Eurasian Reed Warbler Acrocephalus Scirpaceus Research Articles

Morphometric versus genetic variation in the Eurasian Reed Warbler Acrocephalus scirpaceus in Italy

ABSTRACT Capsule The Eurasian Reed Warbler Acrocephalus scirpaceus from breeding sites in Italy has little genetic variability in the mitochondrial cytochrome b gene but shows morphological variation suggesting isolation of some populations. Aims To investigate the morphometric and genetic trait variation of Eurasian Reed Warbler populations at a large temporal and spatial scale. Methods We analysed morphometric traits and the mitochondrial cytochrome b (cyt b) gene of samples collected over 25 years, from populations at seven sites across the Italian peninsula and islands. Results While we found no genetic differences, we detected significantly smaller body sizes in individuals breeding in Sicily and Sardinia compared to those from mainland Italy. There were also significant body size differences between two Sicilian populations: one larger breeding on the southern coast and the other smaller at an inner highland site. Conclusion The morphometric data, together with the site fidelity of individuals, are consistent with there being isolation of some populations. Specifically, our results are consistent with an example of polyphenism, morphological plasticity, in the populations of the main Italian islands, Sicily and Sardinia.

Animal tracing with sulfur isotopes: Spatial segregation and climate variability in Africa likely contribute to population trends of a migratory songbird.

Climatic conditions affect animals but range-wide impacts at the population level remain largely unknown, especially in migratory species. However, studying climate-population relationships is still challenging in small migrants due to a lack of efficient and cost-effective geographic tracking method. Spatial distribution patterns of environmental stable isotopes (so called 'isoscapes') generally overcome these limitations but none of the currently available isoscapes provide a substantial longitudinal gradient in species-rich sub-Saharan Africa. In this region, sulphur (δ34 S) has not been sufficiently explored on a larger scale. We developed a δ34 S isoscape to trace animal origins in sub-Saharan Africa by coupling known-origin samples from tracked migratory birds with continental remotely sensed environmental data building on environment-δ34 S relationships using a flexible machine learning technique. Furthermore, we link population-specific nonbreeding grounds with interannual climatic variation that might translate to breeding population trends. The predicted δ34 S isotopic map featured east-west and coast-to-inland isotopic gradients and was applied to predict nonbreeding grounds of three breeding populations of Eurasian Reed Warblers Acrocephalus scirpaceus with two distinct migratory phenotypes. Breeding populations as well as migratory phenotypes exhibited large-scale segregation within the African nonbreeding range. These regions also differed substantially in the interannual climatic variation, with higher interannual variability in the eastern part of the range during 2001-2012. Over the same period, the eastern European breeding population seemed to have experienced a more steep decline in population size. The link between migratory patterns and large-scale climatic variability appears important to better understand population trajectories in many declining migratory animals. We believe animal tracing using sulphur isotopes will facilitate these efforts and offers manifold ecological and forensic applications in the biodiversity hotspot of sub-Saharan Africa.

The European Tree Frog Hyla arborea as a Probable Reason for Nest Failure in the Eurasian Reed Warbler Acrocephalus scirpaceus

We describe a case in which occupation by the European Tree Frog Hyla arborea caused the nest failure of a pair of Eurasian Reed Warblers Acrocephalus scirpaceus. The frog was observed several times sitting on the reed warblers clutch, on the day when the third egg was laid, thereby preventing the parental birds from incubating. Although small frog species have been reported to rest during the day in bird nests (mainly empty ones), to our knowledge this is the first published report providing evidence for avian nest failure due to nest occupation by an amphibian species.

