Great Reed Warbler Acrocephalus Arundinaceus Research Articles

Improved haplotype resolution of highly duplicated MHC genes in a long-read genome assembly using MiSeq amplicons.

Long-read sequencing offers a great improvement in the assembly of complex genomic regions, such as the major histocompatibility complex (MHC) region, which can contain both tandemly duplicated MHC genes (paralogs) and high repeat content. The MHC genes have expanded in passerine birds, resulting in numerous MHC paralogs, with relatively high sequence similarity, making the assembly of the MHC region challenging even with long-read sequencing. In addition, MHC genes show rather high sequence divergence between alleles, making diploid-aware assemblers incorrectly classify haplotypes from the same locus as sequences originating from different genomic regions. Consequently, the number of MHC paralogs can easily be over- or underestimated in long-read assemblies. We therefore set out to verify the MHC diversity in an original and a haplotype-purged long-read assembly of one great reed warbler Acrocephalus arundinaceus individual (the focal individual) by using Illumina MiSeq amplicon sequencing. Single exons, representing MHC class I (MHC-I) and class IIB (MHC-IIB) alleles, were sequenced in the focal individual and mapped to the annotated MHC alleles in the original long-read genome assembly. Eighty-four percent of the annotated MHC-I alleles in the original long-read genome assembly were detected using 55% of the amplicon alleles and likewise, 78% of the annotated MHC-IIB alleles were detected using 61% of the amplicon alleles, indicating an incomplete annotation of MHC genes. In the haploid genome assembly, each MHC-IIB gene should be represented by one allele. The parental origin of the MHC-IIB amplicon alleles in the focal individual was determined by sequencing MHC-IIB in its parents. Two of five larger scaffolds, containing 6-19 MHC-IIB paralogs, had a maternal and paternal origin, respectively, as well as a high nucleotide similarity, which suggests that these scaffolds had been incorrectly assigned as belonging to different loci in the genome rather than as alternate haplotypes of the same locus. Therefore, the number of MHC-IIB paralogs was overestimated in the haploid genome assembly. Based on our findings we propose amplicon sequencing as a suitable complement to long-read sequencing for independent validation of the number of paralogs in general and for haplotype inference in multigene families in particular.

The structure of songbird MHC class I reveals antigen binding that is flexible at the N-terminus and static at the C-terminus.

Long-distance migratory animals such as birds and bats have evolved to withstand selection imposed by pathogens across the globe, and pathogen richness is known to be particularly high in tropical regions. Immune genes, so-called Major Histocompatibility Complex (MHC) genes, are highly duplicated in songbirds compared to other vertebrates, and this high MHC diversity has been hypothesised to result in a unique adaptive immunity. To understand the rationale behind the evolution of the high MHC genetic diversity in songbirds, we determined the structural properties of an MHC class I protein, Acar3, from a long-distance migratory songbird, the great reed warbler Acrocephalus arundinaceus (in short: Acar). The structure of Acar3 was studied in complex with pathogen-derived antigens and shows an overall antigen presentation similar to human MHC class I. However, the peptides bound to Acar3 display an unusual conformation: Whereas the N-terminal ends of the peptides display enhanced flexibility, the conformation of their C-terminal halves is rather static. This uncommon peptide-binding mode in Acar3 is facilitated by a central Arg residue within the peptide-binding groove that fixes the backbone of the peptide at its central position, and potentially permits successful interactions between MHC class I and innate immune receptors. Our study highlights the importance of investigating the immune system of wild animals, such as birds and bats, to uncover unique immune mechanisms which may neither exist in humans nor in model organisms.

