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Sex-specific niche differentiation is common in marine vertebrates, but how this varies long-term is poorly understood. Here we investigated interannual variation in sexual segregation among breeding northern gannets Morus bassanus, wide-ranging central-place foragers with slight sexual dimorphism. Over 11 breeding seasons, we used GPS tracking and/or stable isotopes to test for sex differences in foraging trip characteristics (range, duration and timing); spatial distribution; habitat selection; and carbon and nitrogen isotopes in blood. When combining data from all years, females foraged further and for longer than males, yet despite this, the foraging areas of the sexes almost completely overlapped. Males and females selected foraging habitats that differed in terms of oceanography but not fishing vessel density. We also detected temporal segregation: females were more likely to be at sea during the day than at night, while males were more likely to be at sea during the night. However, foraging behaviour quantified by all GPS analyses varied interannually, with sex differences detected in some years but not others. Finally, males had consistently higher red blood cell δ13C and δ15N than females across all years, which was not driven by size dimorphism, instead likely by prey choice or very fine-scale habitat selection. We conclude that environmental variation influenced short-term sex differences in movement, but sex differences in stable isotopes that integrate behaviour over longer periods reveal more consistent differences. Our results suggest that inferences drawn from single-year studies may not relate to general patterns, highlighting the importance of long-term studies and combining methods.

The work was funded by the Natural Environment Research Council (Standard Research Grant NE/H007466/1) and donations from Evan Fountain (In Memoriam donations for Issy Fountain) and Mike Matthewson. Marine vertebrates show a diversity of migration strategies, including sex differences. This may lead to differential demography, but the consequences of such between-sex variation are little understood. Here, we studied the migration of known-sex northern gannets Morus bassanus — a partial migrant with females ~8 % heavier than males. We used geolocators to determine wintering areas of 49 breeding adults (19 females and 30 males during 2010 to 2014) from 2 colonies in the northeast Atlantic (Bass Rock and Grassholm, UK). We also tested for sex-specific survival probabilities using capture−mark−recapture methods (n = 72 individuals Bass Rock, n = 229 individuals Grassholm; 2010−2018) and applied sex-specific population projection matrices (PPMs) to quantify population-level effects. Tracked gannets wintered in a range of large marine ecosystems (LMEs): Canary Current LME (CCLME; 69 %), Celtic-Biscay Shelf LME (16 %), Iberian Coastal LME (8 %), North Sea LME (4 %) or Mediterranean LME (2 %). Migratory destination differed between the sexes: 90 % of females vs. 57 % of males wintered in the CCLME. Survival was similar between the sexes at Bass Rock (mean ± 95 % CI = 0.951 ± 0.053 and 0.956 ± 0.047 for females and males, respectively). At Grassholm, there was evidence of slight sex differences in breeder survival: females had lower annual survival (0.882 ± 0.040) than males (0.946 ± 0.026). At Bass Rock, PPMs with no sex effect best fitted the observed population increase (1994−2014). Sex-specific PPMs fitted the population estimates for Grassholm (1995−2015). Our results reveal that female gannets are more likely to travel further than males to winter in the CCLME. This difference is unlikely due to morphological differences, unlike in other bird species. However, the reason for slightly higher over-winter female mortality at Grassholm is unclear. Publisher PDF

Individual foraging site fidelity (IFSF) has been documented in a wide range of species, but few studies have examined the incidence or implications of variation among individuals in levels of fidelity, especially among short-ranging species where costs of travel place fewer constraints on exploring alternative foraging sites. Using combined GPS and dive data for 560 trips by 70 birds, we quantified the repeatability of foraging behaviour including IFSF in a short-ranging, mainly benthic predator, the European shag Phalacrocorax aristotelis, across 3 consecutive breeding seasons at a colony in NE England. There was significant repeatability in a wide range of foraging trip parameters, with highest consistency in those related to foraging location and maximum dive depth, and lowest consistency in those related to trip duration and time spent in different activities. Birds also had high IFSF overall but there was marked variation among individuals in this respect: some were highly consistent in the locations visited over multiple years whereas others frequently changed their foraging locations between successive trips. IFSF was typically higher from one year to the next than within a single year, with most birds retaining similar levels of consistency from year to year. Females with higher IFSF during chick-rearing were in better condition than birds with lower consistency and had earlier hatching dates. These data strongly suggest IFSF may be beneficial even in short-ranging species, at least in benthic feeders where prior knowledge and experience of particular habitat patches and associated prey capture techniques may be advantageous.

Individual specialisations in animals are important contributors to a wide range of ecological and evolutionary processes, and have been particularly documented in relation to multiple aspects of foraging behaviours. Central-place foragers, such as seabirds, frequently exhibit pronounced specialisations and individual differences in a variety of foraging traits. In particular, the availability of fisheries discards alongside natural prey resources provides additional potential for differentiation and specialisation for opportunistically scavenging seabird species. However, the consequences of such specialisations for at-sea distributions and intraspecific interactions are not well known. Here, we investigated the links between the degree of dietary specialisation on natural or discarded prey and the foraging movements and spatial occupancy of northern gannets Morus bassanus in relation to differing intraspecific competition at 6 colonies of differing sizes. We found that, at most colonies, individuals with different dietary strategies concentrated foraging at differing levels of intraspecific competition. In addition, individuals pursuing different strategies were frequently, but not consistently, spatially separated, distinctions that were most acutely seen in females. However, this variation in individual strategy had no significant impact on current body condition. These analyses demonstrate how foraging-associated metrics need not covary within an unconstrained system. They also reveal that specialisation can have important consequences for the competitive regimes individuals experience, highlighting the complexity of examining interacting consequences at large spatial scales.

