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Above all, bird ringing must be safe for birds, for ethical reasons but also to ensure that data collection is not biased by marking effects. Bird ringing schemes are responsible for determining the size of the rings used to mark birds individually. This paper critically reviews the recommended ring sizes in the Aranzadi Ringing Scheme and proposes an objective criterion for ringers. A morphological analysis of tarsus width (MTW) and the width of the tibia–tarsus articulation (MAW), over a sample of more than 4000 individuals of 74 species of both passerines and non-passerines captured in Spain, revealed that the mean difference between the internal diameter of the recommended ring (IDR) and MTW was 32% in relation to MTW (sd 15.8%, 95% confidence interval 28–35%). Experience demonstrates that this clearance is adequate, but recommendations for rings with clearances either too narrow (<6%) or too wide (>50–60%) should be reviewed and changed, if possible, to bring them closer to a 32% standard. [ABSTRACT FROM AUTHOR]

Tawny Owls in southern Europe were trapped and measured. Compared to males, the females were larger, heavier, and more greyish in colour, but there was a large biometric overlap. The variable with the highest Storer's dimorphism index (SDI) was weight, though our population had one of the smallest dimorphisms observed in Europe for weight. It has been proposed that differences in diet could lead to differing behaviour and, in consequence, the observed dimorphism degree (SDI) by natural selection. [ABSTRACT FROM AUTHOR]

Environmental conditions in non-breeding areas cause differential winter distribution patterns between age classes in a Eurasian Woodcock Scolopax rusticola population wintering in northern Iberia. To determine the factors driving spatial distribution of first-winter and adult birds in the non-breeding period of a population of the Eurasian Woodcock, in order to make decisions for the management of the species. Using a data set of 10 consecutive winters (2009/10–2018/19), we tested for the effect of year, geographical and habitat-related variables on age ratio and body condition of Eurasian Woodcocks wintering in two nearby zones with different hunting regimes in northern Iberia. Our results suggest that the age ratio depended mainly on year, month, and altitude. As winters progressed, we observed a higher proportion of first-winter birds occurring at sites with lower altitude. First-winter birds were in poorer body condition than adults, and body condition was higher in mid-winter and spring. The effect of hunting on body condition was unclear. Variation in the population structure and body condition between age classes are discussed in the context of age-dependent wintering strategies, which to a large extent depend on survival thresholds that take into account foraging efficiency and the risk of predation. The lack of detectable effects of hunting on age structure and body condition precludes us from giving any management recommendations for northern Iberia. [ABSTRACT FROM AUTHOR]

Barn Owls Tyto alba and Red-billed Choughs Pyrrhocorax pyrrhocorax show differential breeding outputs as a consequence of sharing nest boxes. To describe for the first time a commensal relationship between the Red-billed Chough and the Barn Owl during the breeding period. During systematic monitoring of 90 nest boxes installed for Barn Owls in Navarre, northern Spain, it was found that in some boxes, Barn Owls and Red-billed Choughs nested simultaneously (inside and on top of the box, respectively). The clutch size, breeding success, and the number of fledglings of each species were compared when nesting together versus separately during the breeding period of 2015 and 2017. About one-third of the nest boxes observed during the study were simultaneously used by both Barn Owls and Red-billed Choughs. No relationship was found between the composition of the habitat around the boxes and the degree of occupancy for either species. Nest box sharing had no significant effect on the breeding output of Barn Owls, but improved the breeding performance of Red-billed Choughs by increasing the number of fledglings in relation to clutch size. Barn Owls can provide benefits to Red-billed Choughs by controlling the populations of rodents, which could act as nest predators. Given that Barn Owls apparently do not obtain benefits nor incur any costs by the association with Red-billed Choughs, this interaction may be defined as commensalism. To our knowledge, this is the first documented nesting association between a corvid and an owl species. [ABSTRACT FROM AUTHOR]

The main goal of this study was to describe in detail the structure and phenology of flocks of the Common Linnet (Linaria cannabina) passing through eastern Spain. Data were compiled during the spring and autumn migrations of 2019 to 2021 (excluding spring 2020), by counting and by capture for ringing. We counted 6151 flocks, comprising 17 732 birds, of which 7638 individuals were captured. The size of flocks ranged from 1 to 30 Linnets (mean ± 95% CI, 2.8 ± 0.1), with a higher proportion of solitary migrants in spring (46.8%) than in autumn (33.8%). Passage in spring peaked during the third hour after dawn, whilst in autumn this peak was just after dawn. There were more females than males (56%), probably as an indicator of latitudinal differential migration. Juveniles were significantly more abundant (c. 80%) than adults. The phenological pattern of the passage was more variable in spring than in autumn, and such variation could be useful to explore potential impacts of climate change on the migration ecology of the species. [ABSTRACT FROM AUTHOR]

