Reed Warbler Research Articles

Migratory birds can extract positional information from magnetic inclination and magnetic declination alone.

Migratory birds are able to navigate over great distances with remarkable accuracy. The mechanism they use to achieve this feat is thought to involve two distinct steps: locating their position (the 'map') and heading towards the direction determined (the 'compass'). For decades, this map-and-compass concept has shaped our perception of navigation in animals, although the nature of the map remains debated. However, some recent studies suggest the involvement of the Earth's magnetic field in the map step. Here, we tested whether migratory songbirds, Eurasian reed warblers (Acrocephalus scirpaceus), can determine their position based on two magnetic field components that are also associated with direction finding, i.e. magnetic inclination and magnetic declination. During a virtual magnetic displacement experiment, the birds were exposed to altered magnetic inclination and magnetic declination values that would indicate a displacement from their natural migratory corridor, but the total intensity of the field remained unchanged, creating a spatial mismatch between these components. The response was a change in the birds' migratory direction consistent with a compensatory re-orientation. This suggests that birds can extract positional as well as directional information from these cues, even when they are in conflict with another component of the magnetic field. It remains to be seen whether birds use the total intensity of Earth's magnetic field for navigation.

Common cuckoo eggs are more resistant to puncture by the host

The puncture resistance hypothesis suggests that thick-shelled eggs of parasitic birds can resist puncture-ejection by the host. However, few experiments have yet been conducted to test this hypothesis in terms of natural host behavior (e.g., pecking at foreign eggs). To explore whether the eggshells of common cuckoos (Cuculus canorus) are resistant to puncture-ejection by their common hosts, Oriental reed warblers (Acrocephalus orientalis), we designed experiments to investigate if and how breeding Oriental reed warblers peck at foreign eggs that includes common cuckoo, Oriental reed warbler and budgerigar (Melopsittacus undulatus) eggs. The results showed that, given the same frequency of egg pecking, the probability of eggshell breakage was 87.5% for eggs of the Oriental reed warbler and 0% for eggs of the common cuckoo, with a significant difference (P = 0.001, Fisher's exact test). Our study shows clearly that common cuckoos' eggshells are less susceptible to puncture-ejection than those of Oriental reed warblers and budgerigars. This indicates that the eggshells of common cuckoos can resist host Oriental reed warblers' puncture-ejection, supporting the puncture resistance hypothesis.

The prenatal assimilation of minerals and metals in nestlings of a small passerine bird

Although certain metals can reach their highest concentrations in tissues of newly hatched nestlings, their sources have yet to be fully elucidated. Evidence for the differentiation of body reserves, including some chemical elements, during avian embryonic life and early post-hatching period is limited mostly to studies on poultry. Here, we present novel findings on the bioaccumulation and egg-to-nestling transfer of 10 minerals and metals, including non-essential potentially toxic chemical elements (Cu, Ni Cd, Pb, Zn, Ag, Mg, Fe, Co and Ca) in the tissues of the gastrointestinal tract (GT), whole body (WB) and carcass (CA) of nestlings of a small passerine bird, the Eurasian Reed Warbler Acrocephalus scirpaceus, breeding in an intensively fertilized fishpond habitat. Our findings show that concentrations of metals that are relevant to contaminated systems (Cu, Ni, Cd, Pb, Zn, Fe and Co) are the highest in GT tissues across all the nestling age classes examined (from hatching to day 9 of life). The important question emerged from our findings: why the hatchlings were characterized by higher metal concentrations in GT, WB and CA compared to older nestlings. We suggest that the prenatal accumulation of chemical elements assimilated from egg contents, and their subsequent utilization for building various tissues/internal organs by the growing embryo, is a reasonable explanation for this finding. More specifically, the potential functional mechanism explaining the highest concentrations of some trace metals in hatchlings seems to be associated with an extremely high rate of utilization of residual yolk metal resources that survived from the embryonic until the post-natal period. On the basis of our own work and literature data, we identified a presumable significant discrepancy in the actual rate of egg-to-nestling and egg-to-embryo metal transfer due to the eggshell being ignored (a commonly occurrence), and only the egg contents being treated as the basic repository of chemical elements for developing embryos. Much more research is needed on the prenatal assimilation and subsequent accumulation of chemical elements in a developing avian embryo, representing both the transition from near-to-hatching embryos to hatchlings, and then to older nestlings.