Remnant Rhine delta population of Great Reed Warblers maintains high diversity in migration timing, stopping sites, and winter destinations

Summary Many Afro-Palearctic songbird migrants have declined, with conservation efforts mainly focused on the restoration of breeding habitat. However, pressures outside the breeding season might play a role. This includes the possibility that local relict populations no longer maintain the original phenotypic variation in migration patterns, with a loss of flexibility. The Great Reed Warbler Acrocephalus arundinaceus kept stable population levels in central and eastern Europe, but was almost extinct in the western part of the breeding range. In the Rhine delta the population declined from 10,000 individuals around 1950, to fewer than 100 at present. Here we document migratory timing, routes, and destinations of members of this remnant songbird population. It turned out that the remaining adults still showed high diversity in migratory phenotypes. This is even true in each of the two last tiny subpopulations in the Netherlands (of 50 and 15 pairs). So, even very small populations maintain the possible adaptive phenotypic variation, and with hindsight this justifies the breeding habitat restoration efforts currently underway.

The role of reed management and habitat quality on brood parasitism and chick survival of the brood parasitic Common Cuckoo.

Despite efforts on ecosystem restoration and management, biodiversity loss remains one of the major environmental concerns of our time. Beyond the focus on threatened species, animals that indicate regional biodiversity hotspots and population trends, such as brood parasites, should also be targeted by conservation actions. We studied how reed habitat quality and management influence brood parasitism rate and offspring survival in Common Cuckoos Cuculus canorus parasitizing nests of Great Reed Warblers Acrocephalus arundinaceus in six reed habitats in an intensive agricultural landscape. Data collected from 45 sites over 13 years showed that the brood parasitism rate was highest on large canals and was positively influenced by the availability of potential perches (Cuckoo vantage points) and the height where host nests were built. Cuckoo chick survival decreased with water depth and was not affected by other factors. Our results suggest that the habitat-dependent detectability of host nests was central in brood parasitism rate and that water level was central in Cuckoo chick survival. Our study shows that a maintenance of intermediate water levels is the most optimal for maintaining Cuckoo populations in intensive agricultural landscapes. Because brood parasites are excellent bioindicators as their presence predicts regional hotspots of taxonomic and functional diversity as well as population trends in bird communities, knowledge on their habitat requirements is relevant in management targeting diverse bird communities.

Reuse of nests in the Great Reed Warbler Acrocephalus arundinaceus: A behavior to save time and energy and to deter nest parasites?

The reproductive period in animals is a demanding part in their life history. In birds, environmental factors, such as adverse weather, predation, or brood parasitism; and/or anthropogenic disturbance, can limit breeding success, resulting in failure of clutches. The nest loss in open‐cup nesting passerines is usually replaced with a new nest with a new clutch, however, in some cases the clutch replacement may occur in unusual forms. In this study, we report on three cases of within‐season nest reuse in the Great Reed Warbler. In the first case, a nest was reused for two times in the same season after unsuccessful nesting attempts (two‐time nest reuse). After the nest was depredated the first time, the female laid new eggs that were depredated again, then again the female laid new eggs that produced four fledglings. In the second case, the first clutch was depredated, after which the female laid a new clutch in the same nest that was again depredated. In the third case, the female laid new eggs among the eggs that failed to hatch previously. Our observations tend not to be consistent with the predator avoidance hypothesis because the depredated nests were reused by the parents. The time/energy saving hypothesis or possible deterrence of nest parasitism could explain nest reuse in this study, but because of low number of nests reused compared to the total number of nests found, this phenomenon needs further clarification.

Fast and furious: host aggression modulates behaviour of brood parasites

Avian brood parasites pose a serious threat to hosts, substantially reducing their fitness, which selects for the evolution of host defences. A classic example of a host frontline defence is mobbing, which frequently includes contact attacking of brood parasites. Here, we investigated how the nest defence of a very aggressive host, the Great Reed Warbler Acrocephalus arundinaceus, influences the speed of egg‐laying and egg‐removing behaviour of its brood parasite – the Common Cuckoo Cuculus canorus. We video‐recorded 168 brood parasitic events at 102 active host nests and found that Cuckoos avoided host mobbing in 62% of cases. If hosts spotted the Cuckoo at their nests, they almost always attacked it (91 of 104 cases; 88%); however, such attacks only rarely and temporarily prevented Cuckoos from parasitizing (11 additional cases). When attacked, Cuckoos parasitized host nests significantly faster and left them immediately after laying. However, when not attacked, Cuckoos frequently stayed at or near the nest, suggesting that host aggression, rather than the risk of being spotted, influences the speed of brood parasitism in this species. Further, we found that Cuckoos performed egg‐removing behaviour in all parasitic events without regard to host aggression. As a result, Cuckoos removed at least one egg during all brood parasitism events except those when the egg slipped from their beaks when they were attacked by hosts and, thus, remained in the nest (in nine of 82 cases when they were attacked; 10.9%). This indicates that egg‐removing behaviour is not costly for the Common Cuckoo and is an essential part of its parasitism strategy, widening our understanding of this elusive behaviour.

