Since first human settlement c. 400 years ago, the Mascarene islands have undergone some of the highest rates of ecosystem transformation and species extinction recorded worldwide. One surviving species, the Mascarene Swiftlet Aerodramus francicus native to the islands of Mauritius and Réunion, is typical among Aerodramus swiftlets in presenting a paucity of distinguishable morphological characters, as well as being particularly vulnerable to anthropogenic activities. In this study, a primary objective was generating genetic data for each island population, allowing genetic divergence between them to be assessed for the first time, employing informative methods regarding the taxonomic status. We find that the two island populations are 0.8% divergent and reciprocally monophyletic in the mitochondrial ND2 gene, with nuclear data (β-fibrinogen intron 7) being largely congruent but showing incomplete lineage sorting and low resolution. Results of barcode gap and coalescent species delimitation analyses are consistent with a comparison of the genetic divergence uncovered within Mascarene Aerodramus with that among other long-established Aerodramus species. The findings of all three approaches are consistent with the two island populations representing two separate species. However, because of lack of resolution in the nuclear locus, we conservatively retain a single species pending future data. To assess the conservation status, we conducted the first quantitative assessment of population size and distribution through extensive nest counts on the islands of Mauritius and Réunion, and by considering the threats to swiftlets on each island. The survey produced estimates of 10,100–10,700 individuals for Mauritius, and 39,600–53,500 individuals for Réunion. Considering the population size data, inferred distributions, and numerous conservation concerns, an updated conservation status to Threatened (Endangered): B2 b(i,ii,iii,iv,v) c(i,ii,iii,iv) is warranted according to IUCN guidelines. Improving protection of the breeding habitat of the Mascarene Swiftlet is crucial to prevent this endemic species from being added to the Mascarene's staggering extinction toll.

The Yellow-eyed Pigeon Columba eversmanni is globally endangered and one of the rarest pigeons with a predominantly Central Asian distribution, nesting in the valleys of the Syr Darya and Ili Rivers in Kazakhstan. The causes of the sharp population decline of the species are unknown, partially due to fragmentary migration data and a lack of information on their migration routes. In this study we aimed to gather novel information on the migration patterns of Yellow-eyed Pigeons by deploying GPS/GSM transmitters. Long-term ringing data showed a clear decline in the number of captured Yellow-eyed Pigeons. Two individuals were tagged in mid-September, during autumn passage at the Shakpak Pass in Kazakhstan. Both could be tracked over a migration distance of more than 1000 km, yielding precise data about the flight direction, speed, distance and stopover behaviour of these birds during their autumn migration. During migration, the two birds achieved maximum travel rates of 257 km/day and 243 km/day, respectively, with an average of 102 km/day. The flyway during autumn migration includes several stopover sites located in south Kazakhstan, east Uzbekistan and north Afghanistan. The birds primarily utilized forest belts, crop fields and trees in rural areas as stopover sites during autumn migration, while preferred night-time roosting in trees in crop fields and on buildings. These findings are significant as they underscore the importance of using GPS/GSM trackers to study Yellow-eyed Pigeon migration on a larger scale, which could provide more comprehensive data on their habitats and movements and facilitate the identification of potential threats.

