Several aspects of the migratory behaviour of birds, notably timing, direction and duration, have been shown, mainly from breeding experiments, to be under genetic influence. Nevertheless, aspects of migratory behaviour can be improved by learning, either from personal experience or from other individuals. This review is concerned with social influence on different aspects of bird migration. Communal migration can provide not only the usual anti-predation and food-finding benefits, but also energy savings (resulting from particular flock structures), greater synchronization of individual journeys and improved route finding. Individuals in a group can pool their knowledge of directions and routes to mutual benefit, and if a group contains different age-groups, naïve youngsters can learn from more experienced individuals. These advantages may be one reason why many birds migrate in flocks or in more diffuse aggregations, even some species that normally live solitarily. Some species, notably swans, geese and cranes, migrate as family units within flocks, so young could learn migratory routes, stopover sites and wintering areas from their parents. And providing the two age-groups migrated together, the young of other species could learn the same details from older, more experienced individuals, whether related or not. In the daytime, some birds can be seen to migrate on their own, while others travel in groups, flocks or other looser assemblages. Night migration has been studied by radar, ceilometers and other artificial lights and by ‘moon-watching’, as well as by sound recording to detect flight calls. While some species travel at night in dense flocks, as in the daytime, others (including some solitary species) seem to travel in loose aggregations which may form once the birds are airborne. Calling maintains contact between individuals and may thereby provide information on optimal directions, flight altitudes and other aspects. Species vary in the extent to which they call at night, but such vocalizations increase at times of change, such as take-offs and landings, altitude changes or as birds enter mist. Many birds show specific behaviours before migration which serve to synchronize departures of those individuals which have reached an appropriate condition.

Individual homing pigeons, released at a distance, can normally find their way back to their home loft, but different pigeons may develop different routes. When pairs of pigeons, which have developed different routes, are released together, they may either take an intermediate route or one pigeon may join the other regarded as ‘leader’. The larger the difference in experience between two pigeons, the greater the likelihood that the more experienced bird will emerge as leader.

Migration in flocks may improve the accuracy of orientation if the direction of the flock is an average of the headings of the individual flock members (the ‘many wrongs’ principle). This notion has been supported by experiments on homing pigeons, and also by observations of wild birds, in which single individuals or small groups showed more variation in their directions than did larger groups. In general, directional scatter decreased as group size increased. In such systems, individual birds benefit from social contact with others, but all individuals have equal influence on the resulting direction. In practice, some birds (leaders) are likely to have more influence on directions than others (experienced more than inexperienced), as shown in experiments with pigeons and tracking of wild goose families on migration.

Large scale field experiments have also shown that birds can be influenced in their movements by others of their species, and that even birds from resident populations can sometimes be induced to migrate when released into areas where migrants prevail. When juveniles are released in the absence of experienced adults, they take a wider range of directions than normal, some of which take them off an optimal route. By training geese, swans or cranes to fly behind ultralight aircraft, new migration routes have been created, re-establishing migratory populations in areas from which they were eliminated in the past. Such experiments further illustrate the role of social influence on bird migratory patterns. Suggestions are made for further research aimed to fill gaps in understanding and to take forward our understanding of social influence on migratory behaviour.

Although a few ringing recoveries indicate that Icelandic breeding Purple Sandpipers Calidris maritima remain in Iceland through the year, there was the possibility that some spend the non-breeding season outside Iceland. To obtain a better understanding of their movements, 22 breeding adults (mainly males) were colour-ringed and tagged with geolocators in Arctic Tern Sterna paradisaea colonies on heathland adjacent to the coast of the Melrakkaslétta in northeast Iceland during 2019 and 2022. Counts, including searches for the colour-ringed birds, were also made on the coast beside the breeding areas through one non-breeding season (2019/20). Based on the recovery of 13 geolocators from males, 11 remained in northeast Iceland for the autumn moult, whilst two moved to northwest Iceland. By mid-winter, only five remained in northeast Iceland, and the rest wintered in the northwest and west of Iceland. Observations of colour-ringed birds confirmed that some remained close to their breeding area in autumn before dispersing to other parts of Iceland for winter. There was no evidence that Icelandic breeding birds left Iceland. Birds that wintered in the northwest and west of Iceland, returned to the northeast mainly during April. Counts during the non-breeding season along the shore adjacent to the breeding areas exceeded the number that could be accounted for by the local breeding population, indicating an influx of wintering birds from elsewhere.