Untangling the colonization history of the Australo-Pacific reed warblers, one of the world's great island radiations.

Island radiations, such as those of the Australo-Pacific, offer unique insight into diversification, extinction, and early speciation processes. Yet, their speciation and colonization histories are often obscured by conflicting genomic signals from incomplete lineage sorting or hybridization. Here, we integrated mitogenomes and genome-wide SNPs to unravel the evolutionary history of one of the world's most geographically widespread island radiations. The Australo-Pacific reed warblers (Acrocephalus luscinius complex) are a speciose lineage including five species that have become extinct since the 19th century and ten additional species of conservation concern. The radiation spans over 10,000 km across Australo-Papua, Micronesia and Polynesia, including the Mariana, Hawaii and Pitcairn Island archipelagos. Earlier mtDNA studies suggested a stepping-stone colonization process, resulting in archipelago-level secondary sympatry of divergent mtDNA lineages in the Mariana Islands and Marquesas. These studies hypothesised that morphologically similar species on neighbouring islands arose from ecological convergence. Using hDNA from historical museum specimens and modern genetic samples, we show that incomplete lineage sorting and/or gene flow have shaped the radiation of Australo-Pacific reed warblers rather than secondary sympatry. The nuclear genome reconstructs a simpler biogeographic history than mtDNA, showing close relationships between species in the Mariana Islands and Marquesas despite their paraphyletic mtDNA lineages. Gene flow likely involved early and late colonizing waves of the radiation before the loss of ancestral dispersive ability. Our results highlight how collection genomics can elucidate evolutionary history and inform conservation efforts for threatened species.

Unravelling the link between productivity and climate for two sympatric Acrocephalus warblers across Spain

ABSTRACT Capsule An accumulated rainfall of around 100 mm favoured the productivity of the specialist Moustached Warbler Acrocephalus melanopogon, but temperature had a negative effect. By contrast, temperature had a positive effect on the productivity of the generalist Reed Warbler Acrocephalus scirpaceus, whereas rainfall had almost no effect, although productivity also peaked when rainfall approached 100 mm. Aims Understanding the extent to which climate and geography are driving the negative trend of Moustached Warbler productivity, and the stable trend of Reed Warbler productivity. Methods Using data from a breeding bird monitoring ringing scheme in Spain (PASER; 1995 – 2021), we investigated the effects of seasonal rainfall and temperature patterns on the productivity of two sympatric warblers: the resident/short-distance migrant Moustached Warbler (occurring at nine ringing stations from a total of 129), and the generalist long-distance migrant Reed Warbler (57 out of 129 ringing stations). Analyses were performed using multi-adaptive regression splines (MARS). Results Between 1995 and 2021 geographical and site-specific factors drove annual productivity for both species, combined with rainfall and temperature. For the Moustached Warbler, geographical and site-related variables, together with the temporal changes (year), were more important than rainfall and temperature. For the Reed Warbler, temperature was more important than year but less than site and longitude. Conclusion Warmer temperatures and a higher frequency of torrential rainfall events may compromise the conservation of the vulnerable Moustached Warbler in Spain due to the adverse effect on productivity. In contrast, increasing temperatures across Spain may benefit the widely distributed Reed Warbler. Thus, Moustached Warbler populations are likely to decrease, whereas Reed Warbler populations might increase. However, further research is needed to establish the impact of climate change on survival, and its implications for the population trends and dynamics of these two sympatric warblers.

Is syllable repetitions a song parameter important for male-male interactions in Eurasian reed warbler (Acrocephalus scirpaceus)?

