Mate choice and nest cavity selection are directly linked to reproductive performance in seabirds, and disruption of these can result in breeding failure. The Red-billed Tropicbird Phaethon aethereus mesonauta breeds on inaccessible and remote islands, nesting primarily in natural rock cavities. I examined the relationship between reproductive performance and mate and nest cavity fidelity on St. Eustatius from 2012 to 2016, corresponding to four successive breeding seasons. The majority of nest failures occurred during the incubation stage. Pairs that failed in their first breeding attempt and laid a replacement clutch were more likely to be faithful to their mate and nest cavity within and between seasons than pairs that bred successfully. Only two replacement clutches (3.8%) were laid after the loss of a chick in the first breeding attempt, whereas the remaining 50 replacement clutches (96.2%) were laid after the loss of an egg. The stage at which pairs split had no influence on mate or nest cavity retention the following season. There were no significant differences in the weight or morphological measurements of chicks from pairs that retained their nest mate and/or cavity and those that switched. The results of this study suggest that switching nest mate or nest cavity does not necessarily improve subsequent breeding success among Red-billed Tropicbirds on St. Eustatius.

Egg size represents a fundamental predictor of chick mass and body condition. Chicks from bigger eggs have significantly increased survival, especially in precocial species, where chicks must forage for themselves and cope with environmental threats, such as bad weather or predators. Therefore, our understanding of the factors influencing egg size is crucial both from the perspective of their breeding ecology as well as of their conservation. However, studies simultaneously addressing multiple factors and quantifying their influence on egg size in large samples are rare. Here, we test the effect of seasonality, clutch size and nesting habitat on egg size, measured as volume, in a ground-nesting shorebird, the Northern Lapwing Vanellus vanellus, using a sample of 4384 eggs from 1125 clutches in South Bohemia, Czech Republic, during the period between 1988 and 2018. We report a significant decline in egg size over the breeding season, on average bigger eggs in larger clutches with a significant difference between 2-egg and 4-egg clutches, and no direct effect of nesting habitat. From our review of the same predictors across 15 Northern Lapwing populations throughout Europe it is apparent that replacement or late clutches have on average 3–7% smaller eggs than first or early clutches. Nesting habitat only rarely affects egg size and there are no significant differences in egg size between 3-egg and 4-egg clutches. Earlier studies showed that chicks hatching from bigger eggs early in the breeding season performed better, and that there was higher food abundance available for chicks at that time. This fact, together with the documented seasonal decline in egg size, sends an important message to conservationists and policymakers that early breeding attempts may play a pivotal role in safeguarding shorebird breeding productivity.

The miniaturization of tracking devices is now rapidly increasing our knowledge on the spatiotemporal organization of seasonal migration. So far, most studies aimed at understanding within- and between-individual variation in migratory routines focus on single populations. This has also been the case for continental Black-tailed Godwits Limosa l. limosa (hereafter Godwits), with most work carried out on individuals from the Dutch breeding population, migrating in relatively large numbers in the westernmost part of the range. Here we report the migratory timing and routes of four adult individuals of the same subspecies from the low-density population in eastern Poland and compare this with previously published data on Godwits breeding in The Netherlands. During northward migration, the birds from Poland departed and arrived later from their wintering and breeding grounds. However, on southward migration the Polish breeding Godwits departed earlier, but arrived one month later than the Dutch birds on their wintering grounds in sub-Saharan Africa. Despite the small sample size of tracked birds from Poland, we find a significantly higher between-individual variation in timing during southward migration in Polish Godwits as compared to the Dutch Godwits. Furthermore, not only did migratory routes differ, but the few Polish Godwits tracked showed a higher level of between- and within-individual variation in route choice during both southward and northward migration. To explain this remarkable discrepancy, we propose that the properties of transmission of social information may be different between Godwits from a high-density population (i.e. the one in The Netherlands) and a low-density population (in Poland) and that this leads to different levels of canalization. To examine this hypothesis, future studies should not only follow individuals from an early age onwards, but also quantify and experimentally manipulate their social environments during migration.

Coastal pastures provide supplementary foraging habitats for migratory shorebirds that rely on intertidal areas outside the breeding season. However, their role as foraging grounds for shorebirds is poorly understood. Here, we assessed the food supply, prey size selection, foraging behaviour and estimated consumption of a migratory shorebird, the near-threatened Eurasian Curlew Numenius arquata, at two temperate coastal pastures in northern Spain. Wintering Curlews only fed on earthworms (Oligochaeta), the main food available at coastal pastures. Average density and biomass were 102.3 ± 22.2 earthworms/m² and 5.8 ± 1.3 g AFDW/m², respectively, and only the density of surface living (i.e. epigeic) earthworms declined at the end of the winter. This food supply was comparatively high with respect to the biomass available in other man-made coastal habitats used by shorebirds wintering in temperate regions. Unexpectedly, the density of Curlews was higher at the pasture with the lowest food availability, though it declined significantly throughout the season at this site. Interestingly, Curlews preferentially selected the smallest size class (<32.5 mm) of epigeic earthworms, implying that a trade-off between energy intake rate and other fitness components could exist. Estimated consumption by Curlews was consistent with observed reductions in earthworm densities at both sites, thus reflecting a robust interaction between Curlews and epigeic earthworms at these agroecosystems throughout the non-breeding season. Our results highlight the potential role of earthworm communities at coastal pastures as food supply for declining migratory shorebirds.