The Aquatic Warbler Acrocephalus paludicola is a globally threatened, long-distance migratory passerine that continues to decline in abundance and range. Studies throughout its annual cycle are needed, including during migration, when information on habitat availability and usage are important for conservation. Here, the autumn occurrence (distribution, abundance, phenology) of Aquatic Warblers in Portugal is described taking advantage of an increase in capture effort with diurnal song playback. The species was captured at six new sites and seen at a further three, increasing the total number of sites to 16. Salreu marshlands appeared to be the most important with 93% of all captures since 2009, although ringing effort was highly variable between sites. We show that song playback did not influence age nor sex ratios, but tended to attract individuals with lower body mass, presumably those that arrived recently at stopover sites. Adult males migrated earlier than all other sex and age categories, in accordance with their limited role in parental care. Furthermore, we show that at least some individuals fuel up at Portuguese stopovers, which increases their importance for conservation. In addition, at Salreu marshlands, the abundance of Aquatic Warblers was significantly greater at an active than at a dry, abandoned (extensive) rice field, suggesting this land-use change may affect the quality of stopover sites. Recommendations regarding habitat management and conservation measures are proposed.

Despite major implications, moult remains understudied compared with other major annual cycle phenomena such as breeding and migration. This is certainly true for Eurasian Spoonbills Platalea l. leucorodia for which even the primary moult has remained poorly described. Using digital photography, we studied primary moult of Spoonbills in the eastern Dutch Wadden Sea from July to October 2021. From the images that were sufficiently clear to quantify primary moult we obtained a total of 168 observations for immatures and 790 for adults. We aged spoonbills by the extent of black on the primary tips and scored their abdominal profile as an index of body mass. Progression of the descendent moult showed a distinct sigmoid pattern, implicating population turnover at the start and end of the study period. Also, few individuals completed moult in the study area. Therefore, we applied Underhill-Zucchini models to actively moulting individuals. As expected, immatures were estimated to initiate moult over a month earlier (11 June) than adults (20 July). Surprisingly, immature spoonbills moulted at a higher rate, which would allow immatures to complete primary moult in 131 days without suspension compared with 170 days in adults. As a fast moult may reduce feather quality, we speculate that the black wing tips of immature spoonbills may compensate for lower feather quality. Birds with low abdominal profile scores were observed only in July and August, but fat birds were seen throughout the study period. The incidence of suspended moult sharply increased through September in both age groups. By late September 30–40% of birds had suspended moult and abdominal profile scores peaked. This means that immature and adult spoonbills appear to synchronize the suspension of moult before migration, despite marked age differences in the onset and rate of primary moult. Spoonbills may benefit from synchronized migration by improving navigational accuracy and flight efficiency through collective decision-making and formation flights.

The Parishan freshwater wetland is the most important wintering site for waterbirds among the inland wetlands of Iran. Using annual wintering waterbird census data collected from 1967 to 2022, we analysed the population trends of wintering Anatidae species. Of the 19 species of Anatidae, only ten species were recorded during at least 20 years, and statistical trend analysis was possible for seven species. The average population of Anatidae over 31 years (1988–2018) was more than 5430 individuals. In the first 21 years it was 7854 individuals, while in the last ten years (2009–2018) it decreased to about 340 individuals. Since 2018, no waterbirds have been counted and recorded in the wetland. The largest number of Anatidae was observed in 1989 and 1990 by counting 32,271 and 26,200 individuals, respectively. The Anatidae family accounted for an average of more than 39% of the waterbird populations and 29% of the species diversity. Results showed that the population of Greylag Goose Anser anser decreased with a slope of –17.9% per year, Ruddy Shelduck Tadorna ferruginea decreased with a slope of –10.4%, Common Teal Anas crecca –10.9% and Common Pochard Aythya ferina –10.7%. The decrease in the abundance and diversity of Anatidae is directly related to the decrease in the water level of wetlands and water bodies. Currently, the Parishan Wetland is completely dry and there are no longer any areas that are permanently flooded. Protection and restoration of the wetland requires the allocation of water resources to wetland, choosing an effective governance method and participation of the stakeholders.

Highly pathogenic avian influenza has emerged as a global menace over the past decades, posing a dramatic threat to birds worldwide, and its spread is of great concern to both conservationists and health authorities. In the 2022 breeding season, the Sandwich Tern Thalasseus sandvicensis was affected by a northwestern Europe-wide pandemic of the highly pathogenic avian influenza A (strain H5N1). This note reports on the occurrence of avian influenza in Sandwich Terns breeding in the Lagoon of Venice (Italy) in 2023 where the outbreak exterminated all chicks, but spared the adults.

