Search

Sharing the same wintering grounds by avian populations breeding in various areas may synchronize fluctuations in vital rates, which could increase the risk of extinction. Here, by combining multi-colony tracking with long-term capture-recapture data, we studied the winter distribution and annual survival of the most numerous Arctic seabird, the little aukAlle alle. We assessed whether little auks from different breeding populations in Svalbard and Franz Josef Land use the same wintering grounds and if this leads to synchronized survival. Our results indicate that birds from the Svalbard colonies shared similar wintering grounds, although differences existed in the proportion of birds from each colony using the different areas. Little auks from Franz Josef Land generally spent the winter in a separate area, but some individuals wintered in the Iceland Sea with Svalbard populations. Survival data from 3 Svalbard colonies collected in 2005-2018 indicated that sharing wintering grounds did not synchronize little auk annual survival rates. However, it is clear that the Iceland Sea is an important wintering area for little auks, and environmental changes in this area could have widespread impacts on many populations.

Foraging ecology of chick rearing seabirds is affected mainly by the food availability on feeding grounds, but it can be also modulated by environmental conditions during the foraging trip, in that wind force. Considering predicted strengthening of surface winds over the Arctic Ocean, this factor may have a growing impact on the foraging performance of Arctic seabirds. Here, we studied how wind speed could affect prey accessibility for the High Arctic zooplanktivorous seabird, the little auk Alle alle breeding in Svalbard in 2015-2019. First, we estimated availability of its preferred prey, a cold water copepod Calanus glacialis, based on wider-scale mesozooplankton biomass model and environmental conditions. Then we estimated prey accessibility by including wind speed, the factor affecting the flapping flight performance of little auks commuting from/to the colony. Finally, we compared reproductive performance of the little auks (chick diet, growth rate and survival and duration of foraging flights of adults) between the studied years differing in wind and food availability conditions. We found that wind speed could affect significantly food accessibility for a zooplanktivorous seabird. Despite high spatial and temporal variability in prey availability and accessibility in shelf waters of SW Spitsbergen, interannual differences in duration of foraging flights and chick growth rate, little auks were able to sustain high breeding success confirming their capacity to buffer suboptimal foraging conditions. Our multidisciplinary work, combining multi-year remote sensing of oceanographic conditions, zooplankton availability and accessibility modelling, little auks diet composition and chick growth and survival emphasizes the importance of including wind conditions in the studies of foraging ecology of seabirds.

Studies on time allocation of various activities are crucial to understand which behavioural strategy is the most profitable in a given context, and so why animals behave in a particular way. Such investigations usually focus on a time window when the studied activity is performed, often neglecting how the time devoted to focal activity affects time allocation to following-up behaviours, while that may have its own fitness consequences. In this study, we examined time allocation into three post-foraging activities (entering the nest with food, nest attendance, and colony attendance) in a small seabird species, the little auk (Alle alle). Since little auks alternate foraging trips of different duration (short and long) and purpose (offspring feeding and primarily self-feeding, respectively) we expected that duration of the following up in-colony activities would also vary, being longer after a long absence in the colony (because of greater need of reassessment of the current predation pressure and social interactions in the colony, and re-establishing the bond with the offspring and/or partner and/or neighbours after longer absence). We found that it was not always the case, as time allocation of the post-foraging in-colony activities was primarily year- and sex-specific. It highlights the need to consider year and sex effects in studies of behavioural ecology, as not doing so may lead to spurious conclusions. Interestingly, and despite a great inter-individual variation in time allocation in the post-foraging in-colony activities, little auk individuals were quite repeatable in their behavioural performance, which suggests these activities may reflect birds behavioural profile. Overall, post-foraging in-colony activity of the little auk, although not much dependent on duration/type of the preceding foraging flights, varies with respect to year and sex, and as such may be a proxy of behavioural plasticity of the population.

Whether variation of winter migration distance among individuals within a population has consequences on their reproductive performance has rarely been assessed. Using individual global location sensor-tracking, we determined variation in migration distance, and its breeding consequences, in a Svalbard colony of the little auk Alle alle, a high-Arctic seabird with high flight costs. We found 2 distinct wintering quarters (north of Iceland and SW Greenland) implying a 2-fold difference in migration distance (1500 vs. 3100 km on average from the colony). This migration route from Svalbard to Greenland is among the longest recorded distances for an alcid species. Birds travelling a longer distance were more likely to be females and smaller, but more importantly, migration strategy had no apparent effect on reproductive performance (either timing or success). Our results contradict predictive studies which suggested that the establishment of migration routes to different winter destinations may be limited by the energetic constraints of flight. Our results show that a longer migration route does not necessarily have carry-over effects on subsequent reproduction.

International audience; Abstract: Tracking data of marine predators are increasingly used in marine spatial management. We developed a spatial dataset with estimates of the monthly distribution of six pelagic seabird species breeding in the Northeast Atlantic. The dataset is based on year-round global location sensor (GLS) tracking data of 2356 adult seabirds from 2006-2019 from a network of seabird colonies, data describing the physical environment, and data on seabird population sizes. Tracking and environmental data were combined in monthly species distribution models (SDMs). Cross-validations were used to assess the transferability of models between years and breeding locations. The analyses showed that birds from colonies close to each other (1000 km) used colony-specific, and in many cases, non-overlapping habitats. Based on these results, the SDM from the nearest model colony was used to predict the distribution of all seabird colonies lying within a species-specific cut-off distance (400-500 km). The uncertainties in predictions were estimated by cluster bootstrap sampling. The resulting dataset consists of 4692 map layers, each layer predicting the densities of birds from a given species, colony and month across the North Atlantic. The dataset represents the annual distribution of 23.5 million adult pelagic seabirds, or 87% of the Northeast Atlantic breeding population of the study species. We show how the dataset can be used in population and spatial management applications, including the detection of population-specific nonbreeding habitats and identifying populations influenced by marine protected areas.

