Acrocephalus Arundinaceus Research Articles

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.

Wing-size related habitat choice of Great Reed Warbler females: the role of habitat quality and management

Wing-size related habitat choice of Great Reed Warbler females: the role of habitat quality and management

Host alarm calls attract the unwanted attention of the brood parasitic common cuckoo

It is well known that avian brood parasites lay their eggs in the nests of other bird species, called hosts. It remains less clear, however, just how parasites are able to recognize their hosts and identify the exact location of the appropriate nests to lay their eggs in. While previous studies attributed high importance to visual signals in finding the hosts’ nests (e.g. nest building activity or the distance and direct sight of the nest from vantage points used by the brood parasites), the role of host acoustic signals during the nest searching stage has been largely neglected. We present experimental evidence that both female and male common cuckoos Cuculus canorus pay attention to their host’s, the great reed warbler’s Acrocephalus arundinaceus alarm calls, relative to the calls of an unparasitized species used as controls. Parallel to this, we found no difference between the visibility of parasitized and unparasitized nests during drone flights, but great reed warblers that alarmed more frequently experienced higher rates of parasitism. We conclude that alarm calls might be advantageous for the hosts when used against enemies or for alerting conspecifics, but can act in a detrimental manner by providing important nest location cues for eavesdropping brood parasites. Our results suggest that host alarm calls may constitute a suitable trait on which cuckoo nestlings can imprint on to recognize their primary host species later in life. Our study contributes to the growing body of knowledge regarding the context-dependency of animal signals, by providing a novel example of a beneficial acoustic trait intercepted by a heterospecific and used against the emitter.

Spatial distribution of feather mites (Astigmata: Analgoidea) on the plumage of passerine birds from the territory of South Dobrudzha, Bulgaria

Feather mites (Astigmata: Pterolichoidea, Analgoidea) are permanent ectosymbionts of birds, found from all avian orders except Rheiformes (Stefan et al. 2015). The feather mites’ spatial distribution on the plumage depends on the morphology and structure of the feathers, aerodynamic and frictional forces, the life cycle and social behavior of the hosts, as well as environmental factors such as temperature and humidity (Dubinin 1951; Fernández-González et al. 2015). Depending on the location of the feather mites on the host’s body, a variety of morphological, physiological and behavioral adaptations are observed, which allow several feather mite species to coexist on the same bird (Dabert and Mironov 1999; Proctor 2003; Mestre et al. 2011). Studies dedicated to the spatial distribution of feather mites on the plumage of birds are relatively scarce. Such data is largely missing also for the territory of Bulgaria. Therefore, the aim of this paper is to present the results of the first specialized study of their spatial distribution on the plumage of passerines in Bulgaria. 379 birds belonging to 47 species of the order Passeriformes were examined in the period 2005–2007. As a result, the locations of 76,000 specimens (including both adults and nymphal stages) of feather mites of 54 species were determined. The distribution on the plumage of each species of bird was presented by generalized schemes. Feather mites were not found on the outermost primary feathers (P10), which are smaller than others primaries and do not offer enough resources. Feather mites located on the wing and tail feathers were mainly observed on the medial part of the feather, close to the rachis, and hence on the feather barbs. Feather mites preferred the wider parts of the primary feathers. On the secondary feathers, the mites were located predominantly in the middle third, on either side of the rachis. We examined if feather mites were symmetrically distributed on both wings using samples from those birds represented by numerous individuals (Passer montanus (L., 1758), Acrocephalus arundinaceus (L., 1758), Panurus biarmicus (L., 1758), Cyanistes caeruleus (L., 1758), Fringilla coelebs L., 1758). In this analysis, χ2-test confirmed no difference in the location of the feather mites on the feathers of both wings of the respective hosts and symmetry. Mites, such as Proctophyllodes pinnatus (Nitzsch, 1818) and Mesalgoides megnini (Oedemans, 1937) (on Chloris chloris (L., 1758)), Proctophyllodes stylifer (Buchholz, 1869) and Ptronyssoides parinus (Koch, 1841) (on Cyanistes caeruleus), Pterodectes rutilus Robin, 1868 and Scutulanyssus hirundicola Mironov, 1985 (on Hirundo rustica L., 1758), can coexist on the feathers of their respective hosts. Probably the different body size or trophology of these mite species reduced the competition between them. We found Trouessartia crucifera Gaud, 1957 and T. appendiculata (Berlese, 1886) on the secondary feathers of Hirundo rustica, but never together. The competition for resources between these two mite species is probably stronger. Feather mites respond to bird molting mainly by moving to adjacent feathers (Dubinin 1951). We monitored the spatial distribution of Proctohyllodes balati on the wing feathers during the molting of Panurus biarmicus.