Censusing breeding Common Redshanks Tringa totanus is hard, in view of the difficulty to access the breeding sites, find the highly camouflaged nests and avoid disturbing nesting birds. Drones are increasingly used to census waterbirds, owing to their capacity to overcome most of these difficulties. The present study aims to compare effectiveness, managerial efficiency and safety for birds of drone-conducted counts on a population of Redshanks breeding on a saltmarsh area of the Lagoon of Venice (NE Italy). Thirty-one drone flights on 29 marsh islands covering 6940 m, were conducted. We divided surveys into transects, each 20 m long and 10 m wide. In 2017–2018, drone flights counted 99 Redshank pairs, versus a count of 75 from ground censuses, in an area of 6.94 ha. Ninety-one breeding pairs of Redshanks were ascertained, combining results obtained with both methods. Drone surveys achieved a far better sensitivity and a slightly worse specificity than ground censuses. Overall, the average coefficient of agreement between methods was good, classified as substantial (Cohen's kappa = 0.76). Mean time spent to survey a transect of 20 m with a drone was far less than with the traditional approach: 4.2 ± 0 vs. 137 ± 29 s. Overall, 13.2 h were spent on censusing the whole area by the traditional approach vs. 0.46 h using the drone (–96.5%). Excluding the cost of the drone, this corresponded to an operational cost reduction of 94%. No apparent negative effects on nesting pairs or clutches were observed. Our study shows that drone flights constitute the least invasive option for studies on the breeding biology of Redshanks nesting at low densities on saltmarshes. Using a drone improved the effectiveness of nest finding and reduced disturbance to incubating birds. Integrating this new tool into the traditional ground searching approach would allow researchers to more rapidly identify nests, while at the same time reducing costs.

In many bird species intraspecific variation in migration strategies is related to sex or size. The Eurasian Bittern Botaurus stellaris is a sexually size-dimorphic heron species with a vast breeding and wintering range spanning a range of climates. Ringing data show that Bitterns from northern populations migrate westwards or southwards over thousands of kilometres, while Bitterns from southern or temperate breeding areas migrate much shorter distances or remain resident. So far, any differences in the migrations of males and females have remained unstudied. In temperate climates, relatively benign winters alternate with the occasional harsh winter, and under these conditions, males as the larger sex might take the risk to stay rather than show seasonal migration. In the years 2010–2012 we equipped three females and three males from breeding areas in The Netherlands with a tracking device, recording their movements over periods of 2–5 years. All three males and one female stayed within or near the breeding area during the non-breeding season, but two females moved to distant wintering sites. One female migrated 4900 km to The Gambia providing the first direct evidence for trans-Saharan migration in this species. Another female migrated 700 km to winter in Devon, UK, for five consecutive years. The Bitterns were site-faithful to their breeding area and, mostly, to their wintering areas as well, although one male and one female progressively wintered closer to the breeding area in three successive years. Our results suggest that larger males from The Netherlands, a breeding area with fluctuating winter conditions, are predominantly resident, while the females are partial migrants with individually different and flexible strategies.

As is the case for most avian species, there is considerable variation in the egg size of Continental Black-tailed Godwits Limosa l. limosa breeding in The Netherlands. It is interesting that egg size has costs and benefits yet varies considerably at the population level. To better understand this variation in egg size, we tested its relationship to a suite of individual and environmental factors. We found that egg size can decrease up to 2.8% throughout a breeding season and that egg size increases with clutch size by 1.4% with each additional egg in the clutch. Female body mass and body size explained 5% of the total variation in egg size observed across the population. Furthermore, females wintering south of the Sahara laid 3% smaller eggs than those wintering north of the Sahara. We also found that egg size increases with age, which may indicate age-related differences in the endogenous and/or exogenous conditions of females. The variation in egg size was, however, mostly the result of consistent differences among individuals across years (repeatability = 0.60). A comparison of daughters with mothers suggested that most of this individual repeatability reflects heritable variation (heritability = 0.64). The actual individual traits that underlie this heritable variation among individuals remain mostly undetermined. Smaller eggs did have a slightly lower chance of hatching, but we found no relationship between egg size and chick survival. Finally, nest and chick survival were strongly correlated with lay date. Thus, in Black-tailed Godwits, lay date may actually reflect a female's endogenous and/or exogenous condition at the moment of egg-laying. This finding may be general across birds, since food supplementation experiments usually result in advanced laying and larger clutch sizes rather than in larger eggs.