Water Vapour Conductance of Passerine Nest Walls

Avian incubation relies on maintenance of the appropriate temperature and humidity within the nest. Previous studies suggested that conductance of the nest wall to water vapour was relatively low, albeit greater than the conductance of the eggshell, but no empirical studies confirmed this. In this study hygrometer loggers were used to measure water vapour inside and outside nest cups to determine conductance of the nest wall. Average water vapour conductance values for nine species of passerine ranged from 479 mgH2O·day-1·Torr-1 for Common Chaffinch Fringilla coelebs nests to 712 mgH2O·d-1·Torr-1 for Eurasian Reed Warbler Acrocephalus scirpaceus nests. Generally, water vapour conductance values were two orders of magnitude higher than that of published values for the eggshell for these species, although humidity in the nest cup were comparable to published data for passerine nests. These results indicate that in the study species the nest wall was not an effective barrier to water vapour loss. Reconsideration of high values for nest wall conductance, nest attentiveness and sources of water to create humidity, have led to a new approach that deals with nest humidity as part of the water budget during the incubation period. Over time the amount of water vapour required to raise nest humidity far exceeds that released by the eggs. It is proposed that nest humidity is driven by cutaneous water loss from the incubating bird, rather than water vapour lost from the eggs, in order to counter loss of humidity through the nest wall and during incubation recesses.

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.

Factors Shaping Breeding Phenology in Birds: An Assessment of Two Sympatric Acrocephalus Warblers with Different Life Histories

Timing of reproduction is a critical life-history trait and implies major fitness consequences. Differences in life history strategies can lead to different responses to the same environmental conditions even among closely related species. Investigating the factors affecting the timing of reproduction in closely related and sympatric species may help understanding the relationship between different life history traits and species-specific responses to ecological factors. We investigated the effects of season (photoperiod), local climatic variability and prey abundance on the breeding season of two sympatric insectivorous birds that differ in the timing of reproduction and moult and have different migration strategies: the Moustached Warbler Acrocephalus melanopogon and the Eurasian Reed-warbler Acrocephalus scirpaceus. During a five-year period, breeding phenology was determined by examining bird incubation patches during standardised captures and food availability was assessed by invertebrate sampling. Time, mean temperatures and the abundance of some prey taxa were inter-correlated and all contributed to influence the breeding phenology of both species according to partial least squares regression analysis. Besides some interspecific differences in the effect of environmental factors on the breeding phenology, we found strong similarities between the patterns observed in the two warblers. This suggests that, in spite of interspecific differences in life history, the ecological mechanisms shaping reproduction phenology are similar in the two warblers and the same environmental factors are involved in determining the timing of their reproduction.—Ceresa, F., Belda, E.J., Brambilla, M., Gómez, J., Mompó, C. & Monrós, J.S. (2020). Factors shaping breeding phenology in birds: an assessment of two sympatric Acrocephalus warblers with different life histories. Ardeola, 67: 371-385.

First record of Common Blackbird Turdus merula nests built in a reedbed

ABSTRACT In May 2020, during our study on the breeding ecology of the Eurasian Reed Warbler Acrocephalus scirpaceus, we found two nests of Common Blackbird built within an unmanaged reedbed. The distance between the nests and dry land, overgrown by deciduous forest, was about 25 m. One of the nesting attempts was successful while the second was deserted during the incubation stage. To our knowledge this is the first record of Blackbirds nesting in reeds. This observation supports the notion of great flexibility of nest-site selection in this species. We conclude that Blackbirds use reedbeds as a nesting site more often, but such nests may be easily overlooked.

Wintering areas on the move in the face of warmer winters

For several decades, there have been changes in the distance of bird migration. The Eurasian Reed Warbler Acrocephalus scirpaceus is a Trans-Saharan migratory species, which, as recent records suggest, is increasingly wintering in the Iberian Peninsula. This study investigated the strength of the influence of climate on the wintering of this insectivorous species in the Iberian Peninsula, and also used geospatial modelling to identify the most favourable areas. The model was constructed to identify the most relevant explanatory factors of this distribution and it was assessed by applying calibration and discrimination criteria, and the influence of each factor was analysed using variation partitioning. The minimum winter temperature is the most relevant factor affecting sites suitable for the wintering of this species in Europe.