Song complexity has been identified as one song characteristic important in female choice, whereas its role in male-male interactions is less clear and not much is known which song characteristics are specifically important for territorial defense. In Acrocephalus warblers, males seem to increase song complexity towards females but reduce it during territorial disputes. One possibility to reduce song complexity could be achieved by repeating individual syllables. In this context here we examined the importance of males repeating syllables in male-male interactions. We hypothesis that repeating syllables signals a males general fighting ability, aggressive status or willingness to attack. In a playback experiment we investigated the behavioral response of unmated, territorial Eurasian reed warbler (Acrocephalus scirpaceus) males towards two simultaneously singing intruders whereby the song of the two simulated intruders differs in the degree of syllable repetitions. The response of the territory owner was determined by using several behavioral parameters. Our results revealed that males approached faster and stayed significantly longer near the song with few syllable repetitions. However, the proportion of males approaching the song with high or low syllable repetitions first, as well as the minimal distance to which males approach to the two song types did not differ as well. Thus, the weaker response towards the high repetitive song playback suggests territorial males to be more intimidated by the aggressive nature of that song type.

Geographic distribution of feather δ34S in Europe

AbstractGeographic distribution models of environmentally stable isotopes (the so‐called “isoscapes”) are widely employed in animal ecology, and wildlife forensics and conservation. However, the application of isoscapes is limited to elements and regions for which the spatial patterns have been estimated. Here, we focused on the ubiquitous yet less commonly used stable sulfur isotopes (δ34S). To predict the European δ34S isoscape, we used 242 feather samples from Eurasian Reed Warbler (Acrocephalus scirpaceus) formed at 69 European wetland sites. We quantified the relationships between sample δ34S and environmental covariates using a random forest regression model and applied the model to predict the geographic distribution of δ34S. We also quantified within‐site variation in δ34S and complementarity with other isotopes on both individual and isoscape levels. The predicted feather δ34S isoscape shows only slight differences between the central and southern parts of Europe while the coastal regions were most enriched in 34S. The most important covariates of δ34S were distance to coastline, surface elevation, and atmospheric concentrations of SO2 gases. The absence of a systematic spatial pattern impedes the application of the δ34S isoscape, but high complementarity with other isoscapes advocates the combination of multiple isoscapes to increase the precision of animal tracing. Feather δ34S compositions showed considerable within‐site variation with highest values in inland parts of Europe, likely attributed to wetland anaerobic conditions and redox sensitivity of sulfur. The complex European geography and topography as well as using δ34S samples from wetlands may contribute to the absence of a systematic spatial gradient of δ34S values in Europe. We thus encourage future studies to focus on the geographic distribution of δ34S using tissues from diverse taxa collected in various habitats over large land masses in the world (i.e., Africa, South America, or East Asia).

Species-specific and age-related migration strategies of three Acrocephalus warblers along the eastern European–African flyway

The species-specific and age-related autumn migration strategies of the sedge, reed and great reed warbler were investigated at stopover sites between central Europe and north-eastern Africa during autumn. Adult reed warblers accumulated larger fat reserves than juveniles and consequently were able to cover longer distances from most stopover sites. Juvenile sedge warblers, on the other hand, left Europe with significantly larger fat reserves than adults as opposed to the populations migrating along the western route. Both juvenile and adult great reed warblers were potentially able to make long flights without refuelling from the northern part of the Mediterranean region onwards. It was the proximity of large ecological barriers that had the most prominent influence on the potential flight distances of adult and juvenile Acrocephalus warblers during their southward migration. In reed and sedge warblers, the differences in mean flight distances covered by adults and juveniles were the largest (ca. 600 km) just before the crossing of the Mediterranean Sea, but smaller (ca. 200 and 400 km, respectively) in front of the Sahara Desert. Juvenile and adult great reed warblers were potentially able to cover 1660 km from Turkey, which was also supported by very low numbers of individuals caught at the Egyptian ringing sites. The results of this study carried out in the key regions of the eastern European–African flyway documented that migration strategies differed between adults and juveniles of three long-distance migrant species, yet this variation depended on the migration stage and the proximity of natural ecological barriers.