Bird migration is commonly defined as a seasonal movement between breeding and non-breeding grounds. It generally involves relatively straight and directed large-scale movements, with a latitudinal change, and specific daily activity patterns comprising less or no foraging and more traveling time. Our main objective was to describe how this general definition applies to seabirds. We investigated migration characteristics of 6 pelagic seabird species (little auk Alle alle, Atlantic puffin Fratercula arctica, common guillemot Uria aalge, Brünnich’s guillemot U. lomvia, black-legged kittiwake Rissa tridactyla and northern fulmars Fulmarus glacialis). We analysed an extensive geolocator positional and saltwater immersion dataset from 29 colonies in the North-East Atlantic and across several years (2008-2019). We used a novel method to identify active migration periods based on segmentation of time series of track characteristics (latitude, longitude, net-squared displacement). Additionally, we used the saltwater immersion data of geolocators to infer bird activity. We found that the 6 species had, on average, 3 to 4 migration periods and 2 to 3 distinct stationary areas during the non-breeding season. On average, seabirds spent the winter at lower latitudes than their breeding colonies and followed specific migration routes rather than non-directionally dispersing from their colonies. Differences in daily activity patterns were small between migratory and stationary periods, suggesting that all species continued to forage and rest while migrating, engaging in a ‘fly-and-forage’ migratory strategy. We thereby demonstrate the importance of habitats visited during seabird migrations as those that are not just flown over, but which may be important for re-fuelling.

Capsule: The Red-backed Shrike Lanius collurio and the Barred Warbler Sylvia nisoria had similar habitat preferences and their territories often overlapped. However, we found that Red-backed Shrikes were more flexible in habitat choice whilst Barred Warblers had more specific requirements. Aim: We aimed to analyse and compare distribution and habitat preferences of Red-backed Shrikes and Barred Warblers breeding sympatrically in semi-natural landscape in a wetland/farmland mosaic. Methods: We examined habitat availability and use by the two species within their breeding territories to identify differences in habitat selection. Results: Territories of both species were similar in habitat composition and used levees, bushes, fallow areas and single trees. However, the spatial characteristics of the territories differed between species. Red-backed Shrikes used a wider range of sizes and shapes of habitat patches, whilst Barred Warblers preferred a more complex landscape structure and a higher diversity of habitat types. We also found that areas of 71% of Barred Warbler and 34% of Red-backed Shrike territories overlapped. Conclusion: Whilst both species showed similar habitat choices, they appeared to differ significantly in terms of landscape structure: Red-backed Shrikes were more flexible and less selective than Barred Warblers in their habitat choice.

Here, we model current and future distribution of a foraging Arctic endemic species, the little auk (Alle alle), a small zooplanktivorous Arctic seabird. We characterized environmental conditions [sea depth, sea surface temperature (SST), marginal sea ice zone (MIZ)] at foraging positions of GPS-tracked individuals from three breeding colonies in Svalbard: one located at the southern rim of the Arctic zone (hereafter ‘boreo-Arctic’) and two in the high-Arctic zone on Spitsbergen (‘high-Arctic’). The birds from one ‘high-Arctic’ colony, influenced by cold Arctic water, foraged in the shallow shelf zone near the colony. The birds from remaining colonies foraged in a wider range of depths, in a higher SST zone (‘boreo-Arctic’) or in the productive but distant MIZ (second ‘high-Arctic’ colony). Given this flexible foraging behaviour, little auks may be temporarily resilient to moderate climate changes. However, our fuzzy logic models of future distribution under scenarios of 1 °C and 2 °C SST increase predict losses of suitable foraging habitat for the majority of little auk colonies studied. Over longer time scales negative consequences of global warming are inevitable. The actual response of little auks to future environmental conditions will depend on the range of their plasticity and pace of ecosystem changes.

Mercury (Hg) contamination is considered a global concern for humans and wildlife, and although the number of studies dealing with that issue continues to increase, some taxonomic groups such as small passerine birds are largely understudied. In this paper, concentration of mercury in the aquatic warbler (Acrocephalus paludicola) feathers, a globally threatened passerine species, was examined. The concentration differences between two ages and sexes were investigated. The comparison of feathers taken on autumn migrants of two age categories act as a comparison of the species’ exposure within the two different areas (European breeding or African wintering grounds). The average Hg concentration for all sampled individuals [2.32 μg/g dw (range 0.38–12.76)] is relatively high, compared with values found in other passerine species. An age difference was found, with first-year individuals displaying higher mercury concentrations than adults. This indicates that birds are exposed to mercury pollution during the breeding season, i.e., in the continental floodplains of eastern Europe. The average Hg concentration in feathers grown on the breeding grounds was 3.88 ± 2.59 μg/g dw, closer to the critical value of 5 μg/g dw, which is considered to impair the health of individuals. The findings suggest that mercury pollution may constitute a threat so far neglected for the endangered aquatic warbler.

An increasing number of studies report coordinated chick provisioning by avian parents. Although the pattern of parental coordination varies across species, broad occurrence of this coordination suggests that it has an adaptive value: it may increase individual fitness via higher offspring survival, faster offspring growth rate and/or higher body reserves of the parents. However, to what extent the pattern of coordinated provisioning in a species represents a flexible response to current foraging conditions remains an open question. Here, we examined coordination of chick provisioning in the Little Auk (Alle alle), a planktivorous seabird species that breeds in the Arctic. Harsh environmental conditions impose bi-parental care on this species, and high variability within and across breeding seasons promotes flexibility in parental involvement to secure breeding success. During the chick rearing period, parents exhibit a dual-foraging strategy (i.e., alternating long foraging trips, serving to maintain the adults' body reserves, with several short trips aimed to provision the chick). We examined coordination of parental provisioning across five breeding seasons varying in terms of environmental conditions and found that the parents indeed coordinate their provisioning, avoiding performing long trips simultaneously and thus enabling a more even distribution of feeding through time. We also examined chick body condition in relation to the level of parental coordination to test the potential adaptive value of coordination, but we found no significant relationship between these two parameters. We found high variability in the level of the coordination between pairs, and this variability was similar across all study seasons, which represented a wide range of experienced environmental conditions. Nevertheless, we found that the energy density of food loads delivered to chicks was associated with the level of parental coordination: when conditions were characterized by the delivery of higher-energy food loads, the level of coordination exhibited by the studied population was higher. These findings suggest that environmental conditions somehow affect parental coordination, but the range of the environmental variation could be still below a critical threshold of extreme conditions that would trigger more pronounced modifications of parental foraging patterns and coordination.