As more and more species are in decline, understanding the various aspects underlying their population dynamics is crucial to optimise conservation efforts. Hereto, we studied the diet composition and reproductive performance of Gull-billed Terns forming the last breeding population in Central Europe by analysing population-level data collected between 2011 and 2022 as part of a conservation program. We show that the main prey types vary among breeding stages and across years. When observed foraging for themselves, Gull-billed Terns were mainly found to eat insects, whereas crabs and worms were the main prey items used for courtship feeding and chick provisioning. The proportion of prey of terrestrial origin used for courtship feeding increased over the study period, whereas no such change was observed in prey used for self-feeding or chick provisioning. Clutch and egg size also increased over the study period but were not correlated with the proportion of terrestrial prey used in courtship. Fledging success did not change over time and was not correlated with variation in chick diet. These results confirm that the Gull-billed Terns in this population are generalist feeders and suggest that their breeding success is not currently threatened by a lack of, or changes in, specific prey types. The shift towards more terrestrial prey in part of their diet may, however, indicate changes in the aquatic ecosystem, which should be further assessed, together with other potential risks for these threatened birds, such as inbreeding and pollution.

The stability of measurement parameters over the years is essential in long-term monitoring of populations. The goal of this study was to evaluate the variation of selected acoustic parameters in male Tawny Owl territorial calls. A total of ten sites were monitored in low mountains mostly covered by deciduous forests. The owl calls were recorded from February to July and in one case in September. Three acoustic parameters were analysed: the duration of the first and third notes and the total duration of the call. The third note showed the lowest coefficient of variation (10.4%) over the years. This parameter also showed the lowest range between minimum and maximum values. All parameters had higher inter-individual than intra-individual variation. The stability of the parameters was analysed in two males between two years. All parameters showed significant differences in one male. The total duration of the call was also different for the second male. Individual recognition based on the duration of notes is therefore not a suitable method for long-term monitoring. These findings may help in selecting appropriate parameters for individual recognition and a more detailed view of the stability of acoustic parameters.

According to the monthly counts of the Wetland Bird Survey (WeBS), the numbers of Eurasian Oystercatchers Haematopus ostralegus wintering in Great Britain has gradually decreased since the late 1980s/early 1990s. Although numbers also decreased in the South West region of England, the decline was much steeper in the Exe estuary population, suggesting that site-specific pressures may have affected this species in this regionally-important Special Protection Area. By combining data from the WeBS with those from 45 years of research on Oystercatchers by the Centre for Ecology and Hydrology, we tested five hypotheses that could explain the relative decline on the Exe estuary: (1) a gradual improvement in estimating Oystercatcher numbers as counting methods were refined, (2) a deterioration in the main food supply, the Blue Mussel Mytilus edulis, (3) disturbance from people on and alongside the estuary, (4) disturbance from Peregrine Falcons Falco peregrinus and (5) an as yet unexplained increase in the frequency of stealing of mussels from Oystercatchers (kleptoparasitism) by Carrion Crows Corvus corone and European Herring Gulls Larus argentatus. The data are consistent only with the fifth hypothesis. Individual-based modelling suggested that kleptoparasitism at the increased frequency that occurred on the Exe estuary could have reduced the foraging success of Oystercatchers sufficiently to have (1) reduced the overwinter survival of the numerically dominant mussel-eating adults and (2) deterred prospecting immatures from choosing the estuary as their future wintering site.

We studied the breeding ecology of Whinchats Saxicola rubetra in four tiny (less than 10 breeding pairs) populations in nature reserves in the Dutch province of Drenthe. We focussed on reproductive rates (laying date, clutch size, hatchling number, breeding success), proportion of unpaired males and nestling diet. Breeding pairs appeared to cluster together, thereby leaving other seemingly suitable parts of the area unoccupied. Clutch size, nest and pair success were similar to or higher than those in other studies. Laying of first eggs peaked during 16–20 May and laying dates had not advanced since the 1950s, in contrast to many other songbirds. We found 34% of the males remained unpaired, limiting the population reproduction rate. Whinchats provided their nestlings primarily with beetles and caterpillars and feeding frequency appeared to depend on prey species. Higher feeding frequencies may lead to a higher risk of nest predation. Being single-brooded, Whinchat populations may be more affected by nest predation than populations of multi-brooded species.

