Polygynous great reed warblers Acrocephalus arundinaceus suffer more cuckoo Cuculus canorus parasitism than monogamous pairs

There is increasing evidence that hosts within a population may not be parasitized by common cuckoos Cuculus canorus with equal probability. Such non‐randomness has been documented, for example, for host nest sites and host quality. In this study we demonstrate association between successful cuckoo parasitism and host social mating system. We found that nests of socially polygynous great reed warblers Acrocephalus arundinaceus were more often successfully parasitized than the nests of their monogamous counterparts. We hypothesize that lack of parental assistance provided by polygynous males to their mates during egg laying period and higher nest activity in their territories could contribute to this discrepancy. These data imply that social mating system should be taken into account in future studies of brood parasitism.     

Responses of great reed warblers Acrocephalus arundinaceus to experimental brood parasitism: the effects of a cuckoo Cuculus canorus dummy and egg mimicry

Egg rejection behaviour towards parasitic eggs was studied in a great reed warbler Acrocephalus arundinaceus population in central Hungary, which was heavily (about 65%) parasitised by the common cuckoo Cuculus canorus . Clutches were experimentally parasitised during the egg-laying period with artificial, moderately mimetic cuckoo eggs or with conspecific eggs that were good mimics of the hosts' eggs. Great reed warblers rejected 76.2% of the artificial cuckoo eggs, mainly by ejection, but accepted most of the conspecific eggs (87.5%). Cuckoo eggs in naturally parasitised clutches were rejected at a lower rate (32%). When, in addition to the egg mimicry experiments, a stuffed cuckoo was placed near the nest, accompanied by the recording of a female cuckoo call, hosts' rejection rate of the artificial cuckoo egg increased from 76% to 96%. The sight of the cuckoo, on the other hand, did not influence host's rejection behaviour when a conspecific egg was used in the experiment. A stuffed collared dove Streptopelia decaocto , accompanied by its call, was used as a control, and did not cause any increased rejection. Great reed warblers were more aggressive towards the cuckoo than to the dove dummy. When the cuckoo eggs in naturally parasitised clutches were exchanged with artificial cuckoo eggs, we observed no increase in the rejection rate. We conclude that great reed warblers in our heavily parasitised population are capable of detecting brood parasitism in their clutch by identifying the parasitic egg. The efficiency of this identification depends mainly on the mimicry of the foreign egg. The sight of the cuckoo at the nest may increase rejection rate by stimulus summation, and this conditional effect is mainly affected by the degree of mimicry of the parasitic egg.     

Factors influencing the risk of common cuckoo Cuculus canorus parasitism on marsh warblers Acrocephalus palustris

Using multiple logistic regression analysis, we investigated the influence of nest site characteristics, laying date and nest size in marsh warblers Acrocephalus palustris on the risk of parasitism by common cuckoos Cuculus canorus . Marsh warblers breed in more diverse and dense herbaceous vegetation than other cuckoo hosts investigated in comparable studies. The "perch proximity" hypothesis was supported as parasitized nests were situated closer to trees than non-parasitized ones. Furthermore, demonstrated for the first time in a cuckoo host, tree height was an important predictor of parasitism, with higher trees increasing the parasitism odds ratio. The "nest exposure" hypothesis was also supported since parasitized nests had a shorter stand of vegetation in the close vicinity than non-parasitized nests. However, visibility of the nest from the nearest potential cuckoo perch (cuckoo view) was not selected by the model, probably because most nests were well concealed. Laying date, height of nest above ground and the distance from the nest to the nearest edge of the vegetation were not important predictors of parasitism. Though smaller nests tended to be parasitized more frequently than larger ones, nest size only approached significance, making its importance unclear.     