The importance of nest characteristics for birds breeding in the extreme climate conditions of polar regions, has been greatly understudied. Nest parameters, like nest orientation, exposure and insulation, could strongly influence microclimate and protection against precipitation of the nest, thereby affecting breeding success. A burrow nesting seabird, the Wilson’s storm-petrel (Oceanites oceanicus) is an excellent model species to investigate the importance of nest characteristics, as it is the smallest endotherm breeding in the Antarctic. Here, we investigated the effects of nest parameters such as internal nest dimensions, nest micro-topography and thermal properties of the nest burrow and the influence of weather conditions on breeding output, measured as hatching success, chick survival, and chick growth. We collected data during the austral summers of 2017 and 2018, on King George Island, maritime Antarctica. Our results showed that the thermal microclimate of the nest burrow was significantly improved by a small entrance size, a low nest height, and insulation and tended to be enhanced by a low wind exposition index and an eastern nest site orientation. In addition, an eastern nest site orientation significantly reduced the chance of snow blocking. However, the relationships between nest characteristics and breeding output were complex and might be affected by other parameters like food availability and parental quality. The relation between chick growth and nest air temperature remained especially indistinct. Nevertheless, our results indicate that nest characteristics that enhance the thermal microclimate and reduce the risk of snow blocking favoured both hatching success and chick survival. Due to climate change in the Antarctic, snowfall is expected to increase in the future, which will likely enhance the importance of nest characteristics that determine snow blocking. Additionally, despite global warming, thermally favourable nest burrows will likely still be advantageous in the highly variable and challenging Antarctic climate.

Data on trace element bioavailability in the south-polar marine ecosystem is still scarce, compared to that relating to temperate zones. Seabirds can be used as indicators of ecosystem health and sentinels of environmental pollution, constituting a link between marine and terrestrial environments. Here, we analysed the concentration of 17 elements (with special emphasis on mercury, Hg) in feathers of adults and chicks of two pelagic seabirds – the Wilson's storm petrel Oceanites oceanicus and the black-bellied storm petrel Fregetta tropica – breeding sympatrically in the maritime Antarctic. Since adult feathers are formed during the non-breeding period away from the breeding grounds, but down and body feathers of chicks grow at the breeding sites, we were able to evaluate the birds' exposure to contaminants at various stages of their annual life cycle and in various marine zones. We found that of the two studied species, adult black-bellied storm petrels had significantly higher mercury, selenium and copper levels (5.47 ± 1.61; 5.19 ± 1.18; 8.20 ± 0.56 μg g˗1 dw, respectively) than Wilson's storm petrels (2.38 ± 1.47; 1.81 ± 0.98; 2.52 ± 2.35 μg g˗1 dw, respectively). We found that Wilson's storm petrel chicks had a significantly different contaminant profile than adults. Arsenic, bismuth and antimony were detected exclusively in the chick feathers, and the Se:Hg molar ratio was higher in chicks than in adults. Our study also suggests considerable maternal transfer of Hg (to down feathers) in both species. As global contaminant emissions are expected to increase, birds inhabiting remote areas with sparse anthropogenic pollution can indicate the temporal trends in global contamination.

Examining body mass and physiological parameters in breeding adults may provide valuable insight into parental efforts provided by males and females at a given stage of breeding. Here, we examined body mass and physiological variables (hemoglobin and cholesterol concentrations, leukocyte profile) in males and females in a small pelagic seabird, the European Storm Petrel (Hydrobates p. pelagicus) during the incubation period. We expected females to be in poorer physiological condition compared to males because of their assumed higher investment (production of costly egg, with all other parental activities performed being similar). Contrary to our expectation, we did not find sex differences in the hematological values, indicating similar non-resource–based costs of reproduction. Also, we found females in good nutritional state, being heavier and having higher cholesterol concentration than males. Although we are not able to explicitly identify mechanisms responsible for the pattern observed, we su...

Different phenological responses to climate changes by species representing preys and predators may lead to mismatch between functionally dependent components of an ecosystem, with important effects on its structure and functioning. Here, we investigate within-season variation in zooplankton availability, chick diet composition and breeding performance of a small planktivorous seabird, the little auk (Alle alle) in two large colonies in Hornsund and Magdalenefjorden, Spitsbergen, differing in synchrony of breeding (11-day vs. 22-day hatching period, respectively). Assuming similar zooplankton phenology and existing differences in duration of the little auk breeding period, we expected lower availability of the preferred food in the less synchronized colony in Magdalenefjorden and in consequence a negative effect on nestling body mass and survival. We found that in both colonies Calanus glacialis (copepodite stage CV) was the most important prey item in the chick diet making up 68–87 % of the biomass and energy of all prey items. The only exception was the end of the chick-rearing period in Magdalenefjorden, when contribution of this prey item was significantly lower (24–26 %). Thus, late breeders in Magdalenefjorden were apparently mismatched regarding C. glacialis CV availability. However, the hatching date did not affect birds fitness (reproductive output and chick pre-fledging mass) significantly. Results of our study indicate that little auks breeding on Spitsbergen can respond to a wide range of environmental conditions and prey availabilities through the plasticity of their foraging behaviour, which may help them to maintain their optimum fitness level in changing and unpredictable environments.

Birds of different sex and age are known to differ in morphology and physiology. We investigated leucocyte profiles and body condition of the Savi's warbler (Locustella luscinioides) in relation to sex, age and moulting status. We caught birds during the post-breeding period at the stopover site in northern Poland. Of the leucocyte profiles, we examined the heterophil/lymphocyte ratio (H/L), and the number of leucocytes, lymphocytes and heterophils per 10 000 red blood cells (RBC). We found a significant effect of the pentad × age interaction on the H/L ratio. In adults, body condition (BC, size-adjusted body mass) was similar in both sexes. Among adults, females had higher numbers of leucocytes and heterophils per 10 000 RBC and a lower relative number of lymphocytes compared to males. It is possible that subtle sex differences in parental care at the end of the breeding period account for these differences. They may be too subtle to affect BC, but are visible at some leucocyte variables. Moreover, BC of...

Acoustic playback has been shown to substantially increase the number of birds trapped. However, other effects of playback have rarely been investigated; the few studies that have been published strongly suggest that when playback is applied some groups of birds may be captured more easily than others, leading to biased results. In this study, we evaluated the experimental effect of acoustic playback (by comparing days when playback was used and was not used) at a stop-over site in central Europe during the post breeding period and the autumn migration. We examined the influence of playback on numbers of birds captured, i.e., two reedbed passerines—the reed warbler Acrocephalus scirpaceus (RW) and the sedge warbler A. schoenobaenus (SW)—with respect to their age, sex and body condition. We found that playback increased the number of birds captured 2.5 times in RW and 3.6 in SW. We did not find evidence for a differentiated response to the acoustic stimuli between adults and immatures. The sex ratio in RW was not affected by acoustic playback, but we did record significant male-bias in SW. The body condition of lured and non-lured birds was similar in both species. The sex bias revealed in one species but not in the other, clearly shows that great caution should be exercised when using playback to attract various species.

