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ABSTRACTThe Gannet Morus bassanusis one of the seabirds considered most at risk from collision mortality at offshore wind farms in UK waters, so a better understanding of migration routes informs assessments of risk for different populations. Deployment of geolocators on breeding adults at the Bass Rock, Scotland, and Skrúður, Iceland, showed that the timing of migrations differed between populations, birds from Bass Rock passing south through UK waters mostly in October and back in February while birds from Skrúður passed south through UK waters mostly later, in November, but returned north earlier, in January. Many birds from both colonies made a clockwise loop migration around Britain and Ireland. Only a minority of birds from the Bass Rock returned northwards to the colony through the southern North Sea. A counter-intuitive consequence is that many Gannets moving northwards through waters to the west of Britain and Ireland in spring may be birds from North Sea colonies. Although Gannets normally remain over the sea, one tracked bird appears to have made a short overland passage in spring from the west of Scotland through central Scotland to the Bass Rock, whereas most returned around the north of Scotland.

ABSTRACTAlthough studies of wing length in many bird species show longer wings in adults than in juveniles, and increases in wing length among immature age classes of long-lived birds, evidence for an increase in wing length among adult small passerines is very limited. Previous studies of Blue Tits Cyanistes caeruleushave concluded that adults have longer wing lengths than juveniles but that there is no change in wing length with age of adults, yet the latter conclusion has been based on very limited data. Here we examine wing-length data from a large number of retraps of Blue Tits over a 30-year period at a single site in west Scotland. We found very little evidence for feather wear affecting wing-length measurements during the non-breeding season and, as expected, adults had longer wing lengths than juveniles. However, when comparing individuals caught as both one- and two-year-olds, wing lengths were significantly more likely to increase than decrease. This result has implications for studies of evolutionary biology of Blue Tits, such as survival of individuals in relation to biometrics. It would be interesting to determine whether such age-related increases in wing length are widespread in passerines.

Large-scale and long-term ringing and cohort colour ringing of chicks, combined with trapping of immatures on clubs and adults on nests, allows the age of first breeding by Great Skuas Stercorarius skuato be investigated. Modal and median age of first breeding of Great Skuas at their largest colony (Foula, Shetland) was seven years, but there was considerable individual variation. Birds may first breed when four to >11 years old. Compared with the situation in Foula, Great Skuas start breeding at a younger age at smaller colonies. When conditions at Foula deteriorated, resulting in reduced adult survival, reduced breeding success and population decline, age of first breeding increased. These patterns imply a density-dependent age of first breeding, but with individual variation because higher-quality birds recruit at a younger age than birds in poor body condition. Density-dependent variation in age at first breeding may have an important influence on population growth rate, and this would merit study also in other species of seabirds through a dedicated ringing and colour-ringing programme.

Given the conservation importance of Gannet populations in Britain, and concerns about possible adverse impacts of offshore wind farms on Gannets as a result of collision risk, we advocate the establishment of strategic monitoring studies at key colonies. Colour ringing adult Gannets to measure survival at colonies close to and distant from offshore wind farms could indicate whether or not collision mortality has a detectable effect on adult survival. Colour ringing chicks could provide information on prospecting movements of immatures and colonies where birds recruit. Tracking studies of both adults and immatures would be highly desirable to complement improved monitoring of demography.

Advances in technology and engineering, along with European Union renewable energy targets, have stimulated a rapid growth of the wind power sector. Wind farms contribute to carbon emission reductions, but there is a need to ensure that these structures do not adversely impact the populations that interact with them, particularly birds. We developed movement models based on observed avoidance responses of common eider Somateria mollissimato wind farms to predict, and identify potential measures to reduce, impacts. Flight trajectory data that were  collected post-construction of the Danish Nysted offshore wind farm were used to parameterize competing models of bird movements around turbines. The model most closely fitting the observed data incorporated individual variation in the minimum distance at which birds responded to the turbines. We show how such models can contribute to the spatial planning of wind farms by assessing their extent, turbine spacing and configurations on the probability of birds passing between the turbines. Avian movement models can make new contributions to environmental assessments of wind farm developments, and provide insights into how to reduce impacts that can be identified at the planning stage.