Breeding ground correlates of the distribution and decline of the Common Cuckoo Cuculus canorus at two spatial scales

Many migratory bird species are undergoing population declines as a result of potentially multiple, interacting mechanisms. Understanding the environmental associations of spatial variation in population change can help tease out the likely mechanisms involved. Common Cuckoo Cuculus canorus populations have declined by 69% in England but increased by 33% in Scotland. The declines have mainly occurred in lowland agricultural landscapes, but their mechanisms are unknown. At both the local scale within the county of Devon (SE England) and at the national (UK) scale, we analysed the breeding season distribution of Cuckoos in relation to habitat variation, the abundance of host species and the abundance of moth species whose caterpillars are a key food of adult Cuckoos. At the local scale, we found that Cuckoos were more likely to be detected in areas with more semi‐natural habitat, more Meadow Pipits Anthus pratensis (but fewer Dunnocks Prunella modularis) and where, later in the summer, higher numbers of moths were captured whose larvae are Cuckoo prey. Nationally, Cuckoos have become more associated with upland heath characterized by the presence of Meadow Pipit hosts, and with wetland habitats occupied by Eurasian Reed Warbler Acrocephalus scirpaceus hosts. The core distribution of Cuckoos has shifted from south to north within the UK. By the end of 2009, the abundance of macro‐moth species identified as prey had also declined four times faster than that of species not known to be taken by Cuckoos. The abundance of these moths has shown the sharpest declines in grassland, arable and woodland habitats and has increased in semi‐natural habitats (heaths and rough grassland). Our study suggests that Cuckoos are likely to remain a very scarce bird in lowland agricultural landscapes without large‐scale changes in agricultural practices.

Conditions at autumn stopover sites affect survival of a migratory passerine

Weather is an important factor affecting many aspects of avian ecology, yet its importance for survival during various periods of the avian annual cycle has received relatively little attention and remains poorly understood. We have investigated the effect of weather conditions at the breeding and wintering grounds and during migration on the survival probability of Eurasian Reed Warblers Acrocephalus scirpaceus, a long-distance Palaearctic-African migrant species. We found that annual survival was significantly affected by precipitation at the autumn stopover sites in Spain and Morocco, where Reed Warblers accumulate energy reserves prior to crossing the Sahara desert: low rainfall was associated with decreased survival. We suggest that arid conditions at these crucial stopover sites may cause limitations in food availability, prevent proper refueling and hence result in lower survival. Survival estimates were not related to weather conditions in Africa during the bird wintering period or the preceding wet season. Likewise, meteorological conditions at the breeding area did not influence survival. Survival estimates for males and females did not differ, although recapture probability was significantly lower for females than for males. Our results indicate that weather significantly influences Reed Warbler survival, although its effect may vary for different stages of the annual cycle. Our findings suggest that marked climatic changes occurring along migratory routes, in particular at important stopover sites, may have far-reaching consequences on bird survival and population size.

The Effects of Supplementary Food on the Breeding Performance of Eurasian Reed Warblers Acrocephalus scirpaceus; Implications for Climate Change Impacts.

Understanding the mechanisms by which climate variation can drive population changes requires information linking climate, local conditions, trophic resources, behaviour and demography. Climate change alters the seasonal pattern of emergence and abundance of invertebrate populations, which may have important consequences for the breeding performance and population change of insectivorous birds. In this study, we examine the role of food availability in driving behavioural changes in an insectivorous migratory songbird; the Eurasian reed warbler Acrocephalus scirpaceus. We use a feeding experiment to examine the effect of increased food supply on different components of breeding behaviour and first-brood productivity, over three breeding seasons (2012–2014). Reed warblers respond to food-supplementation by advancing their laying date by up to 5.6 days. Incubation periods are shorter in supplemented groups during the warmest mean spring temperatures. Nestling growth rates are increased in nests provisioned by supplemented parents. In addition, nest predation is reduced, possibly because supplemented adults spend more time at the nest and faster nestling growth reduces the period of vulnerability of eggs and nestlings to predators (and brood parasites). The net effect of these changes is to advance the fledging completion date and to increase the overall productivity of the first brood for supplemented birds. European populations of reed warblers are currently increasing; our results suggest that advancing spring phenology, leading to increased food availability early in the breeding season, could account for this change by facilitating higher productivity. Furthermore, the earlier brood completion potentially allows multiple breeding attempts. This study identifies the likely trophic and behavioural mechanisms by which climate-driven changes in invertebrate phenology and abundance may lead to changes in breeding phenology, nest survival and net reproductive performance of insectivorous birds.