How strong are eggs of the common cuckoo Cuculus canorus?

The common cuckoo Cuculus canorus is an obligate brood parasite that lays its eggs in the nests of small passerines. It has long been hypothesized that cuckoo eggs should be structurally stronger than host eggs or those of non-parasitic cuckoos to reduce chances of breakage during laying, to prevent accidental damage during incubation and/or to hinder their rejection through puncture ejection by the host. Therefore, we analysed selected characteristics of a sample of freshly laid eggs of the common cuckoo with two of its major hosts, the reed warbler Acrocephalus scirpaceus and great reed warbler Acrocephalus arundinaceus, and a sample of species with known puncture resistant eggs. We found that in puncture resistance tests cuckoo eggs tolerated on average 231 g. The cuckoo eggs were 3.3 and 2.5 times stronger than those of the reed warbler and great reed warbler, respectively. Greater shell thickness can explain only 17% of the total extra strength of the cuckoo eggs (125.97 g). When we controlled for the confounding effects of egg size (using a sample of eggs of normal strength from bird species of varying size), the common cuckoo eggs were 2.2 times stronger than expected for their size. Our results are consistent with the hypothesis that cuckoo eggs are structurally stronger and this trait probably represents an adaptation for a brood parasitic life style.

Common generalist mites do not transmit from foster parents to brood parasitic chicks

AbstractInteractions between avian brood parasites, their hosts and their parasites provide an ideal model system for studying coevolutionary processes across multiple hierarchical levels. Despite this, how brood parasitism affects the ecology and transmission of bird ectoparasites is not well known. Here, we examined the mite fauna of great reed warbler Acrocephalus arundinaceus nests successfully parasitized and unparasitized by the common cuckoo Cuculus canorus to find whether there is vertical transmission of northern fowl mites Ornithonyssus sylviarum, a generalist blood‐feeding parasite of birds, through infected nests. Although there was no difference in the prevalence of mites between successfully parasitized and unparasitized nests, the former nests exhibited two times lower mite abundances than the latter nests. Importantly, none of the common cuckoo nestlings examined shortly before fledging carried mites, whereas the great reed warbler nestlings were infested in almost half of the contaminated nests. Thus, our results indicate that, despite their high dispersal abilities, northern fowl mites may be unable to survive or reproduce on the common cuckoo nestlings. Factors that can contribute to the observed patterns are discussed.

Falling out of the host nest: an overlooked factor decreasing survival of brood parasite chicks

Animals rely on an array of environmental triggers or cues to make their behavioural and life‐history decisions. Brood–parasitic females should prefer the best host to maximize their reproductive success. Despite the fact that Eurasian reed warbler Acrocephalus scirpaceus is one of the most frequent host of the common cuckoo Cuculus canorus, parasitic chicks often prematurely fall out of the nests and drown. To establish whether these events are influenced by the size of the nest, we experimentally replaced original Eurasian reed warbler nests for much bigger great reed warbler Acrocephalus arundinaceus ones and transferred cuckoo chicks into them to record their fate. The chicks in bigger nests showed significantly lower falling‐out rate (4 of 29) compared with those remained in smaller nests (12 of 32). These findings suggest that host nest size plays an important role in the suitability of host species and that smaller host nests may pose a high mortality risk to the relatively big cuckoo chicks. Moreover, cuckoo chicks that fell out of the nests were significantly less fed by foster parents before falling‐out event than those stayed in the nests. This indicates that insufficient parental care may also facilitate falling‐out of cuckoo chicks, probably because hungry chicks are more restless in host nests. The relatively frequent falling‐out events represent a substantial and underestimated cost of brood parasites. Without video surveillance it can incorrectly be confused with predation. We therefore strongly recommend using video‐recording for determining predation costs in birds.

