Browsing by Author "Strange, Ian J."
Now showing 1 - 4 of 4
Results Per Page
Sort Options
- Brood-guarding duration in black-browed albatrosses Thalassarche melanophris: temporal, geographical and individual variationPublication . Catry, Paulo; Phillips, Richard A.; Forster, Isaac P.; Matias, Rafael; Lecoq, Miguel; Granadeiro, José Pedro; Strange, Ian J.In birds, the period spent brooding or guarding young chicks is highly variable, but such variation has seldom been studied. Previous single-year studies of Antarctic petrels Thalassoica antarctica and grey-headed albatrosses Thalassarche chrysostoma revealed a pronounced seasonal decline in brood-guarding duration and gave rise to the ‘synchronisation hypothesis’, which suggests that some of the variation in the length of the brood-guarding stage is related to predictable seasonal changes in the risk of chick predation. We tested the predictions of this and three other hypotheses in a two-site, four-year study of the black-browed albatross T. melanophris. The existence of a pronounced seasonal decline in broodguarding duration was apparent at both sites, and in years of contrasting food availability, providing further support for the ‘synchronisation hypothesis’. Alternative explanations for this pattern are that short brood-guarding periods for latehatched chicks result from a seasonal decline in food availability or from the fact that early nesting birds are of higher individual quality. However, these explanations are at odds with the absence of a seasonal decline in early chick growth or in probability of chick survival. Furthermore, adult quality (measured as past reproductive performance) had a weak and inconsistent effect on the duration of brood-guarding. Weather changes explained some of the variation in broodguarding, but there were no differences between regions of contrasting climates. Individual pairs displayed a degree of inter-annual consistency in brood-guarding duration and, at least in some years, longer brood-guarding resulted in higher fledging probability. We speculate that a higher investment in brood-guarding increases the cost of reproduction, which counteracts other selective pressures that would otherwise lead to longer brood-guarding durations.
- Can thin-billed prions Pachyptila belcheri breed successfully on an island with introduced rats, mice and cats? The case of New Island, Falkland IslandsPublication . Catry, Paulo; Silva, Mónica C.; MacKay, S.; Campos, Ana R.; Masello, Juan F.; Quillfeldt, Petra; Strange, Ian J.Small burrowing petrels nesting on islands rarely survive introductions of mammalian predators. On New Island, a population of around two million pairs of thin-billed prions nests despite the presence of introduced ship rats, house mice and feral cats. Understanding the mechanisms of such coexistence is important, as it is important to establish a baseline for future monitoring. To do this, prion breeding success was determined for 7 years and in several habitats. Breeding success was high, except for the small fraction of the population that nests in tussock Poa Xabellata stands, where several lines of evidence suggest significant predation by rats. Such high breeding success possibly resulted from predator swamping in this highly seasonal environment. This study suggests that introduced mammals do not currently depress thin-billed prion breeding success on New Island. However, cats and rodents might have future harmful eVects if external factors depressed the prion population or allow a signiWcant population growth of predators on New Island.
- Movements, winter distribution and activity patterns of Falkland and brown skuas: Insights from loggers and isotopesPublication . Phillips, Richard A.; Catry, Paulo; Silk, Janet R. D.; Bearhop, Stuart; McGill, Rona A R; Afanasyev, Vsevolod; Strange, Ian J.In the first published study of the wintering ranges and activity patterns of skuas from any colony, we combined tracking (geolocator) and stable isotope analysis in a comparison of migration behaviour of brown skuas Catharacta lonnbergi and Falkland skuas C. antarctica from South Georgia and the Falkland Islands, respectively. Brown skuas, particularly failed breeders, departed and returned to the colony much earlier than Falkland skuas, and 2 of 3 brown skuas performed a pre-laying exodus. During winter, brown skuas were distributed widely over deep, oceanic water within the Argentine Basin (37 to 52° S) between the Antarctic Polar Front and the northern sub-tropical Front. Falkland skuas, by comparison, wintered mainly in subantarctic waters around the central Patagonian shelf-break (40 to 52° S). Much greater overlap existed among core areas within than between species, and sex did not influence distribution. The partial inter-specific spatial segregation was also reflected in a divergence in activity patterns, with brown skuas in flight for a greater proportion, and more time on average, during both daylight and darkness. Both species of skua spent far more time on the water than do foraging albatrosses, and there was limited overlap between their nonbreeding distributions and those of large procellariids from the same archipelagos. Stable isotope signatures of brown skua feathers indicated that distributions of tracked birds were typical of most or all of the breeding population, and were consistent from year to year. None was characteristic of species that winter on adjacent continental shelves or off south-west Europe. Isotope values also suggested a mixed diet for brown skuas of zooplankton, low trophic-level squid and fish, with little or no reliance on seabird predation or fisheries.
- Population census and nesting habitat selection of thin-billed prion Pachyptila belcheri on New Island, Falkland IslandsPublication . Catry, Paulo; Campos, Ana R.; Segurado, Pedro; Silva, Mónica C.; Strange, Ian J.Seabirds have the potential to be used as indicators for monitoring changes in the southern oceans. However, many species and populations are still poorly known. The thin-billed prion (Pachyptila belcheri) is the most abundant seabird species around the Falkland Islands, but this far, no detailed censuses of its populations had been carried out. In this study, we developed a methodology, based on standard field methods combined with a Geographical Information System, to census the biggest known colony (New Island) of this species. Data were also collected on nesting habitat preferences. Results indicate that there are 1,081,000 (95% confidence limits: 815,000–1,346,000) apparently active nest-burrow entrances on New Island South which, given the very high occupancy rates by known breeders, should correspond to a similar number of nesting couples. We present quantitative evidence that this result can be extrapolated for the entire island, giving an overall estimate of 2 million pairs. Prions have an almost ubiquitous distribution on New Island, but they are more frequent in areas with steeper slopes (excluding cliffs). Habitats dominated by introduced grass species generally hold higher densities of nests in comparison to most native formations.