Article

# Reproductive Success of Red-Billed Tropicbirds (Phaethon aethereus) on St. Eustatius, Caribbean Netherlands

Authors:
• Wageningen University & Research/Waardenburg Ecology
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## 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.

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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.''
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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.
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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
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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
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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.