Mimicry cannot explain rejection type in a host–brood parasite system

One of the most effective defensive strategies of hosts against brood parasites is rejection, commonly achieved by ejection of the parasitic egg or desertion of the parasitized nest. Nest desertion should be a costlier strategy than egg ejection, because birds must thesn spend additional time and energy renesting, and therefore we still cannot explain why some individuals desert their nests rather than eject parasitic eggs and continue a given breeding attempt. The great reed warbler, Acrocephalus arundinaceus, is a frequent host of the common cuckoo, Cuculus canorus, and is known to use both types of rejection response. By measuring cuckoo egg mimicry, we investigated the hypothesis that the hosts desert if they cannot reliably recognize the cuckoo egg in their nest. We predicted that we would find better mimicry when hosts deserted rather than ejected. However, we did not find a difference in mimicry between these two groups of nests, implying that host females do not desert because they cannot reliably recognize the parasitic egg. We also showed that neither the date in the season nor the age of the host females influenced the type of rejection. Other factors potentially eliciting nest desertion, including host personality, host, inability to eject, excessive clutch reduction and visibility of the cuckoo female at the host nest, are discussed. Finally, we suggest that desertion may persist as a host defensive strategy against brood parasitism because it is not as costly as previously assumed and/or it is beneficial for host females in good physical condition.

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).

Anti-parasitic egg rejection by great reed warblers (Acrocephalus arundinaceus) tracks differences along an eggshell color gradient

One of the most effective defenses against avian brood parasitism is the rejection of the foreign egg from the host’s nest. Until recently, most studies have tested whether hosts discriminate between own and foreign eggs based on the absolute differences in avian-perceivable eggshell coloration and maculation. However, recent studies suggest that hosts may instead contrast egg appearances across a directional eggshell color gradient. We assessed which discrimination rule best explained egg rejection by great reed warblers Acrocephalus arundinaceus, a frequent host to an egg-mimetic race of common cuckoos Cuculus canorus. We deployed 3D-printed model eggs varying in blue-green to brown coloration and in the presence of maculation. Using visual modeling, we calculated the absolute chromatic and achromatic just-noticeable differences (JNDs), as well as directional JNDs across a blue-green to brown egg color gradient, between host and model eggs. While most model eggs were rejected by great reed warblers, browner eggs were rejected with higher probability than more blue-green eggs, and the rejection probability did not depend on maculation. Directional egg color discrimination shown here and in a suite of recent studies on other host species may shape the cognitive decision rules that hosts use to recognize foreign eggs and affect the course of evolution in parasitic egg mimicry.

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.

Bimodal habitat use in brood parasitic Common Cuckoos (Cuculus canorus) revealed by GPS telemetry

Abstract Obligate brood parasitic birds have evolved a rare avian strategy for reproduction by laying eggs in the nests of other species. In doing so, their breeding ranges, but not necessarily their foraging habitats, have become intimately related to the nesting territories of their hosts. We studied home range sizes and distribution patterns in Common Cuckoos (Cuculus canorus) on their breeding grounds in central Hungary, where cuckoos parasitize only Great Reed Warblers (Acrocephalus arundinaceus) in channel-side reed-beds at a high frequency (>50%). The geographic coordinates of tagged cuckoos were monitored by high-precision, remotely downloadable non-Platform Terminal Transmitter global positioning system (GPS) loggers, attached to 9 females and 6 males. Our results revealed bimodal use of habitat patches: (1) the home ranges of male and female cuckoos were packed densely along the channels where the hosts breed, and their distribution maps had high overlaps between sexes; (2) ~71% of cuckoos also visited nearby woodland patches, presumably for foraging, where the host species was not present. The size of cuckoo home ranges varied to an unusually great extent: 0.3–185 km2 as calculated by the minimum convex polygon method (85%), or 1–17 km2 when calculated by the more suitable kernel density estimation (KDE) method (Utilization Distribution 85%) for patchy habitats. Male and female cuckoos had similar home range sizes as estimated by the KDE method, consisting of 1–4 areas within the 2 habitat types of channel reed-beds and woodlands. No preference was revealed for night roosting locations between the 2 habitats or sexes. Female cuckoos were more likely to use reed-beds in the afternoons, when females parasitize host nests. Remote downloadable GPS methods offer an effective way of tracking cuckoos across large areas, but the estimation of home range sizes requires caution due to this species’ patchy and disconnected habitat use.

