Body size, assortative mating and divorce rates in a little-known skua taxon, the Falkland Skua Stercorarius antarcticus antarcticus

Several studies have attempted to identify the selective pressures leading to reversed size dimorphism (RSD), a characteristic of skuas and of a wide range of bird species with a raptorial lifestyle. One of the prevailing hypotheses posits that females in species of aggressive and well-armed males should select males that are smaller than themselves, to avoid injuries during pair formation and mating. This hypothesis predicts assortative mating by size in such species, and in previous research, this was confrmed to occur in two skua species. This study presents, for the frst time, biometric data for Falkland Skuas Stercorarius antarcticus antarcticus based on large sample sizes. Despite the clear RSD, we found no assortative mating by size, which could be related to the low divorce rates and few opportunities for mate selection in this population. Our results highlight the need for further work to fnd an explanatory hypothesis for the evolution and maintenance of reversed size dimorphism in skuas.Fundação para a Ciência e Tecnologia - FCTinfo:eu-repo/semantics/publishedVersio

Within an between-year winter-site fidelity of Chiffchaffs Phylloscopus collybita

Current knowledge concerning between-year winter site fidelity in passerines suggests there might be important inter-specific and regional variations in recurrence rates (the proportion of birds recaptured in years subsequent to marking). However, there are virtually no studies reporting low recurrence rates that show this not to be simply the result of low recapture probabilities. This problem can be particularly acute for partly nomadic and mostly non-territorial species in their winter quarters. The present study shows that, in spite of the apparently nomadic behaviour of wintering Chiffchaffs Phylloscopus collybita, some individuals can become temporarily sedentary. Temporarily sedentary Chiffchaffs have very small linear home-ranges (ca. 200m) and their calculated recapture probability (by re-sighting of colour-ringed birds) is high. In spite of that, recurrence rate of temporarily sedentary birds is very low, suggesting that the majority of the surviving individuals are not site faithful. This is in contrast to what has been found in several detailed studies in other Sylviidae and also Parulidae involving mostly territorial birds. The type of approach developed here should be used in other studies before any comparisons and generalisations involving winter-site fidelity results are attempted

Ecology and behaviour of great skuas breeding on Foula (Shetland)

Aspects of the ecology and behaviour of Great Skuas breeding on Foula were investigated by studying individually marked birds. Pre-incubation behaviour of Great Skuas was described and predictions that arise from theories of territorial intrasions and copulation behaviour in birds were tested. Partnership composition and behaviour associated with mate change were studied. In particular, an attempt was made to test which of two alternative theories provides a better explanation for the occurrence of divorces in species where mate change is associated with a (short-term) measurable cost. Investigations were made to evaluate the effect of hatching date on different life- history stages in Great Skuas, including chick growth, survival to fledging, post-fledging survival and reproductive performance. The implications for the optimal timing of breeding in adults were analysed. Clutch characteristics, including total volume and within clutch asymmetry, were analysed in relation to parental age and breeding performance. An experiment was designed to infer foraging ability from territorial attendance of adult skuas, and detect small individual differences in those patterns. Results were analysed in relation to adult age and body size. Mathematical models were built to allow simulations of the effects of age on the study of individual repeatability in a breeding parameter: laying date. The consistency of individual laying dates over long time intervals was assessed. Results were discussed in relation to the concept of a permanent individual quality. Studies of the behaviour and breeding performance of individual skuas were analysed in relation to sex and body size. Results were discussed in relation to specific predictions made by several theories that explain the evolution of reversed sexual size dimorphism in the Stercorariidae and other birds with a raptorial lifestyle

Migrants and residents of a long‐lived seabird differ in their behavioural response to a stressor