Leucocyte profiles, especially the heterophil/lymphocyte ratio (H/L), are widely used to assess immune function and stress in birds. In this study, we investigated whether leucocyte profiles in a small colonial seabird, the little auk (Alle alle), are related to age (chicks, adults), sex or biometric traits, including body mass. We blood-sampled chicks at 4–8 and 14–18 days of life and chick-feeding adults with 14–20- and 24–30-day-old chicks. Relative numbers of heterophils and H/L generally differed among chicks and adults, with the highest values in adults with 14–20-day-old chicks and the lowest ones in 4–8-day-old chicks. Females had a higher proportion of lymphocytes than males. The relative numbers of eosinophils were similar in all the groups. The percentage of monocytes tended to differ among the age categories. Analyses of chicks revealed that the percentage of heterophils and the H/L ratios were significantly higher in older than in younger chicks. The opposite pattern was recorded for lymphocytes. In 14–18-day-old chicks, H/L (stress level) was negatively related to body mass, head-bill length and bill width. This time period is energetically stressful as nestlings start to exercise their wings outside the nest chamber. This suggests that the origin of stress in nestlings was mainly nutritional. We found a significant positive correlation between body mass at fledging and the relative number of eosinophils at 14–18 days of life, indicating a higher level of stress in lighter nestlings. The significant relationship between fledging age and the percentage of monocytes at 14–18 days of life suggested a poorer state of health in chicks that fledged at an older age.

Although abnormal or injured legs are not uncommon in Hydrobatidae, they are rarely investigated. We aimed in this study to estimate the frequency of leg abnormalities and determine health status (expressed by leucocyte profile) in molecularly sexed European Storm-Petrels (Hydrobates p. pelagicus) captured on the Faroe Islands. We found that 2.4% of individuals captured during the breding season had some leg abnormalities. Half of the birds with abnormalities had puffinosis-like changes, while the rest were missing some part of the leg. Both types of abnormalities were recorded in the two sexes with similar frequency. The proportion of the birds with leg abnormalities seems to be relatively low compared to other Procellariiformes, and stable over time. Despite the apparent disability of the birds with leg abnormalities, we found no significant effect of abnormality status on the leucocyte profile.

Observed large-scale changes in climate and oceanography, which are especially pronounced in the Arctic, represent profound challenges for upper trophic predators. Knowledge about the extent to which marine predators are able to adjust to environmental variability is essential in order to assess the impact of changing oceanic conditions on the Arctic ecosystem. The main goal of this study was to investigate the impact of oceanographic conditions that varied on an inter-annual and inter-colony basis on the foraging and breeding strategy of a zooplanktivorous alcid, the little auk Alle alle. The study was conducted in two little auk breeding colonies in Spitsbergen (Hornsund and Magdalenefjord) located in different hydrographical regimes in two contrasting seasons (2009 and 2010). This article provides a comprehensive study linking a number of variables describing foraging ground quality, measured directly at sea, with the birds' foraging strategy, parental efforts, body condition and survival of ne...

(1) During the Arctic summer, little auks (Alle alle) deposit considerable amounts of guano on land. Ecosystems subsidised in nutrients are known to hold greater biodiversity and to produce grander biomass of plants and animals compared with areas where seabirds do not nest. (2) The aim of this study was to look into the relationship between guano fertilisation and body size of invertebrates inhabiting tundra. (3) The specimens of Macrobiotus islandicus islandicus Richters, 1904, a tardigrade dwelling in mosses and lichens of the Arctic, from six different populations from Spitsbergen (Hornsund fjord) were measured. Tardigrades were collected from areas different in terms of seabird guano effects on the tundra ecosystem. An overall body size index for tardigrades was calculated using a principal component analysis. (4) Here, we show that the body size of M. i. islandicus is larger in vicinities of the little auk colonies than in areas devoid of bird nesting sites. (5) Given that fitness of many invertebrates is positively correlated with their condition, our study underlines the ecological importance of a side effect of seabirds biology—the transfer of nutrients from the sea to the land.

Growing evidence indicates that producing eggs may constitute a considerable cost of reproduction. If female parental care, which in many species exceeds male performance, is added to these initial costs, it may be concluded that females contribute more to reproduction than males. However, this additional burden on females should reduce their survival and skew the usually equal sex ratio, but this is generally not the case. A resolution of this apparent paradox requires studies estimating the parental investments of both sexes at different stages of breeding, with particular focus on the initial reproductive stage. In the present study, egg composition and its energetic value was estimated in the Dovekie (or Little Auk, Alle alle), a seabird exhibiting bi-parental care except for the end of the chick-rearing period, when the female deserts the brood while the male continues the feeding and escorts the chick during fledging. Condition estimates (size-adjusted body mass and several hematological an...

Initial reproductive investments in birds are energy-demanding. Amount of energy allocated to egg production and incubation may have an important role in determining parental fitness and evolution of clutch size. Here, we investigated whether the Little Auk (Alle alle (L., 1758)), a small alcid with one-egg clutch but two brood patches (probably remnant of the original two-egg clutch), is capable of incubating two eggs and rearing two chicks. We performed an experiment with removing the egg from 20 nests and transferring it to 20 experimental nests (EN). In 25% of EN, both eggs failed to hatch; in 75% of EN, only one egg hatched successfully. Hatching success in EN was 2.5 times lower than in control nests (CN). In EN, chicks hatched only from eggs warmed during >90% of inspections. Owing to the prolonged incubation, the chicks in EN hatched 2 days later than in CN. However, due to the shorter period spent in the nest in EN, fledging dates in both types of nests were similar. The similar body mass and survival of chicks from EN and CN suggest that these traits were influenced mainly by the rearing conditions. Our study emphasize the importance of the incubation phase as a constraint on clutch size.

Offspring sex ratio in birds is adjusted according to the relative fitness payoff of producing sons and daughters, which is known to depend on parental quality. Therefore, spatial patterns in offspring sex ratio should be consistent with the distribution of pair quality within avian colonies. In many colonial birds, central parts of colonies provide greater safety against predators and thus are occupied by high-quality pairs, who relegate conspecifics of poorer quality to the peripheral zones of colonies. For this reason, we expected that offspring sex ratio was likely to follow similar central–periphery gradients. This hypothesis was tested in a colony of tree-nesting Great Cormorants (Phalacorcorax carbo sinensis) in central Poland, where 204 nestlings from 53 broods were molecularly sexed. We found a clear central–periphery pattern in offspring sex ratio within the studied colony. There was a considerable bias toward male offspring in the broods of centrally nesting pairs, while edge pairs inv...