The pattern of distribution and interaction of metals and calcium in eggshells and egg contents in relation to the embryonic development of eggs in a small passerine bird

Differences in element concentrations have been observed between embryonated and non-embryonated eggs of the Eurasian Reed Warbler Acrocephalus scirpaceus, presumably due to the resorption of calcium and other elements from the shell. To investigate this difference, we tested the hypotheses that (1) the distribution of elements in eggshells and egg contents, (2) the individual element–element relationships between shells and contents, and (3) the relationships among the elements in eggshells and egg contents vary between the two egg samples. Based on the levels of 11 elements (Cr, Cu, Ni, Cd, Pb, Mn, Fe, Co, Zn, Mg and Ca) in Reed Warbler eggs, our results confirmed the second and third hypotheses. Importantly, we found that embryonic development tended to blur the lines between two types of relationships: (1) between elements in eggshells and egg contents through an increase in the number of significant correlations, some of which may be enhanced by high levels of certain elements (Cd, Co, Pb, Zn are indicative of trace metal pollution in fish ponds); (2) among the various elements in eggshells and egg contents, as demonstrated by the differential grouping of elements in principal components analysis (PCA). The consistent positive direction of relationships for almost all elements in both the shells and contents of eggs is presumed to result from the initial parallel co-sequestration of all the elements which accompanies intensive calcium transfer from the female body into the incipient egg. Our study highlights the fact that only limited inferences can be drawn from the interactions of metals and calcium in eggshells and egg contents if the fertility status of the eggs is unknown.

Mitochondrial phylogeny of the Eurasian/African reed warbler complex (Acrocephalus, Aves). Disagreement between morphological and molecular evidence and cryptic divergence: A case for resurrecting Calamoherpe ambigua Brehm 1857

A tree based on the mitochondrial cyt b gene for 278 samples from throughout the range of the Eurasian Reed Warbler Acrocephalus scirpaceus – African Reed Warbler A. baeticatus complex shows well-supported geographically structured divergence for eight distinct lineages. The phylogenetic structuring together with the clarification of priority, provided by sequence data from seven type specimens, suggests that both taxonomy and distribution boundaries are in need of revision. The Iberian and Moroccan populations form a well-supported clade, and we propose that these are treated as taxonomically distinct, under the name ambiguus (Brehm, 1857). We propose that the names scirpaceus, fuscus, avicenniae, ambiguus, minor, cinnamomeus, hallae and baeticatus are used for the well supported clades in the complex, which we recommend to treat as one polytypic species, A. scirpaceus, pending studies of gene flow and assortative mating in the contact zones.

Similar dispersal patterns between two closely related birds with contrasting migration strategies

AbstractStudying dispersal is crucial to understand metapopulation and sink‐source dynamics and invasion processes. The capability to disperse is especially important for species living in fragmented habitats like wetlands. We investigated the distribution of natal and breeding dispersal distances and philopatry in Spanish populations of two closely related reedbed‐nesting birds, the Moustached Warbler Acrocephalus melanopogon and the Eurasian Reed Warbler Acrocephalus scirpaceus. These warblers are morphologically very similar, but differ in migration strategy and, in our study area, in population size. Our aims were to find the best model for dispersal distances and to assess the occurrence of intra‐ or interspecific differences in dispersal patterns. We used ringing data from the Spanish marking scheme and selected recaptures to avoid including migrating individuals. In both species, most individuals were philopatric but dispersing birds were able to cross large distances (up to more than 100 km), suggesting the capability to compensate for habitat fragmentation. We found the heavy‐tailed Cauchy distribution to be the best conceptual description for our data, in all cases but natal dispersal of Moustached Warblers. Among Eurasian Reed Warblers, natal philopatry was lower than breeding philopatry. We found no significant interspecific differences. This does not confirm the hypothesis of higher dispersal ability in long distance migrants (like Eurasian Reed Warblers) than in resident/short distance migrant bird species (like Moustached Warblers). The similarity in dispersal patterns among the two warblers may be explained by their close phylogenetic relatedness, similar constraints imposed on both species by a patchy habitat or similar evolutionary pressures.