Inter-individual variation in behaviour has been recognised as a major driver of population ecology, but its relationship to migratory strategy has been ill-explored. Here, we investigated whether male migrant and resident Cory’s shearwaters Calonectris borealis, a long-lived partially migratory seabird, are distinguishable by their temperament at the colony. We tracked a large number of individuals over two winters using GLS devices and assessed whether exploratory behaviour and reaction to extraction from the nest corresponded to migratory strategy over this period. While exploratory behaviour was unrelated to migratory strategy, birds that were resident were more likely to be unreactive towards extraction from the nest. This difference in behavioural stress response, together with previous findings that migrants display higher physiological stress over winter, suggests that migrants and residents may be distinguishable by their stress threshold.Fundação para a Ciência e a Tecnologia - FCTinfo:eu-repo/semantics/publishedVersio

Unexpectedly deep diving in an albatross

Albatrosses are the iconic aerial wanderers of the oceans, supremely adapted for long-distance dynamic soaring flight. Perhaps because of this they are considered poorly adapted for diving1, in contrast to many smaller shearwater and petrel relatives, despite having amphibious eyes2, and an a priori mass advantage for oxygen-storage tolerance3. Modern biologging studies have largely confirmed this view4,5, casting doubt on earlier observations using capillary tube maximum depth gauges1, which may exaggerate depths, and emphasising albatrosses’ reliance on near-surface feeding. Nevertheless, uncertainty about albatross diving remains an important knowledge gap since bycatch in human fisheries (e.g. birds becoming hooked when diving for longline bait fish) is thought to be driving many population declines in this most threatened group of birds6. Here we show, using miniature electronic depth loggers (TDRs), that black-browed albatross, Thalassarche melanophris, can dive to much greater depths (19 m) and for much longer (52 s) than previously thought — three times the maxima previously recorded for this species (6 m and 15 s), and more than twice the maxima reliably recorded previously for any albatross (from 113.7 bird-days of tracking4,5,7). Further evidence that diving may be a significant behavioural adaptation in some albatrosses comes from co-deployed 3-axis accelerometers showing that these deeper dives, which occur in most individuals we tracked, involve active under-water propulsion without detectable initial assistance from momentum, sometimes with bottom phases typical of active prey pursuit. Furthermore, we find (from co-deployed GPS) that diving occurs primarily in the distal portions of long-distance foraging trips, with deeper dives occurring exclusively during daylight or civil twilight, confirming the importance of visual guidance.Fundação para a Ciênica e Tecnologia - FCTinfo:eu-repo/semantics/publishedVersio

The diet of feral cats at New Island, Falkland Islands, and impact on breeding seabirds

We studied the diet of feral cats (Felis catus) on New Island, Falkland Islands, through the analysis of 373 scats collected during the austral summers of 2004/2005 and 2005/2006. The most frequent prey were three introduced mammals (house mice Mus musculus, ship rats Rattus rattus and rabbits Sylvilagus sp.) and the thin-billed prion Pachyptila belcheri (each season present on ca. 21% of the analysed scats). These represent the Wrst systematic data on feral cat diet for the Falklands. A simple bioenergetics model suggests that cats could be eating in the region of 1,500–11,000 prions per season, representing <1% of the local adult and subadult population. Predation on other seabirds nesting on New Island (several penguin species, albatrosses and cormorants) was unimportant, with the possible exception of white-chinned petrels Procellaria aequinoctialis, which nest locally in very small numbers. For each prion eaten, cats were estimated to have killed 1.1–1.9 ship rats during the summer season, and probably more in autumn and winter. Knowing that ship rats are prion predators, it is conceivable that, on the whole, cats are having a positive impact on the prion population, a scenario predicted by general theoretical models. Thus, considering the available information, we would not recommend the implementation of any eradication programme on New Island that would target cats in isolation. Nevertheless, it would be prudent to consider some local action targeting cats and rats around the small New Island white-chinned petrel colony

Seabird diet analysis suggests sudden shift in the pelagic communities of the subtropical Northeast Atlantic

