Norman Ratcliffe

An Emperor Penguin Population Estimate: The First Global, Synoptic Survey of a Species from Space

Our aim was to estimate the population of emperor penguins (Aptenodytes fosteri) using a single synoptic survey. We examined the whole continental coastline of Antarctica using a combination of medium resolution and Very High Resolution (VHR) satellite imagery to identify emperor penguin colony locations. Where colonies were identified, VHR imagery was obtained in the 2009 breeding season. The remotely-sensed images were then analysed using a supervised classification method to separate penguins from snow, shadow and guano. Actual counts of penguins from eleven ground truthing sites were used to convert these classified areas into numbers of penguins using a robust regression algorithm. We found four new colonies and confirmed the location of three previously suspected sites giving a total number of emperor penguin breeding colonies of 46. We estimated the breeding population of emperor penguins at each colony during 2009 and provide a population estimate of ∼238,000 breeding pairs (compared with the last previously published count of 135,000–175,000 pairs). Based on published values of the relationship between breeders and non-breeders, this translates to a total population of ∼595,000 adult birds. There is a growing consensus in the literature that global and regional emperor penguin populations will be affected by changing climate, a driver thought to be critical to their future survival. However, a complete understanding is severely limited by the lack of detailed knowledge about much of their ecology, and importantly a poor understanding of their total breeding population. To address the second of these issues, our work now provides a comprehensive estimate of the total breeding population that can be used in future population models and will provide a baseline for long-term research.

Assessing the diet of great skuas, Catharacta skua, using five different techniques

Abstract. Several different techniques have been used to study the diet composition of skuas and gulls in polar regions. In this study, we assessed the diet of great skuas using five techniques (pellets, prey remains, spontaneous regurgitates, observed feeds and water off-loading). The estimates of diet derived from each sampling technique during a single breeding season were generally similar. Although it is easy to collect large samples of pellets and remains, these over-estimate the amount of indigestible material in the diet. Prey remains provide a high degree of taxonomic accuracy, but may under-estimate prey swallowed whole. Water off-loading provides unbiased estimates of chick diet but only during a short period in the breeding season. It may also stress birds, and is labour-intensive. Spontaneous regurgitates are useful for assessing chick and adult diet but are difficult to collect systematically, and differing prey digestibility may bias results. Observed feeds are time consuming to record and over-represent easily identifiable prey. Technique-dependent biases highlight that sampling methods should be selected on the basis of sample sizes, time, taxonomic detail and age of study birds. Biomass may be estimated using pellets with correction factors, and also spontaneous regurgitates and water off-loading, but prey remains and observed feeds may be more inaccurate.

The interactive effects of age and food supply on the breeding ecology of great skuas

Age-specific variation in the reproductive performance of great skuas in Shetland, U.K., was investigated during a period of fluctuating sandeel availability between 1988 and 1993. Increased sandeel abundance was associated with earlier laying dates, increased clutch volumes and improved fledging success. Sandeel abundance had no effects on clutch size and hatching success. Parental age improved haying date and clutch size in a nonlinear manner. Laying date became earlier in the youngest age classes, with the effect being negligible in older birds. Clutch size increased with age up to 18 and then declined in older birds. Clutch volume increased with age and the probability of nest predation declined with age. There were no significant interactive effects of age and year on laying date nor clutch volume, indicating that birds of all ages benefited equally from increases in sandeel abundance. This suggests that lack of breeding experience rather than deficient foraging skills are responsible for young birds laying later. There was a significant interactive effect of year and age on fledging success that was associated with changes in sandeel availability. Success was uniformly low for birds of all ages in 1988-90 when sandeel abundance was low. Success increased with age in 1991 and 1992 during which time food supply improved. During 1993 food supply was abundant and fledging success was high in all age classes. Age-specific improvements in laying date, clutch size and nest predation were best explained by learning of skills associated with experience of breeding. Improvements in fledging success with age were probably best explained by the learning of skills associated with foraging.