Dependence of clutch predation rate of Eurasian reed warbler Acrocephalus scirpaceus on nesting site selection: a model study

AbstractThe article is focused on describing a possible way of how biological data can be processed, introduces the procedures used and compares the results gained from a field ornithological study. This was carried out using statistical methods of nonparametric regression with binomial classification and probit function together with the method with forward selection, and presents the most significant outcomes. Some of the interesting findings of this study are as follows: first, a lower level of reed warbler (

Shell thinning due to embryo development in eggs of a small passerine bird

Over four study years we have investigated hatching success and failure, the presence of an embryo and other egg biometrics, and eggshell thickness of the Eurasian Reed Warbler Acrocephalus scirpaceus. Of 1354 eggs from 350 nests, 940 (69.4 %) survived until hatching, of which 159 (16.9 %) were incubated but failed to hatch, while 273 (20.2 % of all) were in nests destroyed during incubation. Of 161 intact unhatched eggs from 97 nests, 64 (39.8 %) had no visible embryo and 88 (54.7 %) had a visible embryo. Eggs without visible embryos were present in 42.7 % of normally incubated clutches; among deserted clutches over 50 % had no visible embryo. Eggshell thickness at the egg equator (the widest part) was negatively correlated with clutch size and the presence of an embryo. A similar relationship was found for the blunt end of the eggs but was not statistically significant. Shells at the equator of eggs with visible embryos were significantly thinner (on average 8.0 % less) than the shells of eggs without embryos. Our study clearly implies that shell thickness decreases in the course of embryonic development. It also indicates that comparison of eggshell thickness data without controlling for the presence of an embryo in eggs can produce biased results, and may, therefore, fail to identify the actual causes of eggshell thinning. Our findings also suggest that shell thickness at the blunt end is a reliable estimator of shell thickness, irrespective of embryonic development.

Does fragmentation of wetlands affect gene flow in sympatric Acrocephalus warblers with different migration strategies?

Wetlands are naturally patchy habitats, but patchiness has been accentuated by the extensive wetlands loss due to human activities. In such a fragmented habitat, dispersal ability is especially important to maintain gene flow between populations. Here we studied population structure, genetic diversity and demographic history of Iberian and North African populations of two wetland passerines, the Eurasian reed warbler Acrocephalus scirpaceus and the moustached warbler Acrocephalus melanopogon. These species are closely related and sympatric in our study sites, but the reed warbler is a widespread long‐distance migrant while the moustached warbler's breeding range is patchier and it is resident or migrates over short distances. Using microsatellite and mtDNA data, we found higher differentiation in moustached than in reed warblers, indicating higher dispersal capability of the latter species. Our results also suggest that the sea limits dispersal in the moustached warbler. However, we found evidence of gene flow between the study sites in both species, indicating a capability to compensate for habitat fragmentation. In most cases, the gene flow was restricted, possibly because of the large distances between study sites (from ca 290 to 960 km) or breeding site fidelity. The reed warbler had higher haplotype diversity, likely due to dispersal from different populations, past admixture event and a larger population size. We found also signs of postglacial population growth for both species and evidence of a recent colonization or re‐colonization of the Mallorca Island by the moustached warbler.