Autumn migration strategy of juvenile great reed warblers Acrocephalus arundinaceus on the eastern European flyway: a spatiotemporal pattern of accumulation and utilisation of energy stores

The great reed warbler Acrocephalus arundinaceus is a common migrant through the Western Palearctic. However, its migration strategy, especially the spatiotemporal pattern of accumulation and utilisation of energy stores en route, is poorly known. Using ringing data collected at distant stopover sites located from Central Europe to Asia Minor, we observed that pattern in juvenile great reed warblers migrating along the eastern European flyway in autumn. We analysed body condition and stopover duration and estimated potential flight ranges for this species, taking into account two large ecological barriers – the Mediterranean Sea and the Sahara Desert – which it crosses during its southward migration. In Central and South-Eastern Europe the seasonal trends in body condition were similar and non-linear, with low values in August (i.e., during intensive migration of Acrocephalidae), and then a significant increase from the beginning of September until the end of the month. Estimated potential flight ranges showed that individuals migrating through Central and South-Eastern Europe might generally reach only the northern coast of the Mediterranean Sea. On the other hand, those flying from Asia Minor were able to reach the northern edge of the Sahel zone, crossing two large ecological barriers without refuelling. Therefore, it seems that great reed warblers change their autumn strategy en route: first, they migrate in short steps with low energy reserves through continental Europe; and then, after intensive refuelling before the Mediterranean Sea, they reach sub-Saharan Africa in a long flight. Based on the analysis of ringing data from several stopover sites, we show the complex pattern of how a long-distance migratory species accumulates and utilises its energy stores en route.

Multiple parasitism reduces egg rejection in the host (Acrocephalus arundinaceus) of a mimetic avian brood parasite (Cuculus canorus).

A host that has been targeted by an avian brood parasite can recover most of its potential fitness loss by ejecting the foreign egg(s) from its nest. The propensity for some hosts to engage in egg rejection behavior has put selective pressure on their parasites to evolve mimetic eggshells resembling the host's own shell colors and maculation. In turn, hosts have counterevolved increasingly more sophisticated detection methods such as narrowing visual egg acceptance thresholds or using social cues to recognize parasitism. However, multiple cognitive mechanisms acting simultaneously could theoretically interfere with one another and ultimately decrease egg rejection accuracy, especially if these heuristics yield differing targets for rejection. By painting hosts own eggs, we studied a host species of the common cuckoo Cuculus canorus, the great reed warbler Acrocephalus arundinaceus, and tested its responses to the presence of "foreign" eggs of varying quantity, colors, and uniformity. Using reflectance spectra of egg background coloration and avian perceptual modeling, we then estimated the sensory thresholds triggering egg rejection by this host for each treatment. As previously reported, rejection rates were positively related to the perceptual distance between own and foreign eggs in the nests in all treatments. However, rejection thresholds were more permissive (error prone) both with greater proportions of foreign eggs per clutch and/or when the suite of foreign eggs was perceptually more variable within the nest. These results suggest that parasites, through multiple parasitism, can partially overcome the evolution of hosts' recognition of mimetic parasite eggs. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Unusual Nests in the Great Reed Warbler Acrocephalus arundinaceus: Reuse of Nests in the Same Season and Unusual Forms of Nest Construction

Although previous studies have reported on rare nesting behaviour such as nest reuse and/or unusual nest construction in open nesting passerines, very little is known about these rare phenomena. We found and monitored 1224 Great Reed Warbler nests between 2008 and 2017. In three nests, we found reuse of nests in the same season; two nests were reused after a Cuckoo chick had disappeared and one nest was reused after a damaged egg had been ejected. In two other nests, we observed that replacement nests were built directly under the original nest, using the same reed stems. These observations suggest that there may be cases when breeding Great Reed Warblers benefit from performing unusual yet adaptive behaviour, such as conserving energy and time when nests are reused, or recognizing the advantages of original nests, e.g. they may serve as cover and/or concealment.