The dynamics of the subtropical pelagic ecosystems of the Northeast Atlantic are still poorly known due to the high costs associated with sampling large oceanic areas. Top predators can be used as alternative low-cost samplers and indicators of the temporal variability of such systems. To study the variation in the composition of pelagic species through time in the broad Canary current region, we analysed foraging trips and regurgitations of Cory's shearwaters Calonectris borealis nesting on Selvagens islands, in 2008-2011 and 2016-2018. Fisheries data, oceanographic variables and the North Atlantic Oscillation were explored as possible explanatory variables for trends in behaviour and diet. Cory's shearwaters' diet, complemented by fisheries data, revealed marked changes in the composition of the pelagic fish communities. In 2016 there was a peak in the abundance of the Atlantic chub mackerel Scomber colias, followed by an explosive increase in the abundance of the Longspine snipefish Macroramphosus scolopax in 2017 and 2018, as deduced from the diet composition of the Cory's shearwater, and supported by fisheries data, in the broad oceanic area surrounding the Selvagens islands. Oceanographic variables did not show fluctuations correlated with these marked shifts in pelagic fish availability, the causes of which remain largely unknown. This study highlights the importance of the Atlantic chub mackerel and of the Longspine snipefish in the Madeira/Canary region and exemplifies the efficiency of avian predators in revealing rapid changes in pelagic communities of oceanic domains. Such trends and variations need to be better monitored and understood to measure the impact of ongoing global changes and to sustainably manage the marine environment and resources.Fundação para a Ciência e Tecnologia - FCTinfo:eu-repo/semantics/publishedVersio

Provenance does matter: links between winter trophic segregation and the migratory origins of European robins

Amongst migratory species, it is common to find individuals from different populations or geographical origins sharing staging or wintering areas. Given their differing life histories, ecological theory would predict that the different groups of individuals should exhibit some level of niche segregation. This has rarely been investigated because of the difficulty in assigning migrating individuals to breeding areas. Here, we start by documenting a broad geographical gradient of hydrogen isotopes (δ2H) in robin Erithacus rubecula feathers across Europe. We then use δ2H, as well as wing-tip shape, as surrogates for broad migratory origin of birds wintering in Iberia, to investigate the ecological segregation of populations. Wintering robins of different sexes, ages and body sizes are known to segregate between habitats in Iberia. This has been attributed to the despotic exclusion of inferior competitors from the best patches by dominant individuals. We find no segregation between habitats in relation to δ2H in feathers, or to wing-tip shape, which suggests that no major asymmetries in competitive ability exist between migrant robins of different origins. Trophic level (inferred from nitrogen isotopes in blood) correlated both with δ2H in feathers and with wing-tip shape, showing that individuals from different geographic origins display a degree of ecological segregation in shared winter quarters. Isotopic mixing models indicate that wintering birds originating from more northerly populations consume more invertebrates. Our multi-scale study suggests that trophic-niche segregation may result from specializations (arising in the population-specific breeding areas) that are transported by the migrants into the shared wintering grounds

Spatial and temporal aggregation of albatross chick mortality events in the Falklands suggests a role for an unidentified infectious disease

In the context of environmental change, determining the causes underpinning unusual mortality events of vertebrate species is a crucial conservation goal. This is particularly true for polar and sub-polar colonial seabirds, often immunologically naïve to new and emerging diseases. Here, we investigate the patterns of black-browed albatross (Thalassarche melanophris) chick mortality events unrelated to predation recorded between the 2004/05 and 2019/2020 breeding seasons in four colonies across the species range in the Falklands. The prevalence of these mortality events was highly variable across years, causing the death of between 3 and 40% of all chicks in the studied plots. With few exceptions, mortality was patchily distributed. Using clustering methodologies, we identified the spatio-temporal mortality clusters based on the nest locations and chick death date. Using generalised linear models and generalised additive mixed-effects models we found that chicks nearer the first mortality event were predicted to die before those in more distant nests. The probability of death increased with age and was highest for chicks close to nests where a chick had died previously. Our findings, along with the symptoms consistently exhibited by most deceased chicks in the study, strongly suggest the prevalence of a widespread infectious disease, potentially with a common aetiology, both in areas with regular and with very rare human presence. Understanding the causes driving these disease-related mortality events, which seem different from the outbreaks documented in the literature, is a conservation priority for the Falklands black-browed albatross population, which comprises over 70% of the species global population.Fundação para a Ciência e Tecnologia - FCTinfo:eu-repo/semantics/publishedVersio