The population increase of Lesser Black-backed Gulls in The Netherlands triggered investigations into life-history, migratory movements and foraging ecology during 16 years of nest-monitoring, colour-ringing and GPS-tracking on the island of Texel (Wadden Sea). The main objective was to obtain comprehensive ecological data on breeding performance within the context of the annual cycle, shifts in resources, prey types and habitat use. Migration strategies ranged from short- (France, England), medium- (Portugal, Spain) to long-distance (NW Africa), utilising marine, coastal or terrestrial, region-specific resources. Young birds travelled on average further than older individuals. Strong within-colony philopatry was found, this was strongest in males. Assessments of mate-fidelity indicated serial, social monogamy. Unexpectedly, given increasing population trends when the study commenced, fledging rates were low and declining egg volumes, smaller hatchlings, declining mass at fledging and high levels of cannibalism indicated structural food stress. Fledgling mass was well below that of chicks in historical studies, suggesting insufficient provisioning. Breeding was highly synchronised and early nesters fledged more young than late pairs. The onset of breeding was significantly delayed over the years, chick depredation rates declined, overall breeding success became more variable. Marine, urban and rural habitats, mostly within 80 km from the colony were used for foraging. Marine prey, mostly fisheries discards, formed the principal prey for most birds, supplemented with food found in agricultural areas. Human waste was found in only 7% of prey samples. A consistent decline of marine prey (in line with developing restrictions in fisheries), combined with signals pointing at food stress, suggests that the population is unable to boost reproductive success with currently existing foraging opportunities.

To verify the common assumption that the provisioning of new nesting structures and/or artificial nest cups helps breeding populations of House Martins Delichon urbicum, we examined (1) the use of new self-standing nest site constructions (offering multiple artificial nest cups) erected to replace lost nesting sites or as additions, and (2) the extent to which House Martins breeding on buildings use artificial nests rather than self-built nest cups. We contrasted breeding performances, measured during two country-wide citizen science projects in The Netherlands, on two soil types, clay and sand, with clay being the expected preferred building material for nest cups over sand. The likelihood that artificial nest site constructions erected as replacements for lost nest sites were occupied was on average almost four times higher (occupancy rate 66%) than added structures (17%). Soil type had no significant effect on the occupancy rate. Confirming that clay is a better nest-building substrate than sand, self-built nest cups on sandy soils were more likely to collapse during a breeding season than nest cups built in areas with clay soils. Artificial nests had higher rates of occupation on sandy soils and in colonies where fewer self-built nest cups from previous seasons remained. On clay soils, self-built nests showed higher nesting success than artificial nests, with the reverse on sandy soils. The probability of second breeding attempts was higher in artificial nests than in self-built nests, but surprisingly this was only so on clay soils, with a strongly negative effect of first laying date. We conclude that, especially on sandy soils, the provisioning of artificial nest cups helps House Martins by enabling breeding and/or by improving breeding success. Where old breeding sites disappear, local populations can be maintained by providing ready-made structures instead. Although House Martin populations can thus be helped with nest sites and artificial nests, a comprehensive evaluation of limitations on the population warrants scrutiny of other factors, such as food, i.e. the supply of aerial insects.

Like many other terns, European Sandwich Terns Thalasseus sandvicensis exhibit a period of post-breeding dispersal before migrating to their wintering grounds in Africa. To understand the significance of Danish waters for these movements, we analysed changes in the abundance and distribution of Sandwich Terns throughout Denmark from late June to October using our own count data as well as data from the online bird portal DOFbasen. To understand the geomorphology of, and threats to important roosting sites, we assessed their habitat characteristics and the level of (human) disturbances using data recorded in the field. We used 2422 resightings of individually marked juvenile Sandwich Terns to assess the use of Danish waters by terns of different geographical origin. The results showed that the temporal use of roosting sites differed between the western and eastern part of Denmark, with maximum numbers of Sandwich Terns occurring earlier in the west. Important roosting sites were characterised by sandy shallow coastlines, which reflect suitable habitats for their key food species. It seems most likely that phenological patterns in Sandwich Terns reflect the phenology and relative abundance of their food items. Results from the resighting data suggested that the proximity of breeding colonies to the different post-breeding areas in Denmark influenced the choice of post-breeding area as well as the phenological patterns. Large roosting numbers in combination with resightings of Sandwich Terns dispersing from various European countries highlight the international importance of Danish waters during the period of post-breeding dispersal. Observed average disturbance rates exceeding one disturbance per hour underlines the need for protected roosting locations on beaches with high human activity.