The aquatic warbler Acrocephalus paludicola is the only globally threatened passerine breeding in Europe. We studied morphometric traits of 176 individuals captured at one of the key European autumn stopover sites, the Loire estuary on the Atlantic coast of France, in relation to age (first-year and older) and sex (based on molecular determination). We found significant sex differences in wing length, third primary feather length and body mass, with males being longer-winged and heavier than females; adult females also had deeper bills. However, high overlap in all measurements between the sexes meant the best discriminant functions based on wing length and bill depth (adults) and wing length (inmatures) correctly classified only 87% and 75% of individuals, respectively. The mean potential non-stop flight range of autumn staging birds at the Loire estuary with high fuel load was estimated at 1,178 km for adults and 926 km for immatures. We conclude that in autumn migrant aquatic warblers probably do not fly directly to wintering grounds in west Africa. Instead they gain significant body mass for onward migration at a few key stopover sites in western Europe and the southern Mediterranean region. Conservation of a series of important refuelling stopover sites, especially wet grassland habitats, along the migration route is therefore essential for effective protection of the aquatic warbler.

Body size differentiation may have developed in response to environmental gradients. A pattern of large individuals prevailing in colder areas is often observed and is explained by the heat conservation hypothesis (Bergmann's rule). To understand patterns driving body size variation in a pelagic seabird, the European Storm Petrel Hydrobates pelagicus, we examined the relationship between wing length, body mass and environmental variables in breeding areas (sea surface temperature, air temperature and wind speed). As this species has been divided into two subspecies: Mediterranean H. p. melitensis and Atlantic H. p. pelagicus, we performed the analyses at different scales (species, Atlantic subspecies and regional North Atlantic). At the species and subspecies scales, there was a longitudinal increase in wing length from west to east. At the subspecies and regional scale, we found a latitudinal increase in this variable from south to north. This result and the significant increase of wing length with decreasing sea surface and air temperatures are concordant with Bergmann's rule. In addition, body mass at the species and subspecies scales decreased with increasing wind speed, what may have a functional implication (small body mass may increase manoeuvrability over waves in conditions of stronger wind). Both genetic (two subspecies differing in body size) and environmental factors seem to be important forces driving intercolony variation in body size. Our study on sexual size dimorphism (SSD) revealed that in 156 molecularly sexed adults from the Faeroes, wing and tail length, and body mass exhibited female-biased SSD, while head-bill length showed male-biased SSD. The best discriminant function for sexing based on body measurements correctly classified 75% of individuals. Considering low correctness of proposed functions and geographical variation of body size, use of alternative methods (e.g. molecular tools) is recommended for sex discrimination in the European Storm Petrel.

Energy and time allocation diff ers between incubation and chick-rearing periods, which may lead to an adjustment in the foraging behaviour of parent birds. Here, we investigated the foraging behaviour of a small alcid, the little auk Alle alle during incubation and compared it with the chick-rearing period in West Spitsbergen, using the miniature GPS (in Hornsund) and temperature loggers (in Magdalenefj orden). GPS-tracking of 11 individuals revealed that during incubation little auks foraged 8 – 55 (median 46) km from the colony covering 19 – 239 (median 120) km during one foraging trip. Distance from the colony to foraging areas was similar during incubation and chick-rearing period. During incubation 89% of foraging positions were located in the zone over shallower parts of the shelf (isobaths up to 200 – 300 m) with sea surface temperature below 2.5 ° C. Th ose environmental conditions are preferred by Arctic zooplankton community. Th us, little auks in the Hornsund area restrict their foraging (both during the incubation and chick-rearing period) to the area under infl uence of cold, Arctic-origin water masses where its most preferred prey, copepod Calanus glacialis is most abundant. Th e temperature logger data (from 4 individuals) indicate that in contrast to the chick-rearing period, when parent birds alternated short and long trips, during the incubation they performed only long trips. Adopting such a fl exible foraging strategy allows little auks to alter their foraging strategy to meet diff erent energy and time demands during the two main stages of the breeding.

Long-lived birds often face a dilemma between self-maintenance and reproduction. In order to maximize fitness, some seabird parents alternate short trips to collect food for offspring with long trips for self-feeding (bimodal foraging strategy). In this study, we examined whether temporal and spatial variation in the quality of foraging grounds affect provisioning and fledging success of a long-lived, bimodal forager, the little auk (Alle alle), the most abundant seabird species in the Arctic ecosystem. We predicted that an increase in sea surface temperature (SST), with an associated decrease in the preferred Arctic zooplankton prey, would increase foraging trip durations, decrease chick provisioning rates and decrease chick fledging success. Chick provisioning and survival were observed during three consecutive years (2008–2010) at two colonies with variable foraging conditions in Spitsbergen: Isfjorden and Magdalenefjorden. We found that a change in SST (range 1.6–5.4 °C) did not influence trip durations or provisioning rates. SST was, however, negatively correlated with the number of prey items delivered to a chick. Furthermore, provisioning rates did not influence chick’s probability to fledge; instead, SST was also negatively correlated with fledging probability. This was likely related to the prey availability and quality in the little auk’s foraging grounds. Our findings suggest that predicted warmer climate in the Arctic will negatively influence the ability of parents to provide their chicks, and consequently, the fledging prospects of little auk chicks.