The role of peri-urban wetland and meadow habitats in the protection of trans-Saharan migrant passerine species in a central European city

Densities were estimated for several passerine longdistance migrant species associated with peri-urban wetlands and meadows in the city of Wroclaw (293 km2), SW Poland. Acrocephalus arundinaceus, A. scirpaceus, Locustella naevia and Lanius collurio nested in a crude density of more than 0.2 pairs/territorial males per 100 ha. The group of four other species (Saxicola torquatus, Locustella fluviatilis, L. luscinioides and Acrocephalus schoenobaenus) bred in a crude density between 1.1 and 1.5 pairs per 10 km2. The least numerous were the Sylvia nisoria (0.06 pairs per 100 ha) and Remiz pendulinus (0.02 pairs per 100 ha). Most of these species nested in the city in higher crude density than in a neighbouring rural area, richer of suitable habitats. Most breeding pairs occupied extensive peri-urban wetlands and meadows. These habitats could play an important role in conservation of these trans-Saharan migrants, as well as other water and marshland bird species. Such habitats are postulated to be protected as nature reserves or Special Protection Areas of the Natura 2000.

Estimation of Local Heading Patterns of Nocturnal Migrants using Orientation Cages

Abstract Busse P. 2018. Estimation of local heading patterns of nocturnal Migrants using orientation cages. Ring 40: 31-58 The main aim of this paper is to compare the results of two data evaluation procedures used for presenting the data from the orientation cage field tests. Both procedures accept multimodality of the data and multimodality of the headings of an individual bird as well as migrating population. The goal is to reach acceptable level of migration patterns presentation in biological sense, taking under consideration a flexibility of the real movements, depending on specific weather and landscape parameters. Such knowledge is absolutely necessary for estimating migration bottle-necks and the long-term studies on influence of the climate changes on migration patterns. The material used for the comparison of the procedures was collected in years 2001-2007 by the team of the Bulgarian Ringing Station Kalimok (44°00’N, 26°26’E) within the frame of the SEEN (SE European Bird Migration Network) activity and kindly shared for evaluation. The data were obtained using the standard SEEN methods, with the standard Busse’s cage working procedure of the field tests. The material contains data on four species of nocturnal migrants living in different habitats: the Great Reed Warbler, Acrocephalus arundinaceus (ACR.ARU), the Sedge Warbler, A. schoenobaenus (ACR.ENO), the Willow Warbler, Phylloscopus trochilus (PHY.LUS) and the Whitethroat, Sylvia communis (SYL.COM). There are confirmed earlier conclusions that so called „classic” unimodal procedure is not applicable to the orientation cage data resulted from any field procedure. There are available two evaluation procedures that base on the same general assumptions: multimodality of distributions that reflects combination of several unimodal partial distributions, that can be described both using sophisticated Bayesian „Calculation” method and much simpler „Estimation” procedure. Results of both procedures are enough close to each other that they can be used for describing local and general heading patterns of migration of the nocturnal migratory movements studied using orientation cages.

Miniaturized multi-sensor loggers provide new insight into year-round flight behaviour of small trans-Sahara avian migrants

BackgroundOver the past decade, the miniaturisation of animal borne tags such as geolocators and GPS-transmitters has revolutionized our knowledge of the whereabouts of migratory species. Novel light-weight multi-sensor loggers (1.4 g), which harbour sensors for measuring ambient light intensity, atmospheric pressure, temperature and acceleration, were fixed to two long-distance migrant bird species - eurasian hoopoe (Upupa epops) and great reed warbler (Acrocephalus arundinaceus). Using acceleration and atmospheric pressure data recorded every 5 and 30 min, respectively, we aimed at reconstructing individual diurnal and seasonal patterns of flight activity and flight altitude and thereby, at describing basic, yet hitherto unknown characteristics of migratory flight behaviour. Furthermore, we wanted to characterise the variability in these migration characteristics between individuals, species and migration periods.ResultsThe flight duration from breeding to sub-Saharan African non-breeding sites and back was more variable within than between the species. Great reed warblers were airborne for a total of 252 flight hours and thus, only slightly longer than eurasian hoopoes with 232 h. With a few exceptions, both species migrated predominantly nocturnally - departure around dusk and landing before dawn. Mean flight altitudes were higher during pre- than during post-breeding migration (median 1100 to 1600 m a.s.l.) and flight above 3000 m occurred regularly with a few great reed warblers exceeding 6000 m a.s.l. (max. 6458 m a.s.l.). Individuals changed flight altitudes repeatedly during a flight bout, indicating a continuous search for (more) favourable flight conditions.ConclusionsWe found high variation between individuals in the flight behaviour parameters measured – a variation that surprisingly even exceeded the variation between the species. More importantly, our results have shown that multi-sensor loggers have the potential to provide detailed insights into many fundamental aspects of individual behaviour in small aerial migrants. Combining the data recorded on the multiple sensors with, e.g., remote sensing data like weather and habitat quality on the spatial and temporal scale will be a great step forward to explore individual decisions during migration and their consequences.