Daily energy expenditure of singing great reed warblers Acrocephalus arundinaceus

According to honest signalling theory, signals must be costly to produce to retain information about the signaller's quality. The song produced by male birds during breeding is a vocal "ornament" used for intra- and inter-sexual purposes. The energetic cost of this vocal signal remains a contentious issue. We used the doubly labelled water method to measure field metabolic rate by estimating CO₂ production and then convert this to daily energy expenditure (DEE) in great reed warbler males singing under natural conditions (10 at low to moderate intensity and 7 at very high intensity from dawn to dusk). There was a significant positive relationship between singing intensity and DEE. From this relationship we extrapolated the average DEE for intensely singing males (i.e., males producing song sounds 50% of the time and hence sitting at their elevated song post in the top of a reed stem more or less continuously throughout the ∼20 h of daylight) to 3.3×BMR (basal metabolic rate) and for non-singing males to 2.2×BMR. The mean DEE measured for the seven males singing with very high intensity was 3.1×BMR. The maximum measured DEE for a single male was 3.9×BMR, i.e. close to the maximum sustainable DEE (4×BMR), and the minimum DEE was 2.1×BMR for a male singing at very low intensity. These results imply that producing intensive advertising song in birds may incur a substantial cost in terms of increased energy expenditure.     

Sex ratio variation among broods of great reed warblers Acrocephalus arundinaceus

The sex of 746 great reed warbler fledglings (from 175 broods) was determined by the use of single primer polymerase chain reaction. The reliability of the technique was confirmed as 104 of the fledglings were subsequently recorded as adults of known sex. The overall sex ratio did not differ from unity. Variation in sex ratios between broods was larger than expected from a binomial distribution. Female identity explained some of the variation of brood sex ratio indicating that certain females consistently produced sex ratios that departed from the average value in the population. The theory of sex allocation predicts that parents should adjust the sex ratio of their brood to the relative value of sons and daughters and this may vary in relation to the quality of the parents or to the time of breeding. In the great reed warbler, the proportion of sons was not related to time of breeding, or to any of five female variables. Of five male variables, males with early arrival date tended to produce more daughters. The sex ratio of fledglings that were a result of extra-pair fertilizations did not differ from that of legitimate fledglings. Hence, there is currently no evidence of that female great reed warblers invest in a higher proportion of sons when mated with attractive males.     

Genetic and phenotypic associations in morphological traits: a long term study of great reed warblers Acrocephalus arundinaceus

We present estimates of standardized selection (directional and quadratic) differentials via reproductive success on eight morphological traits in a newly founded long-term study population of great reed warblers Acrocephalus arundinaceus in Sweden. In order to predict the evolutionary response to selection in these traits we present estimates of heritabilities (h²), phenotypic (r_P) and genetic (r_A) correlations among the same traits. We also examined the extent of parental effects in the expression of the phenotypic traits. Overall, the population is subject to low levels of directional selection and higher levels of stabilizing selection. This makes us predict that the population is changing very little if anything, even though h² in many cases are considerable. Midparent-midoffspring h² ranged from 0.14 and 0.94 (mean 0.58) and were significant for seven of eight traits. We found indications of positive maternal effects in tarsus length. Phenotypic correlations between traits ranged from 0.02 to 0.43 (mean 0.15) and showed generally much lower values than the corresponding genetic correlations that ranged between 0.08 and 1.04 (mean 0.46). Overall, the correlation between r_P and r_A was significant, although moderate, but they tended to differ in magnitude, possibly due to overestimation of additive covariance between traits.     