REPRODUCTIVE CONSEQUENCES FOR GREAT SKUAS SPECIALIZING AS SEABIRD PREDATORS

Abstract Most of the Great Skuas (Stercorarius skua) breeding at Hermaness, Shetland, exhibit dietary specialization: a small proportion feed almost exclusively upon seabird prey, a small proportion feed as generalists, and most feed on fishery discards. We investigated the foraging dynamics, reproductive performance, and survival of Great Skuas that specialized in depredating other seabirds compared with those feeding predominantly on fish. Around half of the specialist bird predators defended combined breeding and feeding territories that included a section of seabird colony; the remainder of the predatory skuas foraged away from breeding territories. Specialist bird predators retained their feeding habit and, if present, feeding territory, across years. Time budgets revealed that specialist bird predators spent less time foraging than skuas feeding predominantly on fish. Results of radio-telemetry indicated that bird-specialist skuas have smaller home ranges than other birds. In a comparison of reproductive performance, specialist bird predators consistently hatched earlier among years. They also showed larger clutch volumes and improved chick condition, but these were subject to annual variations. Hatching success and fledging success for specialist bird predators and specialist fish predators were similar. Specialist bird predators showed similar annual survival compared with fish-feeders over the same period. Specializing as a bird predator may be limited to the best birds in the population, but their poorer than predicted breeding success reveals the need for further study into the relationship between diet and reproductive success in this species. Consecuencias Reproductivas para Individuos de Stercorarius skua que se Especializan como Depredadores de Aves Marinas Resumen. La mayoría de los individuos de Stercorarius skua que se reproducen en Hermaness, Shetland, presentan una especialización de la dieta: una proporción pequeña se alimenta casi exclusivamente de aves marinas, otra proporción pequeña generalista y la mayoría se alimenta de desechos de pesqueros. Se investigó la dinámica de forrajeo, el desempeño reproductivo y la supervivencia de individuos de S. skua que se especializan en la depredación de otras aves marinas comparándolo con otros que se alimentan predominantemente de pescado. Cerca de la mitad de los depredadores especialistas de aves defienden de manera combinada territorios para reproducción y para alimentación que incluyen una sección de la colonia: el resto de los individuos depredadores forrajean lejos de los territorios de reproducción. Los aves depredadores especialistas de aves mantienen sus há bitos alimenticios y, en caso de presentarse, el territorio para alimentación, a través de los años. Los presupuestos de tiempo revelaron que los depredadores especialistas de aves pasan menos tiempo forrajeando que los que se alimentan predominantemente de pescado. Los resultados de radio-telemetría indicaron que las aves especialistas tienen rangos de hogar menores que otras aves. En una comparación de desempeño reproductivo, los depredadores especialistas de aves presentaron fechas de eclosión más tempranas, las cuales se repitieron a traves de los años. Así mismo, mostraron volúmenes de puesta más grandes y mejor condición de las crías, pero éstos estuvieron sujetos a variaciones anuales. El éxito de eclosión y el éxito de emplumamiento de los depredadores especialistas de aves y de los depredadores especialistas de pescado fueron similares. Los depredadores especialistas de aves mostraron una supervivencia anual similar a la de los que se alimentan de pescado durante el mismo periodo. La especialización como depredador de aves puede limitarse a las mejores aves de la población, pero el bajo éxito reproductivo encontrado, contrario a lo predicho, revela la necesidad de estudios adicionales sobre la relación entre la dieta y el éxito reproductivo en esta especie.

Pellets as indicators of diet in Great Skuas Catharacta skua

(2001). Pellets as indicators of diet in Great Skuas Catharacta skua. Bird Study: Vol. 48, No. 3, pp. 373-376.

The influence of hatching date on different life-history stages of Great Skuas Catharacta skua

We investigated the effect of hatching date on chick body condition, chick survival, post-fledging survival and eventual breeding performance as an adult (laying date and clutch volume) of Great Skuas in Foula (Shetland). In 10 out of 16 years there was a seasonal decline in chick body condition. In the remaining years, there was no detectable effect of hatching date on chick condition. Differences among years were statistically significant. Chicks in early broods survived better than late-hatched chicks in four years, and no trend could be detected in one year. Annual variation in the seasonal pattern of chick growth and survival, and of adult territorial attendance, suggests that environmental conditions play a role in determining the seasonal decline in performance. Post-fledging survival in relation to hatching date was studied by analysis of ringing recoveries. Hatching date affected post-fledging survival, but only in the first year of life. Breeding parameters of birds of known hatching date were studied when they came back to breed in Foula. Hatching date of an individual did not affect its laying date or volume of the clutches laid later in life. This suggests that laying date has a low heritability, or none at all. Birds that laid early in the season produced more future recruits than later birds. After an early breeding attempt, skuas had a higher overwinter survival probability than after a late season. These results suggest that laying date is an indicator of individual quality in skuas, in the sense that early birds apparently are more competent at gathering resources for reproduction and survival.

Love thy neighbour or opposites attract? Patterns of spatial segregation and association among crested penguin populations during winter