Alternative mating strategies in Eurasian Reed Warblers: extra-pair paternity and egg dumping

Alternative mating strategies, involving mating outside a social pair bond, have been studied intensively during the last few decades. However, we are still far from understanding what factors affect their frequencies in wild bird populations. In 2006–2011 we studied Eurasian Reed Warblers breeding in the nature reserve “Stawy Milickie” in SW Poland, and analysed the frequency of extra-pair paternity (EPP) and intra-specific brood parasitism (IBP). We genotyped a total of 786 nestlings from 259 nests. EPP offspring constituted 13.40% of all young (from 7.63% to 22.82% in different study years), and they were found in 21.62% of the broods (from 14.6% to 32.7% annually). The mean proportion of females with EPP young was 24.2% (from 16.2 to 41.7% annually). The proportion of EPP young in nests containing any EPP offspring ranged from 25% to 100% (on average 60%) and was not related to brood size. EPP young in one nest were sired usually by one or two males (92% of all cases), but we recorded one brood with three EPP fathers and one brood of four, where each young was sired by a different EPP male, and none by a social male. We found only one case of intra-specific brood parasitism (IBP). The brood with IBP constituted 0.39% of all examined broods, while the nestling that hatched from the dumped egg constituted 0.13% of all young whose parents were genotyped. We conclude that extra-pair paternity is a relatively common and regularly occurring alternative mating strategy in our study population of Reed Warblers, while intra-specific brood parasitism is extremely rare.

Population status of the endemic Pitcairn Reed Warbler Acrocephalus vaughani on Pitcairn Island, South Pacific

Summary Reed warblers are widespread throughout Eurasia, Africa, and Australasia, and many species undertake long seasonal migrations. By contrast, other species of the genus Acrocephalus are sedentary and endemic to single oceanic islands. The Pitcairn Reed Warbler Acrocephalus vaughani is confined to the small volcanic island of Pitcairn in the South Pacific Ocean, and no population assessments have ever been conducted for this species. Due to its restricted range, the presence of invasive species, and the loss of natural habitat, the species is considered as globally “Endangered”, but its actual conservation status is entirely speculative. We conducted transect surveys and nest monitoring in the austral summer of 2022/23 and present abundance estimates for the species. We counted between 51 and 158 reed warblers along 54 transects that were each 100 m long and covered all habitats and roughly 13% of the vegetated island area. Using binomial mixture models accounting for imperfect detection and habitat variation in abundance, we estimated that Pitcairn may hold 1,568 (95% confidence interval 812–3,237) Pitcairn Reed Warblers, and that the species appeared to be most abundant in introduced Rose Apple Syzygium jambos stands. Based on the monitoring of 49 nests, of which only four failed to fledge any young, we estimated that Mayfield nest survival rate was 0.69 and estimated productivity was 1.07 (± 0.39 standard deviation) fledglings per nest. Assuming that Pitcairn Reed Warblers have similar annual survival probabilities as other island reed warblers, the productivity appears sufficient to maintain the population and there is no indication that the species has decreased significantly over the past three generations. Given the limited extent of occurrence, and the stable current population size between 442 and 2,774 mature individuals, we recommend that the global conservation status of the Pitcairn Reed Warbler be classified as “Vulnerable”.

Structural effects of reedbed grazing and its cessation on reed-nesting songbird densities

Reedbeds are crucial breeding habitats for vulnerable songbird species. Irrespective of their protection status, these habitats may be threatened by organic matter accumulation, progressively leading to structural homogenization and habitat succession towards woodland. Managers prevent excessive litter build-up with various interventions opening-up the reedbeds, such as grazing, which may be detrimental or suitable for some reed bird species. We assessed the effects of extensive grazing by horses and its cessation in the medium and long terms on reedbed structure, and the consequences on reed-nesting songbird densities in Estagnol Nature Reserve, a protected wetland near the French Mediterranean coast. We compared reedbed structural features between grazed, newly ungrazed and old-ungrazed plots. During nine years, we censused four songbird species in spring and collected water level data in the same survey plots. Grazing reduced reedbed extent, rejuvenated the vegetation with more short green stems on a thinner litter, and produced higher structural heterogeneity and discontinuity compared to long-lasting non grazing. Newly ungrazed plot showed intermediate effects. All surveyed songbird total densities were similar among plots while species numbers and densities differed. Grazed reedbed was more attractive to Moustached Warblers (Acrocephalus melanopogon) and Great Reed Warblers (Acrocephalus arundinaceus), likely due to the large edges and the high vegetation structural heterogeneity. However, Moustached Warblers were more negatively affected by higher water level in grazed reedbed, presumably because flooding prevents litter foraging. The newly ungrazed reedbed was not preferred by any species. All warbler species were found under low density in the old-ungrazed reedbed, where Reed Buntings (Emberiza schoeniclus) were exclusively found. Food availability related to thick litter layer may explain this predilection. Common Reed Warblers (Acrocephalus scirpaceus) were found everywhere in similar densities. We conclude that reedbed management by extensive horse grazing and also its long-term cessation may benefit several songbird species. Within a context of wetland degradation and disappearance, both management strategies are useful to establish and maintain suitable habitats for reed-nesting songbird communities.