Host density predicts presence of cuckoo parasitism in reed warblers

In some hosts of avian brood parasites, several populations apparently escape parasitism, while others are parasitized. Many migratory specialist brood parasites like common cuckoos, Cuculus canorus , experience a short breeding season, and in order to maintain local parasite populations host densities should be sufficiently high to allow efficient nest search. However, no studies have investigated the possible effect of host density on presence of cuckoo parasitism among populations of a single host species. Here, we investigated possible predictors of common cuckoo parasitism in 16 populations of reed warblers, Acrocephalus scirpaceus , across Europe. In more detail, we quantified the effect of host density, number of host breeding pairs, habitat type, mean distance to nearest cuckoo vantage point, predation rate and latitude on the presence of cuckoo parasitism while controlling for geographical distance among study populations. Host density was a powerful predictor of parasitism. We also found a less pronounced effect of habitat type on occurrence of parasitism, while the other variables did not explain why cuckoos utilize some reed warbler populations and not others. This is the first study focusing on patterns of common cuckoo-host interactions within a specific host species on a large geographic scale. The results indicate that if host density is below a specific threshold, cuckoo parasitism is absent regardless of the state of other potentially confounding variables.     

An analysis of hatching success in the great reed warbler Acrocephalus arundinaceus

Hatching success is a potentially important fitness component for avian species. Previous studies of hatching success in natural populations have primarily focused on effects of inbreeding but a general understanding of variation in hatching success is lacking. We analyse data on hatching success in a population of great reed warblers Acrocephalus arundinaceus in Lake Kvismaren in south central Sweden. The effects of a range of covariates, including three measures of inbreeding as well as effects of classifications in the data (such as identities of individuals), on hatching success are analysed simultaneously. This is done by means of fitting Bayesian binomial mixed models using Markov chain Monte Carlo methods. Using random effects for each individual parent we check for unexplained variation in hatching success among male and female individuals and compare it to effects of covariates such as degree of inbreeding. Model selection showed that there was a significant amount of unexplained variation in hatching probability between females. This was manifested by a few females laying eggs with a substantially lower hatching success than the majority of the females. The deviations were of the same order of magnitude as the significant effect of parent relatedness on hatching success. Whereas the negative effect of parent relatedness on hatchability is an expression of inbreeding, the female individual effect is not due to inbreeding and could reflect maternal effects, that females differ in fertilisation and/or incubation ability, or an over representation of genetic components from the female acting on the early developing embryo.     

Low frequency of extrapair paternity in the polygynous great reed warbler, Acrocephalus arundinaceus

We carried out DNA fingerprinting on 553 young (130 broods)great reed warblers (Acrocephalus arundnaceus) in 1987–1991.In the study population, where 40% of the males become polygynous,there was a low frequency of extrapair fertilizations (EPF).When data from all five years were pooled, 3.1% of the youngwere sired by extrapair males (EPF-males) and 5.4% of the broodscontained extrapair young. We found no cases of extrapair maternity;young with 6–17 mismatched DNA bands (n= 17) had highband sharing with their putative mothers (range = 0.52–0.72)but low band sharing with their putative fathers (range = 0.24–0.40).In broods exposed to EPF, on average 53% of the young were siredby EPF-males. We found the genetic father to each of the illegitimateyoung. In all cases the same EPF-male sired all extrapair youngin a brood. Broods containing EPF-young tended to be initiatedlate during the breeding season. Breeding attempts were ratherevenly distributed over two months, thus this breeding asynchronywould have facilitated EPFs. There was no difference in EPFfrequency between broods where the pair males had left theirfemales unguarded during parts of their fertile periods andbroods where males guarded throughout the fertile periods. Nestswith extrapair young had significantly shorter mean distanceto the closest male neighbor and more male neighbors within100 m than nests without extrapair young. We found no indicationthat females engaged in EPF to get parental care from the EPF-males,or because they were forced to copulate with extrapair males.The low frequency of EPF suggested that females did not seekgenetic diversity to their brood. We cannot rule out the possibilitythat females engaged in EPF to insure fertility. However, datasupporting this hypothesis were weak. Instead, our data supportthe conclusion that females engaged in EPF to increase the geneticquality of their offspring, and that females may have used malesong repertoire size as a cue when choosing EPF partners.     