Aim Competition for food among populations of closely related species and conspecifics that occur in both sympatry and parapatry can be reduced by interspecific and intraspecific spatial segregation. According to predictions of niche partitioning, segregation is expected to occur at habitat boundaries among congeners and within habitats among conspecifics, while negative relationships in the density of species or populations will occur in areas of overlap. We tested these predictions by modelling the winter distributions of two crested penguin species from three colonies in the south-western Atlantic. Location Penguins were tracked from two large colonies on the Falkland Islands and one in South Georgia, from where they dispersed through the South Atlantic, Southern Ocean and south-eastern Pacific. Methods Forty macaroni penguins (Eudyptes chrysolophus) from South Georgia and 82 southern rockhopper penguins (Eudyptes chrysocome chrysocome) from two colonies in the Falkland Islands were equipped with global location sensors which log time and light, allowing positions to be estimated twice-daily, from April to August in 2011. Positions were gridded and converted into maps of penguin density. Metrics of overlap were calculated and density was related to remote-sensed oceanographic variables and competitor density using generalized additive models. Results Macaroni penguins from western South Georgia and southern rockhopper penguins from Steeple Jason Island, Falkland Islands, were spatially segregated by differences in their habitat preferences thus supporting our first prediction regarding interspecific segregation. However, southern rockhopper penguins from Beauchêne Island showed a marked spatial overlap with macaroni penguins as the two had similar habitat preferences and strong mutual associations when controlling for habitat. Contrary to our predictions relating to intraspecific segregation, southern rockhopper penguins from Beauchêne Island and Steeple Jason Island were segregated by differences in habitat selection. Main conclusions Morphological differentiation probably allows macaroni penguins from South Georgia and southern rockhopper penguins from Beauchêne Island to coexist in areas of spatial overlap, whereas segregation of the two Falkland rockhopper penguin populations may have arisen from two distinct lineages retaining cultural fidelity to ancestral wintering areas.

Short-Lived Repeatabilities in Long-Lived Great Skuas: Implications for the Study of Individual Quality

Excluding age and experience effects, individual quality is frequently assumed to be a fixed trait. This paper tests whether an individuals apparent quality, as indicated by its timing of breeding, is indeed permanent. This is assessed by studying the repeatability of laying dates of a long-lived seabird, the great skua Catharacta skua. Laying date is known to be an important component of fitness in many seabirds, including great skuas. Mathematical models were constructed to evaluate the effect of age-specific timing of breeding on repeatability estimates. Simulations show that, in short-term studies (lasting 2 or 3 years), age-specific laying dates can produce spurious repeatabilities with values ranging between 0.1 and 0.2, when the value should be zero. Field observations showed that individual consistency of timing of breeding was high over short time intervals, but disappeared over longer periods (4 or 5 years). A comparison between field data and the simulations demonstrated that the observed high repeatabilities in consecutive years, and the fast subsequent declines, cannot be fully accounted for by age effects. The mechanisms underlying this surprising pattern are completely unknown, but results clearly suggest that apparent individual quality is a transient attribute in these birds.

The Incidence of Nonbreeding by Adult Great Skuas and Parasitic Jaegers from Foula, Shetland

Several recent studies of seabirds have found high levels of nonbreeding by experienced adults. By contrast, just 8.9% (range 4-14%) of experienced Great Skuas (Catharacta skua) on Foula, Shetland Islands, deferred breeding between 1989-1996. For Parasitic Jaegers (Stercorarius parasiticus), a corresponding value of 5.5% (range 3-8%) was found between 1993-1994. Only 3% of the territorial pairs of Parasitic Jaegers, including new recruits, failed to lay eggs. Higher incidence of nonbreeding in Great Skuas was recorded in years when fledging production was low. Loss of mate due to death or divorce was the main direct cause of nonbreeding. Loss of territory also was important for male birds. In Great Skuas, more males missed a breeding season than females, but the same did not apply to Parasitic Jaegers. Very young and very old Great Skuas were more likely to defer breeding than mid-aged birds. Evidence is presented that the decline in breeding frequency of old birds was due to senescence as opposed to increased frequency of mate-changes resulting from a high mortality of old partners.

Survival in macaroni penguins and the relative importance of different drivers: individual traits, predation pressure and environmental variability

Understanding the demographic response of free-living animal populations to different drivers is the first step towards reliable prediction of population trends. Penguins have exhibited dramatic declines in population size, and many studies have linked this to bottom-up processes altering the abundance of prey species. The effects of individual traits have been considered to a lesser extent, and top-down regulation through predation has been largely overlooked due to the difficulties in empirically measuring this at sea where it usually occurs. For 10 years (2003–2012), macaroni penguins (Eudyptes chrysolophus) were marked with subcutaneous electronic transponder tags and re-encountered using an automated gateway system fitted at the entrance to the colony. We used multistate mark–recapture modelling to identify the different drivers influencing survival rates and a sensitivity analysis to assess their relative importance across different life stages. Survival rates were low and variable during the fledging year (mean = 0·33), increasing to much higher levels from age 1 onwards (mean = 0·89). We show that survival of macaroni penguins is driven by a combination of individual quality, top-down predation pressure and bottom-up environmental forces. The relative importance of these covariates was age specific. During the fledging year, survival rates were most sensitive to top-down predation pressure, followed by individual fledging mass, and finally bottom-up environmental effects. In contrast, birds older than 1 year showed a similar response to bottom-up environmental effects and top-down predation pressure. We infer from our results that macaroni penguins will most likely be negatively impacted by an increase in the local population size of giant petrels. Furthermore, this population is, at least in the short term, likely to be positively influenced by local warming. More broadly, our results highlight the importance of considering multiple causal effects across different life stages when examining the survival rates of seabirds.