Does host-absent vocalisation of common cuckoo chicks increase hosts’ food provisioning behaviour?

Parent-absent vocalisation is produced by nestlings of several bird families when the parents are away from the nest. An analogous behaviour, host-absent vocalisation, has been found in some species of avian brood parasites and there are several explanations why this behaviour could have evolved. Using playback experiments, we examined whether polygynous great reed warblers (Acrocephalus arundinaceus) adjust their food provisioning behaviour in response to host-absent begging vocalisation uttered by the common cuckoo (Cuculus canorus) chicks. We found that both on monogamous and polygynous nests, host pair members responded to the broadcasted parasite begging signals by increasing their feeding rates; yet, they did not deliver larger volumes of food as a consequence of somewhat smaller prey brought per visit. Nevertheless, we propose that host-absent vocalisation of the common cuckoo chick may still represent a signal of hunger that may compensate for other, deficient components of parasite begging display. However, the efficiency of this signal may be limited by the foster parents’ provisioning abilities and local prey availability. Bird chicks beg loudly for food when their parents are at the nest, as well as when the parents search for food elsewhere. The same applies to young parasites raised by their hosts. Experimental playback of host-absent begging calls of cuckoo chicks increased great reed warbler feeding frequency but had no effect on the volume of food delivered. Host-absent vocalisation may represent signal of hunger; however, its effectiveness may be limited by provisioning abilities of the hosts.

Genetic differentiation between insular and continental populations of migratory and resident warblers, the Great Reed Warbler Acrocephalus arundinaceus and Cetti’s Warbler Cettia cetti

Island populations are frequently smaller than continental populations, have lower genetic diversity, are more inbred and show genetic differentiation from the mainland ones. However, sufficient numbers of immigrants may reduce the differentiation of insular populations and moderate the effects of genetic drift. In this study, we compared insular and continental populations of Cetti’s Warbler Cettia cetti and Great Reed Warbler Acrocephalus arundinaceus. We assessed the degree of genetic differentiation between insular and continental birds, their demographic history and genetic diversity. We compared the results, taking into account the differences in migration strategy and morphology of the two warblers. We found slightly lower genetic diversity in the insular populations than in the continental birds, possibly because of the lower population size or reduced immigration. The genetic differentiation between island and mainland birds was low, but higher in Cetti’s Warbler than in the Great Reed Warbler, suggesting differences in the species’ capability of crossing the sea. We found evidence for a past bottleneck in both the insular and continental populations of Cetti’s Warbler, while for the Great Reed Warbler we found no signs of past population reductions. A high dispersal capability of the Great Reed Warbler may have allowed high gene flow, which may explain the observed interspecific differences in the demographic history of the Great Reed Warbler and Cetti’s Warbler.

Timing of natal nests is an important factor affecting return rates of juvenile Great Reed Warblers

In many bird species, return rates tend to be greatest in the vicinity of their hatching site, thus natal philopatry is an important trait in avian demography. In the present study, we examined how conditions in natal nests affect return rates and dispersal distances of a long-distance migratory passerine. From 2008 to 2016, we studied a colour-ringed population of the Great Reed Warbler Acrocephalus arundinaceus breeding in southeastern Czech Republic. Almost 10% of the 1430 ringed individuals returned to the study site within 2 years after fledging. Birds originating from earlier broods were more likely to return than birds from delayed broods. The variation in local dispersal distances was explained by the sex of the recruits: males dispersed shorter distances than females. There was no evidence that return rates and dispersal distances in Great Reed Warbler are related to other factors closely linked to the condition in the natal nest, namely brood social status and brood size. While the higher return rates of the juveniles from earlier broods is in concordance with the findings of other studies, the sex-related difference in dispersal distances does not correspond with what was found in Great Reed Warbler juveniles in other studies and adults at the same study site. Our results show that early breeding produces more local recruits and that sex-biased dispersal may be site specific.