Mitochondrial phylogeography of the Eurasian Reed Warbler Acrocephalus scirpaceus and the first genetic record of A. s. fuscus in Central Europe

The phylogeography and evolutionary history of the long‐distance migratory Eurasian Reed Warbler Acrocephalus scirpaceus were analysed using sequence data from the mitochondrial DNA COI gene of 405 individuals and cyt b sequences from a subset of those individuals from 20 breeding, migrating and wintering populations over its wide geographical range. Median‐joining network and Bayesian phylogenetic analyses recovered three lineages corresponding to three subspecies of A. scirpaceus that started to diverge approximately 480 000 years ago: one spanning Asia (Acrocephalus scirpaceus fuscus), one encompassing Europe and Northern Africa (Acrocephalus scirpaceus scirpaceus) and a third that included Eastern Africa and Southwestern Asia (Acrocephalus scirpaceus avicenniae). Phylogenetic analyses suggest that the third clade is basal and diverged from its sister species, the African Reed Warbler Acrocephalus baeticatus, around 700 000 years ago. This earliest subspecies (avicenniae) may have survived the Last Glacial Maximum in one of the African refugia, probably the low forest refugium of the Ethiopian Highlands. Sequences of 348 samples from Germany, which were captured for ringing, showed that one bird was genetically assigned to the subspecies fuscus, the first genetic evidence of this Asian subspecies in Central Europe. According to our DNA sequences, 6.8% of Eurasian Reed Warblers were misidentified at the species level, Marsh Warbler Acrocephalus palustris accounting for most of these misidentified specimens. At the subspecies level, of 22 fuscus identified by genetic analyses, only six had been correctly named by the ringers.

Mitochondrial evidence for genetic diversity and low phylogeographic differentiation in the Marsh Warbler Acrocephalus palustris (Aves: Acrocephalidae)

We analyzed the levels of genetic variability in a long-distance migratory reed warbler, the Marsh Warbler Acrocephalus palustris, by using nucleotide sequences of the mitochondrial cytochrome c oxidase subunit I gene (COI; 611 nucleotides [nt]). We obtained sequences from 229 individuals from ten sampling sites that include breeding, wintering, and migrating birds. Overall, 44 haplotypes were detected, which reflect high levels of genetic variation in this species, but most of this variation corresponds to individual differences within collecting sites. We also analyzed 829 nt of cytochrome b (cyt b) from 49 selected individuals of different sampling sites to evaluate the reliability of the COI results. Our analyses based on both mtDNA loci could not detect any population subdivision or phylogeographic structure, indicating high levels of gene flow between breeding sites (Nm = 13.69). The split between the Marsh Warbler and its sister species, the Eurasian Reed Warbler Acrocephalus scirpaceus, could be dated for the Lower Pliocene (about 3.8 million years ago). The time to the most recent common ancestor (TMRCA) among Marsh Warbler haplotypes was estimated as 0.45 million years, indicating their bottleneck during the last glacial periods. Low nucleotide diversity, a shallow phylogenetic tree, a star-like haplotype network, and a unimodal mismatch distribution point to a sudden increase of the effective population size (probably after the last glaciation period) and a recent range expansion likely from a single refuge.

Connectivity in Reed WarblersAcrocephalus scirpaceusBetween Breeding Grounds in Europe and Autumn Stopover Sites in Iberia

We determined the degree of connectivity in the Eurasian Reed Warbler Acrocephalus scirpaceus between breeding regions in central and northern Europe and stopover sites in Iberia, during the autumn migration. We used both recovery data and wing length analyses to achieve this. Biometric data were obtained during 2009 at 11 sampling localities in Iberia, which we amalgamated into four major migratory areas within the region (Central, Western, Eastern and Southwestern Iberia, hereafter CI, WI, EI, SW). From the EURING database, we selected birds captured at their breeding sites in Europe and recaptured in Iberia during the autumn migration. Reed Warblers passing through WI had shorter wings than those in CI, EI and SW, suggesting that birds in WI migrated shorter distances, a fact also supported by recovery data. Although Reed Warblers showed some population overlap when passing through Iberia, we found that birds passing through EI and CI came from areas further to the east (continental Europe) than those passing through WI (mainly British Isles), thus supporting parallel migration. Reed Warblers tended to converge in southwestern Iberia, suggesting an effect of nearby geographical barriers.