Population Of Great Reed Warblers Research Articles

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.

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

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

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.

Do the European Great Reed Warblers (Acrocephalus arundinaceus) reach South Africa during wintering?

Former studies have shown that the Great Reed Warbler (Acrocephalus arundinaceus) has two genetically distinguishable haplogroups (a ‘‘western’’ and an ‘‘eastern’’ clade). The species occurs in South Africa from January to late March, yet, in the whole database of EURING, there are no recoveries to the south of the Congo. There are at least three hypotheses concerning which birds are seen wintering in South Africa: (1) the European breeders reach South Africa, but there are no ringing recoveries; (2) a mixed population of birds originating from Turkey, Iran and Kazakhstan (Near and Middle East) winters in South Africa; and (3) birds from Europe and the Near and Middle Eastern populations both reach South Africa. We have sequenced a 492-bp part of the mtDNA control region II in 146 samples from five European breeding and one South African wintering population of Great Reed Warblers. The results show that over 60% of the wintering birds in South Africa carry European haplotypes, belonging to both ‘‘eastern’’ and ‘‘western’’ clades. Since the exact haplotypic constitution of the Near and Middle Eastern populations are not known to us, we cannot exclude that a mixed wintering population is formed from birds originating from these regions, but it seems more likely that the European breeders reach South Africa. (Less)

Choosing suitable hosts: common cuckoos Cuculus canorus parasitize great reed warblers Acrocephalus arundinaceus of high quality

We investigated the hypothesis that the common cuckoo Cuculus canorus selects host pairs of good phenotypic quality. As there is some evidence that cuckoos may select hosts within a population non-randomly based on external cues reflecting their foster abilities, we predicted that great reed warbler Acrocephalus arundinaceus pairs parasitized by the cuckoo would exhibit higher quality than unparasitized ones. To test this assumption, we evaluated two different parameters indicating host quality: body condition and characteristics of host eggs. We found that parasitized females showed significantly better body condition than unparasitized ones, and the model showed that the probability of being parasitized by the cuckoos increased with increasing body condition. Moreover, the likelihood of being parasitized by a cuckoo within the great reed warbler population increased with decreasing colour variability within clutches: parasitized females allocated costly blue pigments to eggshells more equally compared with unparasitized ones. Our study revealed that cuckoos parasitize great reed warbler females of higher quality, as reflected in host body condition and egg colour characteristics. In highly mimetic systems, cuckoos may choose to parasitize hosts with eggs displaying low intraclutch variation, both because this leads to reduced rejection and because these hosts are of high quality.

No evidence for inbreeding avoidance in a great reed warbler population

Inbreeding depression may drive the evolution of inbreeding avoidance through dispersal and mate choice. In birds, many species show female-biased dispersal, which is an effective inbreeding avoidance mechanism. In contrast, there is scarce evidence in birds for kin discriminative mate choice, which may, at least partly, reflect difficulties detecting it. First, kin discrimination may be realized as dispersal, and this is difficult to distinguish from other causes of dispersal. Second, even within small, isolated populations, it is often difficult to determine the potential candidates available to a female when choosing a mate. We sought evidence for inbreeding avoidance via kin discrimination in a breeding population of great reed warblers (Acrocephalus arundinaceus) studied over 17 years. Inbreeding depression is strong in the population, suggesting that it would be adaptive to avoid relatives as mates. Detailed data on timing of settlement and mate search movements made it possible to identify candidate mates for each female, and long-term pedigrees and resolved parentage enabled us to estimate relatedness between females and their candidate mates. We found no evidence for kin discrimination: mate choice was random with respect to relatedness when all mate-choice events were considered, and, after correction for multiple tests, also in all breeding years. We suggest that dispersal is a sufficient inbreeding avoidance mechanism in most situations, although the lack of kin discriminative mate choice has negative consequences for some females, because they end up mating with closely related males that lowers their fitness.

Partial albinism in a semi-isolated population of great reed warblers.

Albinism in birds is thought to result from the expression of recessive alleles that disrupt melanin pigmentation at feather development. We have studied great reed warblers Acrocephalus arundinaceus in a recently founded and increasing population in Sweden during 15 years for the presence of birds with albinistic feathers. The study population was founded in 1978 and the few cases of albinism was exclusively recorded during the first five years of our study (1985-1989). This fits to the expected pattern if albinism is governed by recessive alleles; we have previously demonstrated that the population suffered from inbreeding during the first years of our study. The albinistic birds experienced a similar lifetime reproductive success as normally coloured birds.