Moulting and breeding are costly stages in the avian annual cycle and may impose trade-offs in energy allocation between both stages or in their timing. Here, we compared feather growth rates (FGR) of rectrices in adults between two pairs of small pelagic Procellariiformes species differing in moult-breeding strategies: the European storm-petrelHydrobates pelagicusand Leach’s storm-petrelOceanodroma leucorhoabreeding in the Northern Hemisphere (Faroe Islands), showing moult-breeding overlap in tail feathers; and the Wilson’s storm-petrelOceanites oceanicusand black-bellied storm-petrelFregetta tropica, breeding in the Southern Hemisphere (South Shetlands), temporally separating moult and breeding. We used ptilochronology (i.e., feather growth bar width) to reconstruct FGR reflecting relative energy availability during moult. Based on previous research, we expected positive correlations between feather length (FL) and FGR. Additionally, we expected to find differences in FGR relative to FL between the moult-breeding strategies, where a relatively higher FGR to FL indicates a higher energy availability for moult. To investigate if energy availability during moult in the studied species is similar to species from other avian orders, we used FGR and FL found in literature (n = 164) and this study. We fitted a phylogenetic generalized least squares (PGLS) model to FGR with FL, group (i.e., Procellariiformes vs. non-Procellariiformes) and the interaction FL * group as predictors. As it has been suggested that Procellariiformes may form two growth bars per 24 h, we fitted the same model but with doubled FGR for Procellariiformes (PGLSadj). The group term was significant in the PGLS model, but was not in the PGLSadj model, confirming this suggestion. Individually predicted FGR by the PGLSadj model based on FL, showed that the Southern species have a significantly higher FGR relative to FL compared to the Northern species. Additionally, we found no correlation between FL and FGR in the Northern species, and a positive correlation between FL and FGR in the Southern species, suggesting differences in the trade-off between feather growth and size between species from both hemispheres. The observed differences between the Northern and Southern species may be caused by different moult-breeding strategies. The Southern species may have had more energy available for moult as they are free from breeding duties during moult, while the Northern species may have had less free energy due to a trade-off in energy allocation between breeding and moulting. Our study shows how different moult-breeding strategies may affect relative nutritional condition or energy allocation during moult of migratory pelagic seabirds.

Many seabirds breed in large aggregations, making it difficult to estimate their population size and habitat preferences. This knowledge is particularly important considering their function in food webs and ecosystem services. In this study, we investigated the factors affecting distribution and abundance of the little auk Alle alle, a seabird considered a keystone species of the Arctic ecosystem. We performed the study on the W and the NW coast of Spitsbergen. Using Generalized Additive Models (GAMs) and Conditional Inference Tree (CIT) we examined factors related to presence/absence and size (estimated number of breeding pairs) of the little auk colonies. We also tested the nesting preferences for geographical features such as aspect, slope angle, altitude, solar radiation, rock type, and distance to foraging grounds. Our findings indicate that the occurrence of little auk breeding colonies is non-random and highly attributed to environmental factors. The probability of colony occurrence was significantly associated with altitude (negative relationship; preference to sites situated lower), solar radiation (positive relationship; the higher radiation, the more likely colony occurrence) and slope (positive relationship; the steeper a slope, the more likely colony occurrence), whilst aspect appeared non-significant (though the probability of colony occurrence peaked at southern slopes). Colony size was significantly associated with rock type (larger colonies in amphibolite and quartzite). The distance to foraging grounds did not appear to affect the probability of colony occurrence and size, implying that birds may choose optimal breeding sites at the cost of longer foraging flights. We estimated the Spitsbergen little auk breeding population at 728 529 (5-95% CI: 479 312-986 352). Spitsbergen comprises ca 1.9% (95% CI: 1.2%-2.7%) of the world breeding population and represents the third most important breeding area for the species, following the W and the E coast of Greenland.

2 Institute of Oceanology, Polish Academy of Sciences, ul. Powstancow Warszawy 55, 81-712 Sopot, Poland ABSTRACT: Knowledge of foraging behaviour is essential to understand both the ecological roles of seabirds and the constraints acting upon them in marine ecosystems. Here, we investigated for- aging trips of a small planktivorous alcid, the little auk Alle alle, using miniature GPS loggers. We performed the study in 2 large breeding colonies in west Spitsbergen (Hornsund and Magdalene- fjorden) with contrasting oceanographic conditions (Arctic and Atlantic environments, respec- tively). Generally, in both locations little auks foraged in areas with low sea surface temperature (Arctic-type water, marginal ice zone, and frontal zones) where preferred zooplankton are com- monly abundant. In the Arctic environment (Hornsund), birds foraged significantly closer to the colony (up to 60 km) compared to up to 150 km in the Atlantic environment (Magdalenefjorden). Hatching and breeding success and chick survival up to 20 d as well as chick body mass para - meters were similar in both studied colonies. However, chicks in the Arctic environment (Horn- sund) achieved both peak body mass and fledging age earlier, suggesting faster chick growth than in the Atlantic environment (Magdalenefjorden). The importance for breeding little auks of nearby cold water foraging grounds may make them sensitive to predicted climate change with serious negative consequences for body condition, future survival and breeding success.

We examined the brood sex ratio and offspring body mass in relation to the timing of breeding and brood size in the Great Cormorant Phalacrocorax carbo sinensis. The brood sex ratio was not related to brood size but it was significantly related to the hatching date, with a decreasing proportion of males in the brood in the course of the season. Male chicks had significantly lower body mass if they hatched later in the season, whereas there was no such relationship for female offspring. Assuming that environmental conditions deteriorate with progress of the breeding season, and male offspring may be more vulnerable to poor environmental conditions, the observed decline in the proportion of male offspring late in the season may be adaptive.

Energetically demanding life-history stages in birds (reproduction, migration) might be associated with sex- and age-specific physiological changes expressed in suppressed immune function or declined body mass. Here, we examined the numbers of red and white blood cells (RBC and WBC), the ratio of heterophils and lymphocytes (H/L)], and body mass of reed warblers (Acrocephalus sirpaceus) and sedge warblers (A. schoenobaenus) during the breeding and migration periods. Higher WBC counts and H/L ratio in adults (both warblers) and lower RBC counts (only the reed warbler) during the prelaying/laying period as compared with those during the incubation/ chick-rearing period suggest greater energetic demands during the latter period. In adults, higher H/L ratio and WBC counts in males of both species as compared with those in females during the prelaying/laying period indicate higher energy expenditures of males, probably associated with mate guarding and chasing intruders. Lower HL ratio and WBC counts in adults...

As diving seabirds use vision underwater, it is presumed they should preferentially select sites where their preferred food items are not only abundant but also clearly visible. To test this, we studied the optical properties of the seawater in the West Spitsbergen Shelf, in combination with zooplankton abundance in the feeding grounds of the planktivorous little auks from the nearby colonies in Hornsund. We estimated the relative attractiveness of the foraging sites using a novel parameter—visual prey availability (VPAv), which relates density and proportion of the preferred food item (Calanus glacialis) of the little auk, in total zooplankton, to the optical properties of the seawater. We found a significant positive correlation between the density of foraging little auks and VPAv values. Birds chose areas where C. glacialis was both abundant and clearly visible, because of the clarity of the water and low proportion of other zooplankton species. The birds avoided foraging over the warmer Atlantic-type waters, characterised by a high abundance of zooplankton taxa mostly ignored by birds and where VPAv values were low. VPAv values could potentially also be applied to other visual planktivores for which prey preference and visual acuity are known.