As on intertidal flats across the globe, the migratory shorebirds that spend most of their annual cycle in the Bijagós Archipelago, Guinea-Bissau, feed on macrozoobenthic prey buried in the intertidal soft sediments. Understanding the diet of shorebirds throughout the period of residence in the Bijagós can help to understand the degree of their trophic flexibility. In this study, we first reconstruct the diet of eight migratory shorebirds in the Bijagós, then investigate how their diet changes throughout the three main periods of the non-breeding season (arrival, mid-winter and fuelling periods) and finally explore the intraspecific dietary overlap between the three periods. We found significant changes in the diet of most shorebirds across the three periods, with some smaller species increasing the proportion of polychaetes in their diet in the fuelling period, while larger species increased the proportion of bivalves. The fuelling period showed the lowest overlap values with the other two periods, confirming that during this time most shorebirds considerably changed their diet, which may either reflect changes in prey availability or in prey selection.

Grassland breeding waders are in strong decline in most agricultural habitats across Western Europe. Studies evidencing the negative effects of agricultural practices on wader populations are numerous, but even in most specially managed areas the decline cannot be reversed. Earlier studies have shown that predation of nests and chicks occurs frequent, and that current predator densities can add to the decline and/or prevent recovery. In this study we experimentally study the effect of different intensities of predator control of Red Fox Vulpus vulpes, Beech Marten Martes foina, Badger Meles meles, Pine Marten Martes martes, Polecat Mustela putorius, Stoat Mustela erminea, Raccoon Procyon lotor and Raccoon Dog Nyctereutes procyonoides on nest predation and chick survival of the Black-tailed Godwit Limosa l. limosa at lake Dümmer, Lower Saxony, Germany. The area was subdivided in two subareas (Ochsenmoor and Osterfeiner Moor), between 2009–2017 343 nests were monitored. Normal predator control was implemented in both subareas in 2009 and 2010; during these years nest survival was low. From 2011 until 2017 intensified year-round predator control was implemented in Ochsenmoor and from 2016 also in Osterfeiner Moor. From 2011 until 2015 nest survival was relatively higher in the subarea with intensified predator control (Ochsenmoor). In 2016 and 2017 predators were intensively controlled in both subareas and nest survival was similarly high in both. From 2009 until 2017 chick survival was measured using radiotelemetry on 243 chicks. Generally, we found that in the subarea with intensified predator control, chick survival was higher. Combining the estimates of nest and chick survival, we estimate that godwit pairs raised between 0.97–1.12 fledglings per season under an intensive predator control regime and only 0.09–0.18 fledglings when predators were hunted at a normal level. As godwit pairs need to produce around 0.6 fledglings per year to sustain their population, the intensified predator control in this study-area has contributed to the recent increase in the breeding population. Our results thus show that the impact of mammalian predators on the breeding productivity of godwits can be reduced by intensified predator control. The intensified control of the targeted ground predators in this subarea could have resulted in increasing densities of unhunted predators, which subsequently could reduce nest or chick survival. Within the timespan of this experimental study, we did not observe a decrease in nest/chick survival that could hint at this meso-predator release.

Migratory shorebirds have suffered considerable declines because their migratory habits make them susceptible to threats in many different parts of the world. Successful conservation initiatives therefore require an understanding of their ecology not just at their breeding sites, but also at wintering and staging sites. Previous studies have shown that relationships between shorebirds and their wintering habitats are complex and context specific, underlining the importance of continued investigation of their ecology in new contexts. In this study, we investigated the relationship between prey density and shorebird abundance at stretches of mudflat spread across two islands in coastal Bangladesh. To explore whether prey density influenced shorebirds' ability to find and ingest prey, we also examined relationships with intake rates and foraging success. We found that the density and richness of shorebird communities increased with increasing polychaete worm density across small stretches of mudflat within each island. We also found evidence that prey intake rates and foraging success increased with available food, implying that food as a resource may be limiting in this landscape. Shorebirds may be required to spend more energy foraging in suboptimal habitats, explaining (in part) why densities are higher in habitats with more available prey. Our study suggests that prey density is an important correlate of shorebird density in this important South Asian wintering ground. Our results may have management implications, but this requires further study.