At St Kilda, Outer Hebrides, a large colony of great skuas Stercorarius skuafeed extensively on one of the largest colonies of Leach’s storm-petrels Oceanodroma leucorhoain Europe, but little is known about the dynamics of this predator–prey system. Recently published population estimates of storm-petrels make it possible to estimate the impact of skua predation for the first time. Although skuas in the southern hemisphere catch petrels attending breeding colonies at night, it is not known whether congeners in the northern hemisphere also forage during the hours of darkness. We found (using radio-transmitters) that skuas regularly forage at night and (using light intensifying equipment) observed them catching storm-petrels at night. However, skuas also foraged during daylight hours, and it is unknown whether they might also catch storm-petrels at sea. Data on diet composition reveals that the proportion of storm-petrels in skua diet declined between 1996 and 1997, but remained constant thereafter. Although a large proportion of the storm-petrel prey is likely to consist of non-breeders, numbers consumed suggest that breeders and an unknown quantity of transients may also been eaten. The numbers of storm-petrels eaten are not sustainable and may result in substantial long-term population declines. Under current conditions, maintenance of large populations of both Leach’s storm-petrels and great skuas at St Kilda appears to be mutually exclusive.

Adaptive sex–ratio theory predicts that parents should overproduce the more beneficial offspring sex. Based on a recent experimental study of lesser black–backed gulls, we tested this hypothesis with the great skua, Catharacta skua, a bird species closely related to gulls but where females are the larger sex. When in poor body condition, the gulls overproduced daughters, the smaller and more viable sex under those circumstances. To discriminate between a mandatory physiological overproduction of female (i.e. non–male) eggs versus the overproduction of the smaller and presumably more viable sex, we conducted an egg–removal experiment with the great skua. Since the males are smaller, larger size and being male are separated. Through egg removal we induced females to increase egg production effort. Eggs were sexed using a DNA–based technique. Manipulated pairs produced a significant male bias at the end of the extended laying sequence, while the sex ratio in the control group did not differ from unity. Our results present an example of facultative sex–ratio manipulation and support the hypothesis that in sexually dimorphic birds parents overproduce the smaller sex under adverse conditions.

The lice, Haffneria grandisand Saemundssoniawere extracted from their hosts, skuas (Aves: Stercorariidae). Lice were extracted from dead birds by combing feathers, while lice from live birds were extracted using a delousing chamber containing chloroform vapour. Lice were measured and the data analysed by canonical discriminant analysis. Lice show variation in morphology that is useful in identifying some of these hosts. This variation is presumably due to micro-environmental pressure provided by each host. Our interpretation is that these chewing lice have evolved to be adapted to each skua taxon; different sizes and morphologies of skua taxa result in different sizes and morphologies of Haffneria grandisand Saemundssonia.

We studied mercury concentrations and amounts in tissues of 19 starved young Common Tern chicks (median age 4 days) and in eggs from the same colony. Concentrations and burden were similar between eggs and newly hatched chicks. Mercury concentrations were highest in down, which contained at least 38% of the body mercury. The mercury burden of the whole body and of the tissues as well as the concentration in down increased with age and body mass, indicating the importance of down as an elimination pathway. Conversion ratios between mercury concentrations in tissues and the whole chick body varied according to the contamination level.

Upwelling regions are highly productive habitats targeted by wide-ranging marine predators and industrial fisheries. In this study, we track the migratory movements of eight seabird species from across the Atlantic; quantify overlap with the Canary Current Large Marine Ecosystem (CCLME) and determine the habitat characteristics that drive this association. Our results indicate the CCLME is a biodiversity hotspot for migratory seabirds; all tracked species and more than 70% of individuals used this upwelling region. Relative species richness peaked in areas where sea surface temperature averaged between 15 and 20°C, and correlated positively with chlorophyll a, revealing the optimum conditions driving bottom-up trophic effects for seabirds. Marine vertebrates are not confined by international boundaries, making conservation challenging. However, by linking diversity to ocean productivity, our research reveals the significance of the CCLME for seabird populations from across the Atlantic, making it a priority for conservation action.