Haemoparasites prevalence in birds can vary among species, age and sex groups and ecological conditions. To comprehend the host—parasite assemblages, it is important to investigate physiological mechanisms underlying the interactions in wild bird species. Here, using blood smears we investigated how leukocyte profiles in reed warblers (Acrocephalus scirpaceus) and sedge warblers (Acrocephalus schoenobaenus) vary in relation to the heamoparasite presence with respect to the host's age and sex. We sampled the birds during their autumn migration in 2008 at a stopover site in northern Poland. We found a significant relationship between the leukocyte profiles and parasite infection. Parasite infection was associated with elevated relative number of heterophils with simultaneous lowered relative number of lymphocytes. These opposite pattern resulted in an elevated H/L ratio in infected birds. Total leukocytes, lymphocytes and heterophils counts were elevated in birds infected with blood parasites as compared wi...

Most seabirds have a small clutch size. Thus, replacement of a clutch after loss can make important contributions to an individual’s lifetime reproductive success. However, in the condition of short polar summer, relaying propensity may be time-constrained. In this study, we investigated rates and consequences of relaying in a small High Arctic seabird, the little auk Alle alle. We performed an experiment in which we removed the single egg from 20 nests of early-laying breeders. We measured relaying rates, and compared chick body mass and breeding success between the experimental and control nests. Despite the narrow window of the Arctic summer and the closely synchronized breeding, 75% of females produced a replacement egg just 2.7% smaller in volume than the first egg. This indicates that in little auks, the demographic effects of disruptions to breeding attempts (by predators, adverse weather or human activity) may be mitigated to some extent by replacement clutches. However, peak body mass and fledging body mass were lower in the experimental than the control chicks. This effect was rather a consequence of late hatching – chicks from replacement clutches followed seasonal decline in peak body mass and fledging mass. Finally, breeding success and chick survival up to 20 d in the experimental nests were respectively 34 and 37% lower than in the control nests. Thus, the quality and post-fledging survival of chicks from the replacement clutches were probably lower compared to the chicks hatched from the first-laid eggs.

Many avian species exhibit a transition from biparental to uniparental care, but the reasons for this pattern are largely unknown. We examined the transition from biparental to paternal-only care in a small Arctic seabird, the Dovekie (Alle alle). We monitored colony attendance of males and females throughout the chick-rearing period in 2008–2010, investigating the sex ratio of breeders and following individually marked parents of both sexes in the colony at Magdalenefjorden (northwestern Spitsbergen). To understand reasons for the female's behavior, we also examined the birds' parental efforts and changes in chick body mass. We found that females fed the chicks less than males but provisioned the chick until very late in the chick-rearing period. Brood desertion occurred in 95% of nests and took place 3 days, 2 days, or 1 day prior to fledging (in 24%, 39%, and 69% of nests, respectively), on the 26th day of the chick's life, on average. Chicks experienced body-mass recession beginning, on aver...

Kwasniewski, S., Gluchowska, M., Walkusz, W., Karnovsky, N. J., Jakubas, D., Wojczulanis-Jakubas, K., Harding, A. M. A., Goszczko, I., Cisek, M., Beszczynska-Möller, A., Walczowski, W., Weslawski, J. M., and Stempniewicz, L. 2012. Interannual changes in zooplankton on the West Spitsbergen Shelf in relation to hydrography and their consequences for the diet of planktivorous seabirds. – ICES Journal of Marine Science, 69: 890–901. The purpose of the work was to determine how atmospheric and oceanic processes (the North Atlantic Oscillation (NAO) and the Arctic Ocean Oscillation (AOO)) influence hydrography and zooplankton on the West Spitsbergen Shelf (WSS), and the impacts of the processes on chick meals of zooplanktivorous little auks Alle alle. There were distinct Atlantic and Arctic oceanographic domains on the shelf resulting from the presence of the West Spitsbergen Current and the Sørkapp Current, which contain different proportions of Calanus finmarchicus and C. glacialis. The abundance of warm- and cold-water species varied as a result of an interplay between processes in the atmosphere and ocean. In the Arctic domain, on which the study focused, the NAO impacted oceanography, zooplankton, and consequently little auk chick meals, with time-lags of 4–7 years. The diet of little auk chicks was more energy-rich when C. glacialis stage 5 in the Arctic community were more easily available. To date, the changes in zooplankton abundance on the WSS have not posed a threat to the ability of little auks to feed their chicks lipid-rich Arctic copepods.

In a number of species with biparental care, the parents of one sex desert the mate and brood before the chick is independent. The reasons for this desertion are poorly understood. In the study reported here we investigated whether sex-specific changes in body mass corrected for size (in short body mass) and the stress-related heterophil to lymphocyte ratio (H:L) during the late incubation and chick-rearing period are related to female brood desertion in a small Arctic seabird, the Little Auk (Alle alle). The body mass of both sexes did not change during the study periods, although females were significantly lighter than males. No sex differences were found in the pattern of changes in the H:L ratio. In both sexes, the H:L ratio, and thus the stress level, increased significantly just after hatching and remained high until the last week of chick rearing when it fell back to the level recorded during incubation. These results support behavioural reports of an equal male and female contribution to parental care. The similar body condition of males and females undermines the hypothesis on the depletion of female body reserves as the main trigger of their earlier brood desertion. It is possible that a sex-specific role in parental care during and after fledging and/or an ancestral pattern of parental care are responsible for the transition to paternal care in Little Auks.

The body condition and haematological parameters of birds are influenced by a multitude of factors. Here we investigated body mass, fat scores, ratio of heterophils and lymphocytes (H/L), red blood cell count (RBC) of reed buntings (Emberiza schoeniclus), expecting differences between spring and summer (different physiological states/energy demands in breeding/post-breeding periods), sexes (different parental duties) and age groups (different experience/social status). Fat scores and size-corrected body mass were similar in all the groups. Adults in spring had higher H/L than in summer, which may have been due to reproduction-related stress or the seasonal elevation of glucocorticoids. The lower RBC and H/L in adults in summer were associated with their complete moult. Lower H/L in adults in summer compared to immatures may have resulted from their lower stress level (greater experience/higher social status) or a complete moult. The H/L ratio was similar in both sexes, despite their different parental duties.