The Bay of Biscay is an important non-breeding area for some Palaearctic gull species and populations. The aim of this work is to describe the spatial and temporal distributions of non-breeding gulls in the Bay of Biscay, and to identify their major areas of origin. With that goal, we analysed 6773 live sightings of colour-ringed gulls within the southeastern part of the Bay of Biscay during the 20 years 2000–19, relating to 2552 individuals of 10 species. Their distribution along the coast was not homogeneous, with numbers peaking in most cases from November to February, though some species were observed to peak around midsummer. Their origins comprised a wide geographical area encompassing western and eastern Europe; overall, the species can be divided into four groups by origin: northwestern France with Britain and Ireland, central western Europe (with a centroid in the Benelux region), eastern Europe, and the western Mediterranean. [ABSTRACT FROM AUTHOR]

Determining the age of specimens is of great help in wildlife management, especially for rare or threatened species in which each individual has a high value. In birds, differences in moulting pattern between juveniles and adults may in some cases allow determination of a bird's age from the examination of its plumage. In the present study, we analyse the moult of 19 breeding individuals of White-backed Woodpecker Dendrocopos leucotos lilfordi captured for GPS tagging in the Pyrenees, in order to describe, for the first time, the moulting pattern of this endangered woodpecker. Two well-differentiated groups of adult birds were identified: those that underwent a partial moult, and were classified as second-calendar-year birds, and those that performed a complete one and were classified as older. The first group had renewed most lesser and median coverts, all primaries and the innermost greater coverts, thus showing clear moult limits between the primaries and secondaries and within the greater coverts. Some individuals also replaced up to two more greater coverts (GC5–6). Individuals classified as older birds showed all feathers to be of a single generation, indicating that they had undergone a complete moult. A few individuals in this category retained a few unmoulted secondaries and primary coverts, however. A recapture of a bird with a complete moult, which had been captured a year before showing a partial moult, would confirm these moult sequences to be age dependent. This moulting pattern is very similar to that described for other spotted woodpeckers, and allows researchers to determine the ages of breeding individuals during the nesting season. [ABSTRACT FROM AUTHOR]

Capsule: Linear censusing and occupancy models based on fixed sampling points are alternative widely used techniques to determine bird densities in riparian ecosystems, although it cannot be always properly executed. Aims: The aim is to assess the survey efficiency for river birds using occupancy models in contexts of impaired visibility owing to dense vegetation along the banks. Methods: We tested whether increasing sampling periods within each survey unit (point) at occupancy models would result in increasing detection probability values. We used two approaches in order to identify the 'best' design for White-throated Dippers Cinclus cinclus along forested river stretches: minimizing survey effort of standard single-season site occupancy modelling and exploratory power analysis. Results: With a detection probability of 0.26 (i.e. much lower than in previous studies), a design with 60 sites surveyed 10 min 6 times a year would be the option to survey White-throated Dippers in forested habitats if an acceptable power is required. Simulations further revealed the consistency of the results. Conclusion: We provide guidelines to establish a cost-effective survey design for any long-term monitoring citizen-based programme of a White-throated Dipper population when detection probabilities are low. A strength and novelty of the method is to take advantage of advanced probabilistic approaches (e.g. GRTS) to select the survey sites providing, among other major interests, a spatially balanced geographic coverage. [ABSTRACT FROM AUTHOR]

Capsule: Northern Gannet Morus bassanus counts obtained from a coastal monitoring network in northern Spain were highly determined by wind conditions. Aims: To determine how external factors including weather conditions affect seabird counts in a land-based seabird monitoring project (Red de observación de Aves y Mamíferos marinos, RAM project; Seabird & Marine Mammal Observation Network). Methods: Seabird counts were obtained from four study capes situated in the southeastern part of the Bay of Biscay in northern Spain. Counts were conducted on a monthly-basis for a period of 3 hours starting at dawn on each sampling day. Data used here were compiled over a 10-year period (2006-15). Results: Northern Gannet numbers were chiefly influenced by meteorological conditions at a local spatial scale. Northwesterly winds were found to positively influence gannet abundance, whereas high relative humidity caused a decrease in abundance. Moreover, gannets were more abundant during the non-breeding period as compared to breeding period. Conclusion: We recommend considering the effect of the wind and relative humidity on land-based seabird counts in the Bay of Biscay when handling data from the RAM protocol in order to avoid biases and misinterpretation of data. Since seabirds approach the coast during harsh weather conditions, the RAM protocol should be improved in order to better account for the distance at which the birds are seen. [ABSTRACT FROM AUTHOR]