In lekking male Ruffs, three genetically distinct morphs compete for copulations with the visiting females. Faeder males are female mimics, whereas Independents and Satellites show marked sexual dimorphism, including an elaborate ornamental plumage. Independent males holding a territory on a lek (Residents) have higher mating success than Independents that do not (Marginals), whereas heterogeneity among Satellites is less well understood. Therefore, we compare variation in lek attendance and copulation success between Independent and Satellite males on five leks in northern Norway, over one mating season. Among Independents, Residents were more likely to return to the same lek between years, had longer tenure, higher visitation rate, longer visit duration, fewer arrivals and departures from the lek together with females, and higher daily and seasonal lek attendance than Marginals. We observed qualitatively similar between-individual variation in behaviour among Satellites. Thus, we define two types of individuals: Central Satellites with high lek attachment and Peripheral Satellites with low lek attachment. A notable difference between the morphs was that some Central Satellites (but no Resident Independents) were highly attached to more than one lek on the same day. Lek attendance correlated with copulation success, both on a daily basis and considering the entire season, and this relationship did not differ significantly between the morphs. Given that the mere presence of a male on the lek when a female visited to copulate predicted his copulation success, the unequal distribution of copulations on a lek can be largely explained by unequal lek attendance. After correction for variation in presence, one male on each lek obtained more than his expected share of copulations. Our observations suggest that endurance rivalry, whereby energetic limitations determine the lek attendance of competing males, could be an important determinant of copulation success on Ruff leks.

In July 2016, six European Bee-eaters Merops apiaster were tagged with radio transmitters in order to determine their space use during breeding and post-breeding. During breeding, home-range sizes were 2.0–5.5 km² (95% kernel contour of minimum convex polygon, MCP₉₅) and the median distance between position points and the nesting burrow was 0.20 km (Q1–Q3: 0.10–0.38 km). After Bee-eaters had fledged, adult birds increased their home range to 10.6–175.2 km² (MCP₉₅) and the median moving distance to the nesting burrow to 3.90 km (Q1–Q3: 1.83–5.25 km). In the last study week, two birds always flew together in larger flocks, while other tagged birds used other locations, this may indicate stable foraging groups of birds from different colonies. Space use was significantly influenced only by cloudiness during breeding and not by temperature or precipitation. In the early breeding season, only males stayed overnight in roosting trees; subsequently, both males and females stayed there. From about the end of the second week after hatching no adult birds were in the breeding colony at night. The results of this pilot study must however be verified with a broader-scaled study approach.

At Anzali Lagoon, on the southwest coast of the Caspian Sea, Great Crested Grebes Podiceps cristatus were observed breeding both in monospecific colonies and in a mixed colony with Whiskered Terns Chlidonias hybridus. In the mixed colony, Great Crested Grebes bred later than in their monospecific colonies, while laying clutches of similar size, but significantly fewer of their nests disappeared during incubation and their chicks had higher survival rates up to at least 20 days old. Human disturbance and the resulting increases in predation likely caused greater losses at monospecific colonies of Great Crested Grebes where there is no common anti-predatory activity; while the strong anti-predatory behaviour of Whiskered Terns in the mixed colony might have offered enhanced protection to the chicks. Unfortunately, we could not separate the effects of the seasonal differences in human disturbance and of the protective presence of terns on the Great Crested Grebe's reproductive output.

Adélie Penguins Pygoscelis adeliae live exclusively in Antarctica and commute between the sea for foraging and the land for breeding. During these movements, they must navigate various environments: underwater, on sea ice and on land; an unusual set of challenges among animals. Navigation mechanisms of this species were intensively studied about 50 years ago, but technological limitation at that time made it hard to fully understand movement patterns under experimental as well as natural conditions. Recent developments in animal-borne data loggers have enabled us to record movement paths and activities of such birds. In this paper, we report the results of displacement experiments combined with bio-logging on Adélie Penguins, conducted for the first time since initial experiments more than 50 years ago. Two chick rearing birds were caught at their nests and data loggers with GPS and accelerometers were deployed on their backs. The birds were artificially displaced and released approximately 1 km from the breeding colony. From the release point, their options to return to the nest were either walking over land or swimming in the sea. The birds successfully returned to the colony 6.0 h and 8.1 h after release, taking 44 min and 41 min, respectively, from the onset of homeward movement. Both individuals took what appeared to be the straightest and shortest course crossing over land. They spent most of the homing phase (51.2% and 66.2%) walking and only entered the water – in the same place – in the last stretch homewards. The results of our study demonstrate the homing ability of Adélie Penguins from a distant location after artificial displacement and the potential use of positional and acceleration data to study the navigation of penguins that travel by both land and sea.

Albinism is a remarkably rare colour deficiency among natural populations of bird species. Here we document the first record of true albinism in a wild population of Collared Flycatchers Ficedula albicollis. We found a nest of two albino and three melanised nestlings, and the former became progressively underdeveloped and perished by the end of the nestling feeding period. It is possible that parents differentiated between chick types when provisioning the brood or that the albinos were of lower competitive ability; these could be additional reasons for the rare occurrence of this phenotype in the wild.