Sexually transmitted diseases have been frequently hypothesised as a cost of multiple avian mating but relatively few studies have investigated the issue in truly wild birds. The main object of this study was to determine prevalence and concordance of fungi in cloacae of pair members of a socially monogamous Arctic seabird, little auk (Alle alle), thereby testing whether the microbes are likely to be transmitted during copulation. Various fungi species, potentially pathogenic, occurred in the little auk cloacae. One-third of the tested individuals were found to be a host for one to three fungi species. However, half of the 19 studied pairs were found to be not concordant in fungal assemblages, suggesting that transmission of the microbes between partners is not straightforward.

In conditions of deteriorating food availability, seabirds may maximize their breeding success by increasing reproductive effort, which compromises body condition, stress level, survival, and future reproductive success. We studied a small planktivorous alcid, the little auk Alle alle, in 2 breeding colonies in west Spitsbergen, Norway (Hornsund and Magdalenefjorden) with contrast- ing oceanographic conditions (Arctic and Atlantic environments, respectively). We hypothesized that the chick diet composition and parental foraging effort differs between these colonies, which results in intercolony variation in body mass and stress level of both adults and chicks. We found dif- ferences in sea temperature and salinity (both lower in Hornsund) and the abundance of Atlantic copepod Calanus finmarchicus (3.6 times less abundant in Hornsund) between foraging areas. Birds from both colonies foraged selectively on Calanus glacialis CV. Composition, total biomass and energy content of food loads were similar in both colonies, though food delivered to nestlings in Mag- dalenefjorden was more diverse. The frequency of feeds was also similar in both colonies. Parent lit- tle auks in Magdalenefjorden, however, performed longer foraging trips than in Hornsund (medians 183 vs. 124 min). Longer foraging trips suggest that these birds traveled longer distances to find abundant prey at the marginal sea ice zone and/or spent more time foraging close to the colony but in poorer foraging areas. Despite increased parental efforts in Magdalenefjorden, body mass and stress levels of adults and chicks were similar in both colonies. This suggests that little auks from northwest Spitsbergen did not reach a threshold requiring prioritization of self-maintenance over chick provisioning.

Composition and abundance of zooplankton were studied simultaneously with feeding ecology of planktivorous Little Auks (Alle alle) in two different sea shelf areas of West Spitsbergen, Norway, in summer 2007. Zooplankton was collected adjacent to bird colonies in Magdalenefjorden (influenced by Atlantic West Spitsbergen Current) and Hornsund (dominated by the Arctic Sorkapp Current). In spite of different hydrological situations, the abundance of prey preferred by Little Auks, Arctic Calanus glacialis copepodids stage V, among zooplankton was similar in both localities. However, there was much more of Atlantic Calanus finmarchicus on the shelf outside Magdalenefjorden compared to Hornsund, resulting in different abundance ratios of Arctic to Atlantic copepods in the two areas (1:14 and 1:1, respectively). Even greater differences between the two areas occurred in the ratio of C. glacialis CV to other zooplankters, amounting to 1:40 in Magdalenefjorden and 1:6 in Hornsund. In both Little Auk colonies food brought by parents to their chicks contained mainly C. glacialis CV, albeit the proportion of this copepod in meals was significantly higher in Hornsund. Meals delivered to Little Auk chicks in Hornsund had also higher zooplankton numbers, biomass and energy content. In Magdalenefjorden, on the other hand, a higher number of feedings and longer duration of foraging trips were recorded. These differences became more apparent with increasing energy requirements of the fast growing nestlings. This was probably a consequence of lower relative abundance of the Little Auks’ preferred prey in the sea adjacent to Magdalenefjorden colony. It seems that searching for the preferred food items, such as C. glacialis, among abundant but less favored C. finmarchicus, may require more time and energy demanding foraging behavior. As a consequence, foraging effort of the Little Auk parents from Magdalenefjorden was higher, and feeding efficiency lower, than those of birds from Hornsund. Increasing influx of warm Atlantic waters that bring boreal zooplankton to the Arctic Seas may have negative impacts on Little Auks’ time and energy budgets and eventually on their breeding success and range of breeding distribution.

In seabirds, variation in breeding phenology and success is often induced by macro- and mesoscale fluctuations in oceanographic conditions, which is recordable at the regional or colony scale. Variation in breeding parameters may also exist at the microscale—among discrete breeding aggregations and subcolonies. The aim of this study was to compare breeding phenology and parameters (hatching success, chick survival, chick body mass indices) among little auks (Alle alle) nesting in three subcolonies differing in their microtopographic and microclimatic features, situated in Magdalenefjorden (north-western Spitsbergen). The hatching phenology varied significantly among the subcolonies. This was probably due to the different duration of snow persistence in spring, as nests are occupied as soon as the snow cover melts sufficiently to allow access to them. The earliest hatching was recorded in the subcolony located on steep slopes at low altitude in the vicinity of the fjord, which favours early ice and snow melting in spring. Hatching success differed significantly among subcolonies, which could also have been due to the microclimatic features of the subcolonies. Hatching success was the lowest in the subcolony where birds started to breed while patchy snow was still persisting. Water from the melting snow could have flooded some of the nests. Alternatively, the low hatchability could have been caused by a higher frequency of less experienced breeders (e.g., first-time breeders) among the individuals nesting in this subcolony. Other breeding parameters (chick survival and growth) were generally similar in all the subcolonies.

The migration strategies of birds may vary strongly between species and also between age and/or sex groups. We studied the autumn migration and body condition of molecularly sexed Reed Warblers Acrocephalus scirpaceus and Sedge Warblers Acrocephalus schoenobaenus (211 and 208 ind., respectively) at a stopover site on Lake Druzno, Northern Poland, in 2008. Immature male Reed Warblers were caught significantly later than females (median dates 9 days later), but in the Sedge Warbler, both sexes of immatures migrated at about the same time. Adult males and females of both species did not differ in their time of migration. Adult and immature males of both species were larger (wing length and body mass) than females. In both species, fat reserves were similar in both sexes of both age classes. Adults of both sexes of Reed and Sedge Warbler were generally caught earlier than immatures. In both species, the body mass and fat reserves of immatures were generally less than in adults. The autumn protogyny of immature Reed Warblers may allow smaller females to limit competition with bigger males during migration and at the wintering grounds. In the Sedge Warbler, which tends to match its migration to peak of occurrence of superabundant food at stopover sites, both sexes gain an advantage from migrating at the same time. Since part of the measured wing length variation in both species was explained by sex differences, temporal trends in wing length recorded at stopover sites should be interpreted with caution.