The migratory behaviour of European Song Thrush (Turdus philomelos) populations ranges from resident in the south and west to partly or totally migratory in northern, central and eastern Europe. This varied migratory behaviour could be reflected in morphological adaptations, with migrants having a proportionally longer wing, lower tail:wing ratio, and, if foraging more on fruits, deeper and wider bills. We used data from Song Thrushes captured during the breeding season and in winter in northern Iberia to test for morphological variations between these two periods. In three consecutive years (2014-16), Song Thrushes were captured with mist nests in a region of southern Navarra (Spain) during the breeding season (from May to July) or in the winter period (October to February), during a period of four hours starting at dawn per sampling day. We consider the winter population to be composed of a high proportion of visitors. Breeding Song Thrushes showed proportionally longer tails for given wing lengths, even though wing length did not vary between the two periods. Moreover, winter visitors showed narrower and less deep bills than breeding Song Thrushes. Potential explanations for these results are discussed. [ABSTRACT FROM AUTHOR]

Capsule: Nocturnal passerine migration patterns were studied by a network of weather radars within the East-Atlantic flyway providing large-scale information on the effect of a geographical barrier. Aims: The aim of this study was to obtain a large-scale spatial overview of the effects of a sea barrier on migratory flyways in northern Spain/western France. Methods: Weather radar data were used from five sites at the Bay of Biscay during nights in spring 2015 to calculate flight directions and migration traffic rates (MTRs). Results: The highest MTRs were registered by the radars at the southeastern edge of the bay, with a gradual decrease northwards. Spring migration direction was generally NNE/NE. Continuous nocturnal migration pattern indicated migration over land in the south. The radar half way up the French coast exhibited bimodal migration intensities at night, indicating sea crossing, and the northernmost corner of the bay showed little migration. Conclusions: Radar patterns indicated migration over land and sea during spring migration. Sea crossing occurred with flight distances of up to about 500 km. Most migration activity was observed in the radars along the southeastern section of the bay, indicating that the general migration flyway from Spain funnels through the eastern side of the north Iberian Peninsula. [ABSTRACT FROM AUTHOR]

Capsule:Pre-fledging survival in a Yellow-legged GullLarus michahellispopulation in northern Iberia is negatively correlated to hatching date. Aims:To explore which factors have more importance in determining the pre-fledging daily survival rates in a Yellow-legged Gull population from northern Iberia. Specifically, we tested for the effect of hatching date and order, body size and condition and meteorological conditions on pre-fledging survival. Methods:Cormack–Jolly–Seber models with mixtures were used to model daily survival rates. Results:Daily survival rates were mostly negatively affected by hatching date. Conclusions:Hatching date was the most important factor affecting survival of chicks during the pre-fledging period in a Yellow-legged Gull colony from northern Iberia. [ABSTRACT FROM PUBLISHER]

Capsule:There were only weak effects of winter flooding episodes on apparent annual survival of a White-throated Dipper breeding population in northern Iberia. Aims:To test whether extreme winter flooding episodes affected survival in a breeding population of White-throated DippersCinclus cinclus. Methods:Dippers were ringed during the breeding seasons of seven consecutive years on rivers in northern Iberia. Cormack–Jolly–Seber models were used to estimate apparent annual survival in relation to flooding during the winter periods. Results:We obtained weak evidence for an effect of flooding during the winter on the surveyed population. Two of the six winters were characterized by having an exceptionally high discharge. Our models showed that survival in the subsequent breeding period tended to be lower, although models assuming constant survival were equally well supported. Conclusion:Extreme flooding in winter may affect survival of some White-throated Dippers in northern Iberia, but its impact at the population level seems to be weak. [ABSTRACT FROM PUBLISHER]