Does conspicuous colouration of Magpies Pica pica influence aggressive behaviour in nesting Great Reed Warblers Acrocephalus arundinaceus?

ABSTRACTCapsule: Using dummy experiments at nests of Great Reed Warblers Acrocephalus arundinaceus, we found no evidence to support the hypothesis that the contrasting black and white plumage colouration in Magpies Pica pica has evolved to reduce mobbing defensive attacks by potential prey species. We suggest instead that it might have evolved in the context of sexual selection or as a signal for flock formation.

Inability to regain normal body mass despite extensive refuelling in great reed warblers following the trans-Sahara crossing during spring migration

Migratory birds wintering in Africa face the challenge of passing the Sahara desert with few opportunities to forage. During spring migration birds thus arrive in the Mediterranean area with very low energy reserves after crossing the desert. Since early arrival to the breeding grounds often is of importance to maximize reproductive success, finding stopover sites with good refuelling possibilities after the Saharan passage is of utmost importance. Here we report on extensive fuelling in the great reed warbler Acrocephalus arundinaceus on the south coast of Crete in spring, the first land that they encounter after crossing the Sahara desert and the Mediterranean Sea in this area. Birds were studied at a river mouth and due to an exceptional high recapture rate (45 and 51% in two successive years), we were able to get information about stopover behaviour in 56 individual great reed warblers during two spring seasons. The large proportion of trapped great reed warbler compared to other species and the large number of recaptures suggest that great reed warblers actively choose this area for stopover. They stayed on average 3–4 d, increased on average about 3.5 g in body mass and the average rate of body mass increase was 4.8% of lean body mass d–¹. Wing length affected the rate of increase and indicated that females have a slower increase than males. The results found show that great reed warblers at this site regularly deposit larger fuel loads than needed for one continued flight stage. The low body mass found in great reed warblers (also in birds with high fat scores) is a strong indication that birds staging at Anapodaris still had not been able to rebuild their structural tissue after the strenuous Sahara crossing, suggesting that rebuilding structural tissue may take longer time than previously thought.

Annual dynamics, nature-conservancy evaluation and an overview of data on the occurrence of waterbirds at Šaleška Lakes (N Slovenia)

Abstract Between September 2014 and August 2015, 32 systematic surveys of waterbirds were carried out in 10-day periods in the area of Šaleška Lakes. The main objectives of the surveys were to establish the birds’species structure and abundance as well as to assess the area’s significance for their breeding, migration and overwintering. A total of 8927 individuals belonging to 53 species were recorded. Most species were observed at the end of March (27), whereas the highest number of individuals were registered in early January (535). The lowest number of individuals were counted in May (73). Mallard Anas platyrhynchos, Great Crested Grebe Podiceps cristatus, Mute Swan Cygnus olor and Grey Heron Ardea cinerea were observed during all counts. An additional 10 species were registered in at least 50% of the 10-day periods. The highest number of individuals (2891) and species (34) were counted at Lake Šoštanj, the highest number of individuals (295) and species (21) in a single 10-day survey period at Lake Gaberke. Most nestlings and nesting species were recorded at Lake Velenje. The majority of species occurred on all lakes within the research area, while some of them frequented certain lakes only exceptionally. The lowest numbers of birds were observed in central parts of all major waterbodies, particularly at Lake Velenje. Distribution of waterbirds at Šaleška Lakes is greatly influenced by the presence and distribution of people along them, as well as by the depth and riparian vegetation of the lakes and number of suitable roosting places. The number of breeding species is higher than in the 1994-2003 period. Little Bittern Ixobrychus minutus, Kingfisher Alcedo atthis and Great Reed Warbler Acrocephalus arundinaceus are new breeders for the area. The most abundant breeder is the Mallard with up to 16 pairs. The area’s breeders of the greatest conservation concern are the Little Bittern, Great Reed Warbler and Reed Warbler Acrocephalus scirpaceus, with the latter boasting the greatest share of the national breeding population (5%) in the area. Generally, most species occur during the migration season, although none of them in the numbers of conservation concern. Among the 33 waterbird species observed during the International Waterbird Censuses, five were recorded in all 18 years. Pochard Aythya ferina and Coot Fulica atra are of the greatest conservation concern, whereas the greatest share of the national population is reached by the Coot and Great Crested Grebe. In spite of the fact that the numbers of overwintering waterbirds in Slovenia are increasing, their numbers are declining at Šaleška Lakes. Among the 13 species recorded during more than ten International Waterbird Counts in the area researched, eight species are experiencing negative trends.