Understanding raptor diet is important when managing raptors of conservation concern, especially those experiencing population declines across parts of their range. Most traditional methods to study raptor diet, such as indirect analysis of pellets and prey remains, tend to be biased towards larger and more easily identifiable prey items. However, such biases can be partly addressed by combining more traditional methods with recent direct approaches such as stable isotope analysis. Here, we combine and compare the use of pellet dissection and stable isotope analysis to explore spatial variation in the breeding season diet of Eurasian Kestrels Falco tinnunculus across a southern England farmland landscape. We analysed 84 pellets and 31 nestling feathers from nest boxes situated throughout Dorset, England. Spatial variation in breeding season diets occurred across our study region. Mammalian prey dominated all sampled nest sites, however, the relative importance of avian and reptilian prey varied spatially. Spatial variation in Kestrel diet is likely partly reflected by the key habitat types throughout our study area, e.g., more avian prey in sites closer to arable and horticultural land. Our results suggest that individual prey preferences of parent birds may account for some of the variation in nestling Kestrel diet across our study region. Continued research focusing on the processes that drive spatial variation in Kestrel diet is required to further our understanding of spatiotemporal patterns in prey selection for this raptor of conservation concern.

The study of moult has received increased scientific attention due to its interdependence with other life-history aspects and its value as a reliable proxy of a bird's fitness. Here, we perform a detailed description of post-juvenile partial moult of a Red Crossbill Loxia curvirostra population exploiting Scots Pine Pinus sylvestris forests in the Pyrenees, southern Europe, with data from more than 1700 moult records. Crossbills within the tested population had replaced all or most median coverts and the six innermost greater coverts (GCs; 70–87%), decreasing outwards. Just 10% of the birds had renewed GC1. Tertials were renewed in half the cases and, exceptionally, some tail feathers, secondaries, primaries or primary coverts were renewed (less than 10%). Post-juvenile partial moults were more extensive in Pyrenean crossbills compared to crossbills in northern Europe, and in females compared to males. Among males, there were differences in moult extent dependent on colour, from greater to lesser extent: patchy, red, orange and yellow. Several factors could underlie these differences, such as climate, food availability or stability (explaining latitudinal differences), hormonal and sexual selection processes (explaining sex-dependent variation), hatching date or oxidative stress associated with the expression of colour patterns.

The marked variation in egg coloration within many bird species has attracted the interest of evolutionary biologists for decades. Many species, such as the Pied Flycatcher Ficedula hypoleuca, use biliverdin pigment to lay vibrant blue-green eggs that function as a signal of female condition to increase parental care from their partner. Here we report on the first case of complete shell pigment loss in this species. So far, we have monitored thousands of Pied Flycatcher clutches over 33 breeding seasons, and we have never observed such a phenomenon before. We observed and studied a complete clutch of five eggs with shell pigment loss. We evaluated the condition of the female and performed blood smears to discuss the potential causes of this phenomenon. The most plausible hypothesis that explains the finding is a genetic mutation affecting the biliverdin production pathway.

Red Kites Milvus milvus and Black Kites M. migrans, two raptor species, occasionally hybridise in Europe. Yet the ecology of hybrids is virtually unknown. Here, I describe migration patterns of a hybrid offspring of the two species. In 2022, a juvenile hybrid male was GPS-tracked from its nest site in Estonia to southern Greece. Between 23 August and 18 September, the hybrid covered 3213 km and made three short stops lasting 1–3 days. The mean daily flight distance was 128 km (21.5–283 km). The route used by the hybrid is a minor flyway for both parental species. The quantitative migration patterns were more consistent with those of the Black Kite but were within the wide range of variation among Red Kites.