Influence of habitat suitability and sex-related detectability on density and population size estimates of habitat-specialist warblers.

Knowledge about the population size and trends of common bird species is crucial for setting conservation priorities and management actions. Multi-species large-scale monitoring schemes have often provided such estimates relying on extrapolation of relative abundances in particular habitats to large-scale areas. Here we show an alternative to inference-rich predictive models, proposing methods to deal with caveats of population size estimations in habitat-specialist species, reed warblers (Acrocephalus scirpaceus and Acrocephalus arundinaceus). Reed warblers were only found in pure reedbeds within riparian woodlands or in riparian vegetation scattered within or around reedbed patches, as expected according to their habitat specialization. The proportion of individuals located in reedbed associated with lotic and lentic waters differed between species, and no reed warbler was recorded in reedbed located along dry streams. This indicates that microhabitat features or their effects on reedbed structure and other factors made a proportion of the apparently available habitat unsuitable for both warbler species. Most warblers detected were males performing territorial singing (females seldom sing and do not perform elaborate territorial song, and are undistinguishable from males by plumage). The regional population sizes of the warbler species (~4000 individuals of A. scirpaceus and ~ 1000 individuals of A. arundinaceus) were much smaller than those estimated for the same area by transforming relative abundance obtained at a national scale to population size through extrapolation by habitat at a regional scale. These results highlight the importance of considering the habitat actually used and its suitability, the manner of sex-related detection, population sex-ratio and their interactions in population estimates. Ideally, the value of predictive methods to estimate population size of common species should be tested before conducting large-scale monitoring, rather than a posteriori. Although logistically challenging, this can be achieved by designing monitoring programs including an intensive sampling of abundance in ad hoc reference areas of variable size.

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.

Advanced Breeding Time in Line with Climate Did Not Affect Productivity of Great Reed WarblersAcrocephalus arundinaceusDespite the Shortening of the Nestling Period

Recent climate warming has resulted in advancement in laying dates and changes in various bird breeding parameters. The study of the breeding biology of Great Reed Warblers Acrocephalus arundinaceus was carried out at Milicz fishponds (SW Poland) during 18 discontinuous breeding seasons between 1970 and 2017. Over the study period, Great Reed Warblers started breeding progressively earlier; both the earliest and median first egg laying dates advanced, whereas the latest laying dates remained unchanged. Egg volume and clutch size did not change between 1972 and 2017. Mean clutch size of all nesting attempts was 4.8 (SD = 0.69, n = 1135). Nestling growth rates were similar in 1971 and 2016. Nestling period shortened during the study period. The decrease amounted to 0.7 days between 1970–1984 and 2004–2017. No temporal trends in fledgling production were observed. The proportion of successful broods varied considerably between years (range: 37%−76%) but did not show any consistent trend throughout the study period. More cases of Cuckoo Cuculus canorus parasitism were observed in 1997−2017 than in 1970−1984 (7% of nest losses versus 0 respectively), whereas starvation of nestlings was more frequent in 1970−1984 (10% versus 2% of nest losses). Breeding densities varied between seasons with no significant trend across the study years. This study concludes that the advanced start of breeding has not significantly influenced basic parameters of breeding ecology of the Great Reed Warbler.

The effects of reed density, surface and management on the probability of cuckoo (Cuculus canorus) parasitism on great reed warbler (Acrocephalus arundinaceus) nests

While some previous investigations have been made into the effects of vegetation structure on the probability of brood parasitism in reed passerines, the effects of vegetation management remain unclear. Furthermore, we possess little information on how vegetation density influences the probability of brood parasitism in wide-surfaced reed-beds. The aim of this study was to test how vegetation density and reed management by burning influence the probability of cuckoo (Cuculus canorus) brood parasitism in great reed warbler (Acrocephalus arundinaceus) nests in a wide-surfaced reed-bed with pure reed stands. We found that the probability of parasitism was highest in nests constructed in low reed density, with low annual variation of reed density and low annual variation of nest height. Reed management had no effect on the probability of parasitism. Our results suggest that, in wide-surfaced reed-beds with pure reed stands, nests in sparser reed are more exposed to brood parasitism than those concealed in dense reed. Furthermore, low spatio-temporal variation in reed density and nest height benefit brood parasitism, as it provides a relatively constant number of host nests in a similar spatial distribution in the reed-bed from year to year. Vegetation structure was suggested to have an important role in adaptation of hosts’ defence against brood parasites.