Capsule:Bird migration was recorded by an infrared device at three sites in the southeastern Bay of Biscay, indicating seasonal east–west differences in migration flow. Aims:The main aims of this study were to quantify and describe nocturnal migration dynamics in proximity of a sea barrier, and to assess seasonal and geographical drivers of migration patterns. Methods:A thermal-imaging camera was used at two coastal study sites (Punta Galea, Cape Higuer) in spring and three study sites (coast: Punta Galea, Cape Higuer; inland: Iregua Valley) in autumn for four hours from sunset over 90 nights in 2014 and 2015. Results:Migration was strong at both coastal sites in early spring. Autumn migration was weak at the western coast, but strong at the eastern coast and inland. Tailwind had no significant effect on migration intensity, but migration ceased during strong cross- or headwinds despite clear skies. The majority of the targets were passerines. Conclusions:The patterns observed suggest spring migration occurs on a broader front, potentially involving sea crossing further to the west, while autumn migration concentrates more eastwards over land. In both seasons, there was no significant response to wind conditions. [ABSTRACT FROM PUBLISHER]

Capsule:Chick growth rates was sex-dependent in an Atlantic Yellow-legged GullLarus michahellispopulation. Aims:To describe the growth rate of Yellow-legged Gull chicks and sex-associated variations, and obtain a discriminant function to sex them using morphological data. Methods:Fifty-two Yellow-legged Gull chicks from a colony in northern Iberia were measured for body mass, head and bill length, tarsus length and three other bill-length associated variables, from the hatching date up to the age of 36 days. Birds were sexed using DNA analysis. Results:Using logistic models, chick growth rates was observed to be similar between the sexes, while the asymptote was higher among males for the majority of the variables. Discriminant analyses showed that the variable head+bill was very reliable to predict the sex of >80% of chicks at an early developmental stage, and 100% of chicks if combined with tarsus length and two more bill-length associated variables at the age of 35 days. Conclusions:This is the first study using a discriminant analysis to sex Yellow-legged Gull chicks, and also the first to describe the growth function for the species. The growth rate varied between sexes because males showed higher asymptote values within the growth function. [ABSTRACT FROM AUTHOR]

Capsule: Stable isotope analyses reveal some degree of migratory connectivity of Bluethroat populations wintering from Iberia to West Africa. Aims: To identify the probable breeding origins of Bluethroats wintering from Iberia to Senegal. Methods: Bluethroat feathers (P1) were sampled from individuals at their wintering areas. These feathers were then analysed for stable H isotopes (δ2H). We assigned individual Bluethroats to approximate geographic origin using likelihood-based assignment procedures. Results: We observed spatial segregation between different Bluethroat populations. At wintering sites north of the Sahara Desert, Bluethroats wintering to the west came from further west origins than those which overwintered to the east. Bluethroats from central-eastern Europe overwintered either within the circum-Mediterranean region or in Senegal. We found no clear evidence supporting a sub-Saharan wintering range for birds breeding in Iberia (Luscinia svecica azuricollissubspecies). Conclusion: North of the Sahara Desert, we found what might be a parallel migration pattern. The apparent lack ofLuscinia svecica namnetumBluethroats in Senegal suggests to some extent some kind of leap-frog migration between someLuscinia svecica cyaneculapopulations and theL. s. namnetumsubspecies. [ABSTRACT FROM AUTHOR]

Capsule:Common sandpipers stopping over in a tidal marsh in northern Iberia during the autumn migration period showed a moderate mean fuel load and low fuel deposition rate, but relatively long stopover periods, suggesting a ‘hopping’ strategy of migration. Aims:The main objectives of this paper were to analyse the stopover ecology of migrant Common Sandpipers at Txingudi coastal marshes, northern Iberia, in autumn. Methods:Common sandpipers were captured during the autumn migration of 2007–2013 at the Txingudi marshlands (province of Gipuzkoa, northern Iberia). Data were obtained from a constant effort ringing station working on a daily basis. Results:We observed a moderate fuel load and fuel deposition rate and long stopovers. Conclusion:Our results suggest overall a ‘hopping’ migration strategy. When moving along the coast of northern Iberia, Common Sandpipers may not use key wetlands to gain much fuel, as found in other waders or in some wetlands of inland Iberia, but all coastal marshes seem to be potentially used in the same way. [ABSTRACT FROM AUTHOR]

CapsuleThe effects of playback use on number of captures, recaptures, fuel load, and age and sex ratios, and so potential bias in stopover studies in migrant Bluethroats was investigated. Playback promoted biases in the number of captures (although this was site-dependent) and fuel load. We strongly advise against the systematic use of playback to sample Bluethroats at constant effort sites or other type of ringing station, especially if studying fuel loads. [ABSTRACT FROM AUTHOR]