Information on a species' reproductive ecology can contribute to the pool of knowledge about avian natural history and life-history evolution. This is particularly true for species inhabiting remote and inaccessible regions. Here we report on the reproductive ecology of the Mountain Chiffchaff Phylloscopus sindianus, a warbler endemic to the Pamir Plateau (Central Asia), based on data from 22 nests observed on the eastern Pamir Plateau. The birds nested in thorny shrubs and their breeding season took place between mid-May and late June, during which a pair might make a single nesting attempt. Clutch size averaged 3.9 (3–5) eggs. Both parents undertook incubation and provisioned nestlings and the nestling period lasted 15.0 (13–18) days. The logistic growth rate constant of nestling body mass was 0.49. Nesting success, measured as the proportion of nests with at least one fledgling, was 68.2%, with predation explaining 57.1% of nest failures. Despite possible constraints on reproductive investment at high altitudes due to the harsh environment, comparisons of reproductive data from current and earlier studies of different warbler species show that higher-altitude birds do not produce different clutch sizes or eggs of different size compared to their congeners at lower altitudes.

Caterpillars are important as food for many insectivorous birds. Their availability is highly seasonal in temperate regions. Global warming is known to advance this seasonality and can change the selection pressure on the seasonal timing of avian reproduction. Time series on caterpillar abundance and timing are often based on quantifying caterpillar faeces (called frass). Accurate estimates of caterpillar abundance and timing within and between years require temperature correction, the topic of this study. To estimate caterpillar abundance, we measured the rate of caterpillar frass production under tree crowns. To calculate the frass to caterpillar biomass conversion factor we kept wild caught caterpillars of a number of abundant species in cages outside and fed them daily with fresh leaves (Quercus robur or Betula sp.) in two studies, Dwingelderveld (Dv) and Hoge Veluwe (HV), in different time periods. We monitored caterpillar body mass and frass production on a 24-h time base to calculate the conversion factor as a function of temperature, study site, tree species and caterpillar species. One study (HV) concerned a reanalysis of earlier published data. The conversion factor strongly depended on ambient temperature, almost doubling from around 8 to 15 when daily temperatures reduced from 15 to 10°C, and differed somewhat between study areas and caterpillar species. Yet in 20 of 34 published studies that estimated seasonality in caterpillar abundance on the basis of frass fall, temperature was not corrected for. We illustrate the importance of the temperature correction of frass to mass conversion factors by comparing variation in raw frass data and temperature corrected caterpillar biomass estimates during the spring for 12 nest boxes in 16 years in the Dwingelderveld. We advise temperature correction of frass fall based estimates of caterpillar abundance in woodland, preferably based on local measurements in the study area and period.

Alfred Russel Wallace (1823–1913) and Carl Benjamin Hermann, Baron von Rosenberg (1817–1888) were naturalists with similar goals in their exploration of the Malay Archipelago. Their collaborations have not been the subject of any previous study. They first met in May 1858 on the coast of Dorey, the northwestern part of New Guinea. Together they discovered two birds new to science, the Dusky Lory Pseudeos fuscata (Blyth, 1858) and the Fairy Lorikeet Charmosynopsis pulchella (G.R. Gray, 1859). Wallace did not propose new names himself, while the Chalcopsitta leucopygialis Rosenberg, 1859 (nomen nudum) and Charmosyna pectoralis Rosenberg, 1862 lost their priority. Wallace visited Aru Island for six months in 1857, where he collected 113 bird species as enumerated in his private notebook, now transcribed in extract for the first time. Wallace's collections from Dorey were examined by George Robert Gray (1808–1872) in 1858 after they were received by the Natural History Museum in London. When Rosenberg was working out the distribution of various birds in the archipelago in 1862, he had not yet visited the Aru Islands. His list of 86 bird species from Aru was contributed by Wallace, most probably during their discussions when they met in Ceram in November or December 1859. The historical narrative provides two rare instances where Wallace's presence was acknowledged by fellow naturalists in the field, and at the same time gives insight in the cooperation and potential scientific rivalry of the period.

Using a high-speed surveillance PTZ camera mounted to a wind turbine in the offshore waters of Belgium, we observed frequent use of the wind farm area by Sandwich Terns Thalasseus sandvicensis and Common/Arctic Terns Sterna hirundo/paradisaea during the spring migration seasons of 2021 and 2022. The terns used a buoy in the wind farm for resting and as a foraging fallback point but were also seen perching on the railing and the platform of the wind turbine itself. In the latter case, terns were foraging close to the base of the turbine. On a few occasions captured sandeel (Ammodytidae) were transported to awaiting females at the turbine. While the potential implications of these behaviours on collision risk remain uncertain, the observed activities underscore the need for comprehensive investigations into the three-dimensional behaviours of terns and other seabirds within offshore wind farm environments.