The importance of nest cleaning in egg rejection behaviour of great reed warblers Acrocephalus arandinaceas

We tested the importance of nest cleaning in egg rejection behaviour of the great reed warbler Acrocephalus arundinaceus in a highly parasitised population in which about 64% of nests are parasitised by the common cuckoo Cuculus canorus . Three types of objects of the same weight, texture and colour but with different shapes (dummy cuckoo eggs, sticks and disks) were placed into great reed warbler nests. We investigated the response of hosts in two stages of breeding: pre-incubation when the risk of brood parasitism is high, and during incubation when the risk of parasitism is low. The dummy cuckoo eggs were rejected less often than the other objects in both breeding stages, although we did not find any difference in the frequency of rejection between pre-incubation and incubation. We integrate these results into current views on the evolution of host–parasite interactions, and propose a hierarchical concept to understand egg rejection behaviour: (1) hosts reject all non-egg shaped objects as a general cleaning mechanism; (2) adaptations for the hosts' ability to recognise their own eggs allows them to distinguish these eggs from similar objects and parasitic eggs.     

Dynamics of parasitemia of malaria parasites in a naturally and experimentally infected migratory songbird, the great reed warbler Acrocephalus arundinaceus

Little is known about the development of infection of malaria parasites of the genus Plasmodium in wild birds. We used qPCR, targeting specific mitochondrial lineages of Plasmodium ashfordi (GRW2) and Plasmodium relictum (GRW4), to monitor changes in intensities of parasitemia in captive great reed warblers Acrocephalus arundinaceus from summer to spring. The study involved both naturally infected adults and experimentally infected juveniles. The experiment demonstrated that P. ashfordi and P. relictum lineages differ substantially in several life-history traits (e.g. prepatent period and dynamics of parasitemia) and that individual hosts show substantial differences in responses to these infections. The intensity of parasitemia of lineages in mixed infections co-varied positively, suggesting a control mechanism by the host that is general across the parasite lineages. The intensity of parasitemia for individual hosts was highly repeatable suggesting variation between the host individuals in their genetic or acquired control of the infections. In future studies, care must be taken to avoid mixed infections in wild caught donors, and when possible use mosquitoes for the experiments as inoculation of infectious blood ignores important initial stages of the contact between the bird and the parasite.     

Within‐ and between‐season repeatability of eggshell colouration in the great reed warbler Acrocephalus arundinaceus

While avian eggshell colouration has attracted biologists for decades, little is known about its variation within individuals. The main goal of this study was to explore within‐ and between‐season repeatability of eggshell appearance in the great reed warbler Acrocephalus arundinaceus. To achieve this, we measured eggshell reflectance of first and replacement clutches of individual females within one breeding season, and the reflectance of their first clutches in two following breeding seasons. As environmental conditions may affect egg colouration, repeatability was estimated from linear mixed‐effects models where we initially included temperature and rainfall during egg laying, as well as year, clutch order and egg‐laying date as fixed effects, and clutch identity nested in females as random effects. Eggshell appearance within clutches showed moderate repeatability, while both within‐ and between‐season repeatability in clutch colouration for individual females was low. Our findings indicate that low intra‐clutch variation may have a function within the context of brood parasitism, i.e. in facilitating host recognition of alien eggs. With variable eggs between successive clutches, however, host females may need to relearn the appearance of their eggs with every clutch they lay. This could represent a significant constraint for host egg‐discrimination abilities. Yet, whether environmental or intrinsic physiological factors are responsible for the variation in eggshell colouration between successive clutches of the same females still remain to be discovered.     