Brood parasite and host eggshells undergo similar levels of decalcification during embryonic development

AbstractCommon cuckoos Cuculus canorus are obligate brood parasites that lay their eggs in the nests of other (host) species. To increase the likelihood of successful parasitism, common cuckoos lay eggs with thicker and structurally stronger eggshells than those of their hosts and non‐parasitic relatives. Although hatching from thicker eggshells requires greater effort and may impose physiological costs on cuckoo embryos during hatching, it is unclear whether cuckoo eggshells are indeed thicker at the time of hatching. This is because avian embryos decalcify the innermost eggshell layer (mammillary layer) for organ development during embryogenesis, reducing eggshell thickness and making hatching easier. Therefore, common cuckoo eggshells may undergo a greater degree of decalcification during embryonic development to facilitate hatching from an initially thicker shelled egg. We used scanning electron microscopy to test this hypothesis by comparing the thickness and degree of decalcification of eggshells collected either before incubation or after hatching. We found that cuckoo eggshells undergo similar degrees of decalcification during embryonic development as the thinner eggshells of a host that lays similarly sized eggs, the great reed warbler Acrocephalus arundinaceus. Cuckoo eggshells hence remain thicker than eggshells of this host throughout embryogenesis, supporting the predicted trade‐off between the benefits of laying puncture‐resistant eggs and the physiological costs associated with hatching from thick shelled eggs.

Cross‐continental migratory connectivity and spatiotemporal migratory patterns in the great reed warbler

Migratory connectivity describes to which degree different breeding populations have distinct (non‐overlapping) non‐breeding sites. Uncovering the level of migratory connectivity is crucial for effective conservation actions and for understanding of the evolution of local adaptations and migratory routes. Here we investigate migration patterns in a passerine bird, the great reed warbler Acrocephalus arundinaceus, over its wide Western Palearctic breeding range using geolocators from Spain, Sweden, Czech Republic, Bulgaria and Turkey. We found moderate migratory connectivity: a highly significant spatial structure in the connections between breeding and sub‐Saharan non‐breeding grounds, but at the same time a partial overlap between individual populations, particularly along the Gulf of Guinea where the majority of birds from the Spanish, Swedish and Czech populations spent their non‐breeding period. The post‐breeding migration routes were similar in direction and rather parallel for the five populations. Birds from Turkey showed the most distinctive migratory routes and sub‐Saharan non‐breeding range, with a post‐breeding migration to east Africa and, together with birds from Bulgaria, a previously unknown pre‐breeding migration over the Arabian Peninsula indicating counter‐clockwise loop migration. The distances between breeding and sub‐Saharan non‐breeding sites, as well as between first and final sub‐Saharan non‐breeding sites, differed among populations. However, the total speed of migration did not differ significantly between populations; neither during post‐breeding migration in autumn, nor pre‐breeding migration in spring. There was also no significant relationship between the total speed of migration and distance between breeding and non‐breeding sites (neither post‐ nor pre‐breeding) and, surprisingly, the total speed of migration generally did not differ significantly between post‐breeding and pre‐breeding migration. Future challenges include understanding whether non‐breeding environmental conditions may have influenced the differences in migratory patterns that we observed between populations, and to which extent non‐breeding habitat fluctuations and loss may affect population sizes of migrants.