CapsuleThe spatio-temporal schedule provided by geolocators suggests that a pair of Barn Swallows could have remained together during the non-breeding period. Data from four birds, of which two were a breeding pair, showed winter quarters from West to Central Africa. The tracks of the paired individuals coincided both spatially and temporally at a scale of <200 km throughout the non-breeding period, in contrast to the tracks of an unpaired male and female from the same region. [ABSTRACT FROM AUTHOR]

CapsuleArrival fuel load of migratory passerine birds after the Sahara crossing in spring varied between oases of different habitat quality. AimsTo test: (1) if spring migrants reaching northern Africa with low fuel stopover in oases of species-specific suitable habitat; (2) whether migrants have sufficient fuel loads to reach southern Europe; and (3) if there is local variation. MethodsWe collected data from passerines caught at three different oases in southeast Morocco during the spring of 2011. ResultsIn species typical of mesophilic vegetation in Europe, birds carrying lower fuel loads occurred at a stopover site with lusher vegetation than two oases located further south. By contrast, in species adapted to xerophilic vegetation, birds carrying high fuel loads were found in drier habitat. ConclusionWhen arriving in northern Africa, migrants stopover in oases of different habitat quality in a species-specific fashion. Fuel load is, except for a few Mediterranean species, sufficiently high to reach the south of Europe without needing to refuel. Significant differences between oases suggests that fuel loads of birds from a single site are non-representative of the entire region. [ABSTRACT FROM PUBLISHER]

During the winter, Bramblings Fringilla montifringilla are known for concentrating in huge communal roosts, normally situated along a NE–SW axis from west Russia to France in southern Europe. Irruptions of Bramblings in Iberia are rare and normally only in an attenuated form. Exceptionally, in November of 2010 a large roost of c 900,000 Bramblings formed in the Basque mountains of northern Iberia. We caught and ringed Bramblings from this roost from November 2010 to February 2011, and compared age and sex ratios, wing length and body mass to data reported in the literature. Overall, first-year birds comprised more than half the roost and the sex ratio in first-year birds was biased to females. This is in agreement with the prediction that first-year birds and females migrate to regions further south than males and adult birds. Wing length was constant across the season, but not body mass. Body mass was also negatively correlated with temperature and positively correlated with snowfall. [ABSTRACT FROM AUTHOR]

The movements of Mediterranean Yellow-legged Gulls Larus michahellis michahellis to the Bay of Biscay were analysed using sighting data of colour-ringed birds. A total of 1,270 sightings corresponding to 567 individuals was compiled over a 14-year period. Gulls originated mainly from colonies in the east of Iberia (52.7%), southern France (28.2%) and the Balearic Islands (17.3%). The proportion of the year spent in the Bay of Biscay decreased with age: first-year birds occurred on the coast of the Bay of Biscay throughout the year while older individuals were seen only from July to December. Overall, 39.7% of the birds were resighted in more than one calendar year. The return rate of adults and sub-adults was similar. The results confirm the importance of the Bay of Biscay as a non-breeding area (mainly in autumn and winter) for Yellow-legged Gulls from the Mediterranean, and suggest that this population should be considered as partially migratory. [ABSTRACT FROM AUTHOR]

Capsule Intra-year survival during the winter is high for some insect-eating passerines in northern Iberia. Aims To estimate intra-year survival of insect-eating passerines wintering in northern Iberia. Methods Capture–recapture data from four passerines (European Robins Erithacus rubecula; Common Chiffchaffs Phylloscopus collybita; Firecrests Regulus ignicapillus; and Dunnocks Prunella modularis) captured during four consecutive winters were analyzed using Cormack–Jolly–Seber models. Results Intra-year survival during the winter differed between species, possibly revealing different survival or wintering strategies. Thus, Dunnocks and Firecrests showed survival rates close to 1, whereas Chiffchaffs and Robins had lower survival rates (0.4 and 0.3–0.6, respectively). Transients were detected only in Dunnocks (60%). This strategy in which some birds are resident and others are vagrant is already known for other species, particularly those that feed on plants or change to a more vegetarian diet during the winter. Conclusions Northern Iberia, and particularly its coast, is a good wintering area for several insect-eating species. [ABSTRACT FROM AUTHOR]