Stability of a behavioural syndrome vs. plasticity in individual behaviours over the breeding cycle: Ultimate and proximate explanations

Animals often show correlated suites of consistent behavioural traits, i.e., personality or behavioural syndromes. Does this conflict with potential phenotypic plasticity which should be adaptive for animals facing various contexts and situations? This fundamental question has been tested predominantly in studies which were done in non-breeding contexts and under laboratory conditions. Therefore, in the present study we examined the temporal stability of behavioural correlations in a breeding context and under natural conditions. We found that in the great reed warbler (Acrocephalus arundinaceus) females, the intensity of their nest defence formed a behavioural syndrome with two other traits: their aggression during handling (self-defence) and stress responses during handling (breath rate). This syndrome was stable across the nesting cycle: each of the three behavioural traits was highly statistically repeatable between egg and nestling stages and the traits were strongly correlated with each other during both the egg stage and the nestling stage. Despite this consistency (i.e., rank order between stages) the individual behaviours changed their absolute values significantly during the same period. This shows that stable behavioural syndromes might be based on behaviours that are themselves unstable. Thus, syndromes do not inevitably constrain phenotypic plasticity. We suggest that the observed behavioural syndrome is the product of interactions between behavioural and life history trade-offs and that crucial proximate mechanisms for the plasticity and correlations between individual behaviours are hormonally-regulated.

Contrasting results from GWAS and QTL mapping on wing length in great reed warblers.

A major goal in evolutionary biology is to understand the genetic basis of adaptive traits. In migratory birds, wing morphology is such a trait. Our previous work on the great reed warbler (Acrocephalus arundinaceus) shows that wing length is highly heritable and under sexually antagonistic selection. Moreover, a quantitative trait locus (QTL) mapping analysis detected a pronounced QTL for wing length on chromosome 2, suggesting that wing morphology is partly controlled by genes with large effects. Here, we re-evaluate the genetic basis of wing length in great reed warblers using a genomewide association study (GWAS) approach based on restriction site-associated DNA sequencing (RADseq) data. We use GWAS models that account for relatedness between individuals and include covariates (sex, age and tarsus length). The resulting association landscape was flat with no peaks on chromosome 2 or elsewhere, which is in line with expectations for polygenic traits. Analysis of the distribution of p-values did not reveal biases, and the inflation factor was low. Effect sizes were however not uniformly distributed on some chromosomes, and the Z chromosome had weaker associations than autosomes. The level of linkage disequilibrium (LD) in the population decayed to background levels within c. 1 kbp. There could be several reasons to why our QTL study and GWAS gave contrasting results including differences in how associations are modelled (cosegregation in pedigree vs. LD associations), how covariates are accounted for in the models, type of marker used (multi- vs. biallelic), difference in power or a combination of these. Our study highlights that the genetic architecture even of highly heritable traits is difficult to characterize in wild populations.

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.

Low intensity blood parasite infections do not reduce the aerobic performance of migratory birds.

Blood parasites (Haemosporidia) are thought to impair the flight performance of infected animals, and therefore, infected birds are expected to differ from their non-infected counterparts in migratory capacity. Since haemosporidians invade host erythrocytes, it is commonly assumed that infected individuals will have compromised aerobic capacity, but this has not been examined in free-living birds. We tested if haemosporidian infections affect aerobic performance by examining metabolic rates and exercise endurance in migratory great reed warblers (Acrocephalus arundinaceus) experimentally treated with Plasmodium relictum pGRW04 and in naturally infected wild birds over consecutive life-history stages. We found no effect of acute or chronic infections on resting metabolic rate, maximum metabolic rate or exercise endurance in either experimentally treated or free-living birds. Oxygen consumption rates during rest and while undergoing maximum exercise as well as exercise endurance increased from breeding to migration stages in both infected and non-infected birds. Importantly, phenotypic changes associated with preparation for migration were similarly unaffected by parasitaemia. Consequently, migratory birds experiencing parasitaemia levels typical of chronic infection do not differ in migratory capacity from their uninfected counterparts. Thus, if infected hosts differ from uninfected conspecifics in migration phenology, other mechanisms besides aerobic capacity should be considered.