Rapid decline of host defences in response to reduced cuckoo parasitism: behavioural flexibility of reed warblers in a changing world

On Wicken Fen and nearby watercourses eastern England, parasitism by cuckoos, Cuculus canorus, declined from 26% and 16% of reed warbler (Acrocephalus scirpaceus) nests in 1985 and 1986, respectively, to 2 to 6% of nests in 1995 to 1997, owing to a decline in cuckoos. Experiments with model eggs showed that over this 12-year period there was a marked decline in host rejection of non-mimetic eggs, from rejection at 75% of reed warbler nests in 1985 to 1986 to 25%, nests in 1997. Calculations suggest that this decline in host defences is too rapid to reflect only genetic change, and is more likely to be the outcome of adaptive phenotypic flexibility. Two other results show flexibility in host responses. First, there was a seasonal decline in rejection, which accompanied the seasonal decline in parasitism. Second, although rejection did not vary with proximity to a naturally parasitized nest within the 3.4km² of Wicken Fen and its surrounds, there was no rejection at a small unparasitized population 11km away. Flexible host defences will be advantageous when there are costs of rejection as well as short-term temporal changes and small-scale geographical variation in parasitism rate. Other recent studies reporting changes in host defences may also reflect phenotypic flexibility rather than evolutionary change.     

Hybridization but no evidence for backcrossing and introgression in a sympatric population of great reed warblers and clamorous reed warblers

Hybridization is observed frequently in birds, but often it is not known whether the hybrids are fertile and if backcrossing occurs. The breeding ranges of the great reed warbler (Acrocephalus arundinaceus) and the clamorous reed warbler (A. stentoreus) overlap in southern Kazakhstan and a previous study has documented hybridization in a sympatric population. In the present study, we first present a large set of novel microsatellite loci isolated and characterised in great reed warblers. Secondly, we evaluate whether hybridization in the sympatric breeding population has been followed by backcrossing and introgression.We isolated 181 unique microsatellite loci in great reed warblers. Of 41 loci evaluated, 40 amplified and 30 were polymorphic. Bayesian clustering analyses based on genotype data from 23 autosomal loci recognised two well-defined genetic clusters corresponding to the two species. Individuals clustered to a very high extent to either of these clusters (admixture proportions ≥ 0.984) with the exception of four previously suggested arundinaceus-stentoreus hybrid birds that showed mixed ancestry (admixture proportions 0.495-0.619). Analyses of simulated hybrids and backcrossed individuals showed that the sampled birds do not correspond to first-fourth-generation backcrosses, and that fifth or higher generation backcrosses to a high extent resemble 'pure' birds at this set of markers.We conclude that these novel microsatellite loci provide a useful molecular resource for Acrocephalus warblers. The time to reach reproductive isolation is believed to be very long in birds, approximately 5 Myrs, and with an estimated divergence time of 2 Myrs between these warblers, some backcrossing and introgression could have been expected. However, there was no evidence for backcrossing and introgression suggesting that hybrids are either infertile or their progeny inviable. Very low levels of introgression cannot be excluded, which still may be an important factor as a source of new genetic variation.     

Factors influencing the appearance of spider (Araneae) and beetle (Coleoptera) assemblages in nests of great reed warbler Acrocephalus arundinaceus

The appearance of spider (Araneae) and beetle (Coleoptera) assemblages found in nests of great reed warbler Acrocephalus arundinaceus was studied, firstly to investigate breeding success and the amount of precipitation as potential factors which might affect the abundance and species richness of both groups. In addition, we compared the diversity of spider and beetle assemblages between nests found in different reed habitats, and considered the position of nests (above water or dry ground). In this study we selected five different randomly chosen reed habitats: two mining ponds, two small canals and one large canal. Great Reed Warbler nests were collected either shortly after fledging, or after the clutch had failed. Altogether, 12 species of spider and 19 species of beetle were collected. In both groups there was no significant difference in abundance between successful, lost and cuckoo-parasitized nests; however, there was a significant difference in species richness between the three nest categories in spider assemblages, which was not the case in beetle assemblages. The amount of precipitation did not affect beetle or spider abundance; only the species richness of spiders showed significant growth. Furthermore, we found no significant relationship between vegetation cover and the species richness and abundance of spiders and beetles. The diversity of both groups differed significantly according to reed habitat: beetle assemblages were most diverse by the large canal and spiders at the mining ponds.     