Sexual segregation, common in many species, is usually attributed to intra-specific competi- tion or habitat choice. However, few studies have simultaneously quantified sex-specific foraging be- haviour and habitat use. We combined movement, diving, stable isotope and oceanographic data to test whether sexual segregation in northern gannets Morus bassanus results from sex-specific habitat use. Breeding birds foraging in a seasonally stratified shelf sea were tracked over 3 consecutive breeding seasons (2010�2012). Females made longer trips, foraged far- ther offshore and had lower δ 13 C values than males. Male and female foraging areas overlapped only slightly. Males foraged more in mixed coastal waters, where net primary production (NPP) was relatively high (>3 mg C m �2 d �1 ) and sea-surface temperature (SST) was relatively low ( 15°C) more than females, possibly as a consequence of foraging in productive mixed waters over offshore banks. Females foraged most frequently in stratified offshore waters, of intermediate SST (12�15°C), but exhibited no consistent response to NPP. Sex-specific differences in diving behaviour corresponded with differences in habitat use: males made more long and deep U- shaped dives. Such dives were characteristic of inshore foraging, whereas shorter and shallower V-shaped dives occurred more often in offshore waters. Heavier birds attained greater depths during V-shaped dives, but even when controlling for body mass, females made deeper V-shaped dives than males. Together

Impacts of anthropogenic climate change on marine ecosystems are now widely acknowledged. In the NE Atlantic, abundant evidence from the partly enclosed waters of the North Sea indicates that recent climate-induced changes in primary and secondary productivity have been propagated up the food chain, with marked consequences for higher vertebrate predators such as seabirds. In contrast, however, there is much less indication of such impacts on higher predators in the more open Atlantic waters around the west coast of the British Isles. Through an annual compar- ison of chick growth and adult food provisioning behaviour of Manx shearwaters in SW Wales, we found that birds bred later and chicks attained lower peak and fledging masses in 2007 and 2008 than in any previous recorded year dating back to 1965. These changes were accompanied by a reduction in parental attendance at the colony, which was probably the result of parents switching to a dual for- aging strategy in 2007 and 2008. These events were linked to higher sea surface temperature in the preceding winter and to a reduction in prey quality, as indicated by the mean body mass of 2 yr old herring. These are the first such findings for the west coast of Britain and indicate that within the NE Atlantic region, adverse impacts of climate change on higher marine predators are no longer restricted to the vicinity of the North Sea.

Many seabirds nesting in areas bordering the North Sea have recently experienced large annual variation in breeding success, including reproductive failures in some cases. In contrast, the breeding success of northern gannets Morus bassanus has remained remarkably stable. The present study examines data from the large gannet colony at the Bass Rock (southeast Scotland) across 3 years, to assess the extent to which such stability may reflect both flexibility and consistency in diets and foraging behaviour. Adults exhibited great flexibility both in the species and sizes of prey consumed and in foraging trip durations, ranges and total distances travelled. They also showed a high degree of consistency in bearings of foraging trips and in behaviour at sea; the sinuosity of foraging tracks and average speed of travel was very similar each year and birds in all years spent about half their time at sea in flight. Adults returned to the nest at higher speeds from more distant foraging locations up to ca. 300 km from the colony, but speeds decreased for the farthest destinations (>ca. 400 km). Moreover, the relationship between trip duration and distances travelled at sea was asymptotic beyond ca. 60 h. These non-linear relationships probably reflected constraints on energy expenditure during flight. As a result, nest attendance was low in years with long average trip durations and chicks were left unattended and vulnerable to attack by conspecifics. These data suggest that while adults have so far been able to maintain high reproductive success in years of low prey availability, they may not be able to do so in future years if providing sufficient food for chicks entails any further increases in trip duration or foraging effort.

Seabirds may be able to increase their foraging efficiency by learning the whereabouts of predictable sources of prey and returning repeatedly to these locations. The occurrence of such foraging area fidelity has been little studied, particularly for pelagic species. We used satellite telemetry to study foraging behaviour and foraging area fidelity of individual chick-rearing gannets in the North Sea (at the Bass Rock, SE Scotland) and in the Celtic Sea (at Great Saltee, SE Ireland), 2 areas that differ in the distribution of prey. Foraging ranges of adults covered a wide area of ocean up to a maximum range of 540 km at the Bass Rock and 240 km at Great Saltee. At the Bass Rock, individual birds foraged in a single direction or at most in 2 distinct directions, with very similar bearings on successive trips in each direction, significant differences in bearings between individuals and significant repeatability in distances travelled. These results strongly suggest that individuals learned and remembered the locations of feeding sites and used that knowledge on subsequent foraging trips. By contrast at Great Saltee, bearings of successive trips by individuals were much less similar, with highly variable distances to destinations, no differences in bearings among individuals and no significant repeatability in distance travelled. These results indicate a much lower degree of foraging area fidelity at Great Saltee, probably due to a more uniform or less predictable distribution of prey in the Celtic Sea than in the North Sea. Despite marked differences between colonies in distances to trip destinations, durations of foraging trips and foraging area fidelity, the behaviour of birds during foraging trips was very similar at the 2 sites: the average speed of travel during foraging trips was almost identical and birds at both colonies spent about half their time at sea in flight.

We used satellite telemetry to examine the foraging ranges, feeding locations and travel speeds of 17 chick-rearing gannets Morus bassanus from the Bass Rock, SE Scotland. Regurgitates indicated that birds at the colony exploited a wide range of prey, frequently including 0-group sandeels (