundreds, and tens of kilometres was estimated from the data bank of over 40 expeditions to the Atlantic and Indian Oceans and seas of the Mediterranean basin. Thirty oceanographic grid surveys were used to estimate variability on a scale of hundreds of kilometres. Continuous records within these grids were used to estimate zooplankton variability on the scale of tens of kilometres, and high-resolution sampling was used to investigate variability of kilometre scale. In the multiscale variability of zooplankton biomass, the maximum variability was observed on the thousands of kilometres scale, with the quantitative values of biomass represented in a form of normalized variance. The local peak of variability of zooplankton biomass in the range of hundreds of kilometres, in the tropical ocean, was probably due to the enhanced water dynamics with the same scale range. This activity is linked with open-ocean mesoscale eddy fields in the tropical zones of both oceans. The other typical feature of the structure of the zooplanktonic fields is its spatial anisotropy, which indicates different properties of spatial variation of parameters along directions through space. The anisotropy was evaluated by means of two-dimensional spatial autocorrelation functions. Two-dimensional correlation ellipses of the zooplankton biomass fields were orientated by their main axes in accordance with the direction of transport of the main water mass, the direction of motion of the eddies, and the orientation of divergence or convergence zones. The spatial heterogeneity of zooplankton biomass distribution in the horizontal plane can be characterized by the frequency of occurrence of patches. On a log scale the frequency of occurrence of different size patches diminishes proportionally with their linear size. Zooplankton biomass is distributed more heterogeneously than that of phytoplankton (chlorophyll a). Synchronous measurements of the three-level system ''phytoplankton-mesozooplankton-flying fish'' (where each component acts as a prey item for the next one) exhibit the same trend of spatial autocorrelation functions, diminishing over the trophic levels on a scale from hundreds to tens of kilometres. This means that relatively heterogeneous fields of predators exist on more uniformly distributed fields of their prey. (C) 1995 International Council for the Exploration of the Sea
The Lincoln-Petersen model (Chapter 2) and closed population models (Chapter 3) are presented briefly. The Jolly-Seber open population model is covered in detail in Chapter 4. In Chapter 5, the authors consider the "enumeration' or "calendar of captures' approach, which is widely used by mammalogists and other vertebrate ecologists, and recommend that it be abandoned in favor of analyses based on the Jolly-Seber model. One restricted version of the Jolly-Seber model, which allows losses (mortality or emigration) but not additions (births or immigration), is likely to be useful in practice. Another series of restrictive models requires the assumptions of a constant survival rate or a constant survival rate and a constant capture rate for the duration of the study. In Chapter 5, the authors consider 2 generalizations of the Jolly-Seber model. The temporary trap response model allows newly marked animals to have different survival and capture rates for 1 period. The other generalization is the cohort Jolly-Seber model. Ideally all animals would be marked as young, and age effects considered by using the Jolly-Seber model on each cohort separately. Chapter 6 presents a detailed description of an age-dependent Jolly-Seber model, which can be used when ≥2 identifiable age classes are marked. Detailed description of the "robust' design is given in Chapter 7, in which each primary period contains several secondary sampling periods. Chapter 8 gives detailed discussion of the design of capture-recapture studies. A new program has been written to accompany the material on the Jolly-Seber model (Chapter 4) and its extensions (Chapter 5). Another new program has been written for a special case of the age-dependent model (Chapter 6) where there are only 2 age classes. In Chapter 9 a description of the different versions of the 2 programs is given. Chapter 10 gives a description of some alternative approaches that were not considered in this monograph. -from Authors
Feare, Christopher J. 1981. Breeding schedules and feeding strategies of Seychelles sea-birds. Ostrich 52:179–185.A comparison of the breeding schedules of Seychelles seabirds with their feeding strategies showed no relation between synchronous breeding and the ranges over which the birds forage, but there was a relation with flock feeding. Flock feeding birds are largely dependent upon schools of predatory fishes that drive prey species to the surface, and food is thus localized and transient. Synchronous colonial breeding probably helps individuals to locate these localized food sources, but species that are dependent mainly on dispersed prey would derive no benefit from synchrony, and they do, in fact, breed throughout the year.Within shoals of prey, food is probably superabundant, but feeding space may be limiting. The dark backs of flock feeding birds render them inconspicuous to foraging birds (conspecifics and other species), thereby tending to reduce competition for space within the feeding flocks. The white plumage of species that feed on dispersed prey appears to act as a spacing out mechanism, reducing interference between feeding birds.