Responses of breeding Reed Warblers Acrocephalus scirpaceus to mounts of Sparrowhawk Accipiter nisus, Cuckoo Cuculus canorus and Jay Garrulus glandarius

Mounts of a Cuckoo Cuculus canorus , a Sparrowhawk Accipiter nisus and a Jay Garrulus glandarius were presented at nests of Reed Warblers Acrocephalus scirpaceus at all breeding stages. Response strength could best be classified according to approach distance (to the mount) and vocalizations used. Warblers reacted much more aggressively to a mount on the nest than to one 3 m adjacent; they approached much more closely, were less likely to sing but often gave a frenzied-sounding rasp (which was never given to an adjacent mount). Reaction was more aggressive to a Cuckoo, which was often attacked, than to a Sparrowhawk, of which they were wary; response to a Jay was intermediate. Reaction was more aggressive after the clutch had been completed than before. After the brood had fledged, parents still responded strongly to the Jay and Sparrowhawk, but responded to the Cuckoo only briefly before ignoring it. These results are consistent with the Reproductive Value-Stimulus Value hypothesis. They also demonstrate that Reed Warblers can tell Cuckoos from Sparrowhawks, as can many non-host species. The close resemblance of these two species is often suggested to be mimicry of the Sparrowhawk by the Cuckoo but, if this is so, it is not successful. Alternatively, the similarity could be mimicry to reduce predation on the Cuckoos themselves, a chance resemblance, or due to similar selective forces acting on both species, as each would presumably benefit from reduced conspicuousness.     

Narrow-Front Loop Migration in a Population of the Common Cuckoo Cuculus canorus,as Revealed by Satellite Telemetry

Narrow migration corridors known in diurnal, social migrants such as raptors, storks and geese are thought to be caused by topographical leading line effects in combination with learning detailed routes across generations. Here, we document narrow-front migration in a nocturnal, solitary migrant, the common cuckoo Cuculus canorus, using satellite telemetry. We tracked the migration of adult cuckoos from the breeding grounds in southern Scandinavia (n = 8), to wintering sites in south-western Central Africa (n = 6) and back to the breeding grounds (n = 3). Migration patterns were very complex; in addition to the breeding and wintering sites, six different stopover sites were identified during the 16,000 km annual route that formed a large-scale clockwise loop. Despite this complexity, individuals showed surprisingly similar migration patterns, with very little variation between routes. We compared observed tracks with simulated routes based on vector orientation (with and without effects of barriers on orientation and survival). Observed distances between routes were often significantly smaller than expected if the routes were established on the basis of an innate vector orientation programme. Average distance between individuals in eastern Sahel after having migrated more than 5,000 km for example, was merely 164 km. This implies that more sophisticated inherent guiding mechanisms, possibly involving elements of intermediate goal area navigation or more elaborate external cues, are necessary to explain the complex narrow-front migration pattern observed for the cuckoos in this study.     

Nocturnal migratory flight initiation in reed warblers Acrocephalus scirpaceus: effect of wind on orientation and timing of migration

We used radio-telemetry to study autumn migratory flight initiation and orientation in relation to wind and air pressure in a nocturnal passerine migrant, the reed warbler Acrocephalus scirpaceus at Falsterbo, southwest Sweden. The majority of the reed warblers departed in the expected migratory direction towards south of southwest, while a low number of the birds took off in reverse directions between north and east. Flight directions at departure correlated with wind directions. These correlations were particularly prominent at higher wind speeds but were absent at wind speeds below 4 m/s. Birds departing in the expected migratory direction compensated completely for wind drift. The reed warblers preferred to depart during nights with tailwinds and when air pressure was increasing suggesting that reed warblers are sensitive to winds and air pressure and select favourable wind conditions for their migratory flights. Since air pressure as well as velocity and direction of the wind are correlated with the passage of cyclones, a combination of these weather variables is presumably important for the birds' decision to migrate and should therefore be considered in optimal migration models.