Capsule Counting and mist-netting provided different estimates of abundance. Aims To compare the efficiency, constraint and bias of mist-netting versus line transects for the estimation of species richness and abundance of passerines. Methods Mist-nets (126 linear metres) placed crossing a 420 m-long hedgerow line, open for four hours starting at dawn, four days per month from June 2006 to May 2007. During this same period, we conducted a transect parallel to a 1.3-km hedgerow line, four times per month. Results Estimation of species richness did not vary between methods, even when controlling for ecological groups, whereas the abundance estimations did vary. Conclusions Overall, line transects are better than mist-netting for estimating species richness and abundance since they are less costly, less invasive, and less time-consuming. However, if fine ecological analyses are required, including species abundance, mist-netting is preferred for solitary passerines that feed on insects and forage in the foliage, and line transects are better for gregarious passerines that feed on seeds or forage on the ground. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the relationship between breeding, moulting and fuel load of Common Crossbills Loxia curvirostra curvirostra inhabiting a Scots pine Pyrenean forest. Data were collected from two locations (Sierra de Uztarroz and Bigüezal) in northern Iberia by systematic mist netting between 2000 and 2008. Breeding females were detected in most months of the year except June, October and November, and with two main peaks, one in late winter (March), and another in the summer (August). Between years, reproduction was more variable in some months than in others, particularly in summer months. This suggests that breeding during the summer was more likely to be influenced by external factors and relatively opportunistic compared to winter breeding. Moulting adults of both sexes were captured during almost every month with a unimodal pattern peaking in June, similar to the pattern described for Mediterranean Crossbills. Moulting did not overlap with breeding, and our data do not support suspended moult patterns. Mean fat scores overall were low and, as was the case for body mass, showed little month-to-month variation. [ABSTRACT FROM AUTHOR]

The article discusses a study on the patterns of migration and wintering of European Robins Erithacus rubecula in northern Iberia, Spain focusing on seasonal patterns of captures and the relationship among migrating and wintering Robins, age and sex ratios and biometrics, fuel load and body condition. Robins were mist-netted weekly from July 2004 to May 2005 and data from robins trapped between September and April were considered. It suggests that the Robins are either birds moving at short distances or at migrating in short steps at longer distances.

The article discusses a study that determines whether plumage coloration is a valid criterion for defining and identifying White-throated Dipper types and its distribution in Spain. It refers to two types of White-throated Dipper existing in Spain, the Cincluse cinclus (C.c.) in the northwestern and central regions and C.c. aquaticus in the eastern and southern regions. Dippers were captured with mist-nets in ten representative zones within the Dippers' range in Spain. It suggests that plumage features alone cannot determine these birds.

Capsule Fuel load is correlated with fuel deposition rate; stopover duration is affected by arrival fuel load. Aims To determine the stopover duration, fuel management and flight ranges at departure of Blackcaps stopping over in northern Spain. Methods Systematic mist-netting and ringing allowed the use of mark-recapture Cormack-Jolly-Seber models for the estimation of stopover duration. Trapped birds were measured and weighed in order to estimate mass gain. FLIGHT software was used to estimate flight ranges. Results Stopover duration ranged from 3.6 to 13.6 days, and was negatively correlated with arrival body mass (assessed by body mass at the first capture event). On average, arrival body mass was 1 8.4 g, whilst body mass at departure was 19.8 g. No significant differences in arrival body mass and departure body mass were observed between age or sex classes. Mass deposition rate did not differ between age or sex classes (mean = 0.20 g/day). Birds recaptured one day after the first capture event lost mass, whilst recaptures from the second day onwards had a mean gain of mass; mass was observed to increase linearly with the stopover duration. Mass deposition rate was positively correlated with departure body mass. Finally, with a mean departure body mass of 19.8 g, a Blackcap stopping over in northern Spain should be able to fly up to 1100 km. Conclusions Stopover duration assessed by Cormack-Jolly-Seber models was longer than that observed in birds recaptured more than once (`minimum stopover duration'). Stopover was longer for birds arriving with less fuel. The positive relationship between departure body moss and mass deposition rate suggests a time-minimizing strategy. The lack of difference in fuel deposition rate between age and sex classes suggests a relatively abundant food supply at the study site, but other explanations might also account for the lack of age and sex differences, for example if competition for food was not determined by social hierarchies but by scramble competition. Departing fuel load would allow these birds to arrive at their wintering areas in southern Spain under still-air conditions, without needing to refuel. [ABSTRACT FROM AUTHOR]