The effect of Common Cuckoo parasitism on the annual productivity of a host population

Avian interspecific brood parasitism is an excellent system for studying coevolutionary processes in nature because brood parasites directly affect host reproductive success. Most research on avian brood parasitism has, therefore, focused primarily on specific host anti-parasite behaviours and parasite counter-adaptations. However, the population-level consequences of brood parasitism for host species are poorly understood. This study investigated the effect of Common Cuckoo (Cuculus canorus) parasitism on the productivity of a local population of the Great Reed Warbler (Acrocephalus arundinaceus) host in south-western Slovakia. A total of 495 females nesting at the studied site successfully fledged 1321 of their own young and 63 young of the Common Cuckoo over the course of 15 years (2008–2022), and the annual rate of successful parasitism ranged from 18% to 44% over this period. As predicted, higher rates of successful cuckoo parasitism significantly reduced the annual productivity of the local Great Reed Warbler population, and its contribution to decreased host reproductive success was at about the same rate as overall nest failure caused by predation and other environmental factors. Such a heavily parasitized population was probably maintained by immigrants from other populations and density-dependent selection, which is also consistent with source–sink dynamics. However, further long-term studies monitoring parasitized populations are required to confirm these findings.

Oriental Reed Warblers do not abandon Common Cuckoo chicks during prolonged nestling periods

The Oriental Reed Warbler (Acrocephalus orientalis) is one of the most commonly used hosts for the parasitic Common Cuckoo (Cuculus canorus). However, as hosts that feed unrelated parasitic nestlings may suffer extra reproductive costs, they may be less willing to care for nestlings that have prolonged nestling periods. To test this hypothesis, the duration of feeding by Oriental Reed Warblers under natural conditions for their own nestlings was compared with the duration of feeding under natural conditions for Common Cuckoo nestlings and for artificially prolonged cuckoo nestlings. The results showed that Oriental Reed Warblers did not starve, drive away, or desert any of the nestlings in the experiment, and neither parent was left alone. Our experimental study indicates that both Oriental Reed Warbler parents were willing to care for nestlings with a prolonged nestling period (up to 30 days, twice the average duration time that the Oriental Reed Warblers fed their own chicks in natural conditions). However, further experiments and observations are required in other host bird species to examine whether both parents or one of the parents may exhibit the behavior of abandoning nestlings with a prolonged nestling period.

The association between haemosporidian infection and non-breeding moult location in great reed warblers revisited by combining feather stable isotope profiles and geolocator data

Stable isotope analysis provides valuable insights into the ecology of long-distance migratory birds during periods spent away from a specific study site. In a previous study, Swedish great reed warblers (Acrocephalus arundinaceus) infected with haemosporidian parasites differed in feather isotope ratios compared to non-infected birds, suggesting that infected and non-infected birds spent the non-breeding season in different locations or habitats. Here, we use a novel dataset comprising geolocator data, isotopes, and haemosporidian infection status of 92 individuals from four Eurasian populations to investigate whether parasite transmission varies with geography or habitats. We found that the probability of harbouring Plasmodium and Leucocytozoon parasites was higher in birds moulting in the eastern region of the non-breeding grounds. However, no geographic pattern occurred for Haemoproteus infections or overall infection status. In contrast to the previous study, we did not find any relationship between feather isotope ratios and overall haemosporidian infection for the entire current dataset. Plasmodium-infected birds had lower feather δ15N values indicating that they occupied more mesic habitats. Leucocytozoon-infected birds had higher feather δ34S values suggesting more coastal sites or wetlands with anoxic sulphate reduction. As the composition and prevalence of haemosporidian parasites differed between the old and the current dataset, we suggest that the differences might be a consequence of temporal dynamics of haemosporidian parasites. Our results emphasize the importance of replicating studies conducted on a single population over a restricted time period, as the patterns can become more complex for data from wider geographical areas and different time periods.

Cooperative parental performance at chick provisioning in a small passerine, the Reed Warbler Acrocephalus scirpaceus

Sexual conflict has long been perceived as a solid background for the evolution of parental care. Recent studies, however, highlight the importance of cooperation between the parents, especially in socially monogamous systems. Here, we examined parental performance of a small monogamous passerine with bi-parental care, the Eurasian Reed Warbler (Acrocephalus scirpaceus, RW), looking at the issue from the perspective of parents’ cooperation. Using accurate and non-invasive video data on parent chick provisioning from 78 nests varying in brood size, we found some evidence of parents cooperation: both parents similarly adjusted their feeding rate to brood size, and the higher was their both contribution, the better was chicks condition (expressed by defecation rate). However, contrary to our expectations (based on premises from similar studies on other bird species) we did not find evidence for frequent and active synchronisation of nest visits. Importantly, we found a negative relationship between synchronisation level and chick body condition, suggesting that synchronisation may actually not be that beneficial in the study system. The results are surprising and highlight the importance of studying various species to understand mechanisms behind cooperation of partners in the bi-parental care system.Significance statementBiparental care has long been viewed in the context of conflict between the breeding partners, and only recently they are considered cooperating agents caring together for the common offspring. Nevertheless, studies applying such a perspective are still scarce and there is a pressing need to test different species. Using video data on chick provisioning by Reed Warbler parents, we found evidence of partner cooperation: both parents adjusted their feeding rate to the number of nestlings, and the higher was their joint contribution, the better was offspring condition. Parents did not synchronise their nest visits more compared to randomly generated visits, but we found that synchronisation level negatively affected chick condition. Our results highlight the need to consider various species to fully understand mechanisms behind cooperation of breeding partners.

Timing of reed management affects habitat use and breeding success in Great Reed Warblers: A field experiment on agricultural drainage canals

Freshwater wetlands and their biodiversity have suffered substantial losses globally, mainly due to the drainage of wetlands for agriculture. Recent studies suggest that small drainage and/or irrigation canals established in lowland areas can function both as partial replacement of wetlands lost and as biodiversity hotspots in agricultural landscapes. However, small canals are usually not protected or managed for biodiversity. We studied whether and how the timing of reed management (removal in winter, in spring, no removal) on small canals affects nest site selection and breeding success in Great Reed Warblers, Acrocephalus arundinaceus, a flagship species of small canals in an intensive agricultural landscape in a five-year quasi-experiment. We found significantly more nests in winter-managed than in non-managed canal sections and hardly any in spring-managed sections. The number of nests also increased with the proportion of reed removed and with reed density. Breeding success measured as the probability of fledging chicks was positively influenced by reed density only and was not affected by management type, nesting height, water depth and water level fluctuation. Reed density had a central role as it mediated the effect of management both on nest site selection and breeding success, likely through better concealment of the nests from predators. Our results suggest that winter management should be favored as much as possible in the conservation of Great Reed Warblers and other reed-nesting passerines, whereas spring management should be avoided as it results in a complete lack of reed or poorly grown reed unsuitable for nest-building by the start of the breeding season. Our study draws attention to the importance of proper management of small canals that can serve as local hotspots of biodiversity in heavily human-modified landscapes, and which thus deserve more conservation attention than they currently have.

Solar heating may explain extreme diel flight altitude changes in migrating birds

Great reed warblers, Acrocephalus arundinaceus,1 and great snipes, Gallinago media,2 exhibit a diel cycle in flight altitudes-flying much higher during the day than the night-when performing migratory flights covering both night and day. One hypothesis proposed to explain this behavior is that the birds face additional heating by solar radiation during daytime and hence must climb to very high, and thus also very cold, altitudes to avoid overheating during daytime flights.1,2 Yet, solar heat gain in birds has been shown to drastically decrease with wind speed,3,4 and the quantitative heating effect by solar radiation on a bird flying with an airspeed of 10m/s or more is unknown. We analyzed temperature data from multisensor data loggers (MDLs)5,6 placed without direct exposure to solar radiation on great reed warblers (the logger covered by feathers on the back) and great snipes (the logger on the leg, covered from the sun by the tail). We found that logger temperatures were significantly higher (5.9°C-8.8°C in great reed warblers and 4.8°C-5.4°C in great snipes) during the day than during the night in birds flying at the same altitudes (and thus also the same expected ambient air temperatures). These results strongly indicate that the heat balance of the flying birds is indeed affected by solar radiation, which is in accordance with the hypothesis that solar radiation is a key factor causing the remarkable diel cycles in flight altitude observed in these two long-distance migrant bird species.1,2.

Factors influencing low incidence of double brooding in the Great Reed Warbler

Double brooding, the laying of a second clutch after successfully fledging young from a first nest in a season, is a common reproductive strategy among short-lived birds to increase annual breeding success. Nevertheless, there is widespread intra- and interspecific variation in the probability of producing a second clutch. Given that the frequency of double brooding could affect population growth rate, knowing what factors contribute to this phenomenon is also important in the conservation management of threatened facultative double-brooded species. Here, we examine the frequency of double brooding and the factors affecting the probability of this reproductive strategy in a population of the Great Reed Warbler, Acrocephalus arundinaceus, in south-western Slovakia. Results of a 15-year study (2008–2022) revealed low overall rate of double brooding in the Great Reed Warbler (5.6% per year, 0–15.4%). The length of breeding season, timing of the first clutch laying date and the size of the first broods were key factors influencing the female's decision to initiate a second clutch. On the other hand, successful brood parasitism did not affect second clutch initiation, which contrasts with our prediction. Given that only monogamous or primary females initiated a second clutch after successfully rearing a first one, we assume that producing two broods within the same season may be an alternative strategy for monogamous males to increase their annual reproductive output in this facultatively polygynous species. Therefore, due to relatively high rate of polygyny, we probably cannot expect higher rate of double broodings in the Great Reed Warbler in the future either.

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.

Food Provisioning and Body Mass of Nestling Meadow Pipits and Cuckoos

Common Cuckoos Cuculus canorus are obligate brood parasites, laying eggs in nests of other species. Cuckoo nestlings are often thought to be insatiable, compared to host broods. However, in Reed Warblers Acrocephalus scirpaceus and a few other species, cuckoo nestlings are fed at most only as frequently as a host brood. To add to the small body of knowledge on feeding frequencies and body mass development of Cuckoo nestlings, I studied Cuckoos using another host, the Meadow Pipit Anthus pratensis, in the Dutch dunes (2019–2021). By filming feeding parents (hosts) I determined the provisioning frequency for broods of Pipits and for nestling Cuckoos. I found that nestling Cuckoos receive on average fewer feeds per hour than broods of Meadow Pipits. Furthermore, overall Cuckoos received as many feeds during their 22-day nestling period as a brood of Pipits during their 13 days in the nest. At 13 days of age (day 1 is the day of hatching), the single Cuckoo was as heavy as a brood of four Meadow Pipits. At 22 days, the maximum weights of the nestling Cuckoos were greater than the Meadow Pipit broods and varied between 91 and 105 g. Thus, although Cuckoos are being fed less frequently, young Cuckoos are heavier at fledging than a whole brood of Pipits. This could be because Cuckoos are fed larger prey. Alternatively, Cuckoos may require less food because their thermoregulatory costs could be smaller: they have almost black skin which absorbs solar radiation efficiently, do not have to compete with siblings in the nest and, once older and feathered, have a smaller surface-to-volume ratio than a Pipit brood of four nestlings.