ADAPTATIONS BY GREAT REED WARBLERS TO BROOD PARASITISM: A COMPARISON OF POPULATIONS IN SYMPATRY AND ALLOPATRY WITH THE COMMON CUCKOO

Abstract Egg rejection behaviour, as an antiparasite defence against brood parasitism was compared in two great reed warbler Acrocephalus arundinaceus populations. In central Hungary the host population lives in sympatry with the common cuckoo Cuculus canorus, and it is heavily parasitised (ca 66%). In central Greece the great reed warbler occurs in allopatry with the cuckoo, so it is not parasitised at all. Experimental parasitism with poorly mimetic (often called 'non-mimetic') artificial cuckoo eggs revealed a 100% rejection rate towards the foreign eggs in the Greek population, but the Hungarian hosts accepted 29.4% and rejected 70.6% of the model cuckoo eggs. No other rejection type than ejection was observed. The significantly higher level of rejection in Greece than in Hungary suggests that the Greek great reed warbler population is an abandoned host population of the cuckoo, which kept its egg recognition ability. A computer-based image analysis of host's clutches revealed that interclutch variation in egg colour was greater than intraclutch variation in the Hungarian population, but not in the Greek population. Spottedness of eggs were similar in both populations. Generally, a tendency for a higher interclutch variation was found in Hungary. Intraclutch variation was more similar in the two populations, but for the colour variables it showed a lower value in Hungary than in Greece. The heavy cuckoo parasitism in Hungary is suggested to be the main cause for the increased interclutch variation in relation to intraclutch variation in egg appearance among great reed warbler clutches. The high interclutch variation reduces the chance that the parasite egg's appearance matches that of the hosts', which facilitates hosts' egg discrimination.

Frequencies of multiple paternity in three Acrocephalus species (Aves Sylviidae) with different mating systems (A. palustris, A. arundinaceus, A. paludicola)

Genetic parentage was determined by DNA-fingerprinting with multilocus probes and microsatellite PCR in three Acrocephalus species with different mating systems: the socially monogamous marsh warbler (A. palustris), the polygynous great reed warbler (A. arundinaceus) and the promiscuous aquatic warbler (A. paludicola). The amount of extra-pair paternity (EPP)/multiple paternity (MP) was then related to ecological parameters (increasing productivity) and parental care (decreasing male participation) of Acrocephalus species. Marsh warblers (33 broods, 131 young plus putative parents) were studied over 3 years in a German population. No case of social polygyny but four extrapair young (EPY) were detected in three socially monogamous broods. The low frequency of extra-pair fertilization (EPF, 9.1% of broods, 3.1% of offspring) may be best attributed to intense male mate-guarding and restrictions on mate assessment due to an extremely short breeding window. The rate of social polygyny in a German great reed warbler population varied between 8 and 35% of males during a 5-year period. In 48 broods (27 monogamous and 21 polygynous males) with 194 young, 5 nests (3 of polygynous, 2 of monogamous males) with 19 EPY were recorded: EPF-rates are 10.4% of broods and 9.8% of offspring. In none of the nests were neighbouring males the genetic fathers, but in 2 nests, “bachelor” males from distant territories had sired the offspring. The EPP-rate in this species seems to be related to an incomplete correlation between male and territory characteristics leaving some females to correct their choice. Aquatic warblers were studied during three years in a Polish population. In each of the years, MP was recorded in more than half of a total of 75 broods (mean 76%). The proportion out of 340 young sired by one to five “additional” males varied between 25 and 57% (mean 39.1%). Food resources are neither critical nor economically defendable in the species' habitat and paternity assurance behaviours of males and female choice are yet not fully understood.

Do females adjust the sex of their offspring in relation to the breeding sex ratio?

Abstract When the adult sex ratio differs between years in local populations, but still is predictable between adjacent years, it has been proposed that the best strategy would be to bias the offspring sex ratio in favour of the rare sex. We tested this hypothesis using a data set of great reed warbler offspring, sexed by molecular techniques, that were collected over 11 breeding seasons at two adjacent reed marshes. Three important assumptions for this hypothesis are fulfilled in the studied great reed warbler population. First, a substantial proportion of great reed warblers are living in small local populations where sex ratio distortions would be sufficiently large and common. Second, breeding adults and their offspring return to breed in the local population to a high degree. Third, females have a possibility to assess the breeding sex ratio before laying their eggs. At our study site, the breeding sex ratio was positively correlated between successive years. However, contrary to our prediction, female great reed warblers seemed not to adjust their offspring sex ratio in relation to the local breeding sex ratio.

Frequency of complete moult in adult and juvenile Great Reed Warblers(Acrocephalus arundinaceus) in Spain

We examined a large data set of adult and juvenile Great Reed Warblers (Acrocephalus arundinaceus), caught over five years, to study the occurrence of complete moult in a Spanish population. A total of 27 adults and 5 juveniles were in active moult. The proportion of birds moulting each year varied between 0.03 and 0.19. Nearly half the adults were finishing moult. At least one bird was certainly of local origin, rather than on passage, and four more were moulting when retrapped in subsequent years, suggesting that they too were resident breeders in the area. Some other birds were probably resident also, because they were in moult when caught at the beginning of August. Two of the five juveniles finished their moult in the area. In conclusion, our data seem to show that complete moult before departure to the wintering quarters is a regular process undertaken by some of the breeding population of Great Reed Warblers in north-east Spain.

Higher Fitness for Philopatric than for Immigrant Males in a Semi-Isolated Population of Great Reed Warblers

Higher Fitness for Philopatric than for Immigrant Males in a Semi-Isolated Population of Great Reed Warblers

Demography and Lifetime Reproductive Success in the Polygynous Great Reed Warbler

I analysed demographic parameters and lifetime reproductive success in a Swedish population of Great Reed Warblers. The population was followed on a daily basis in 1985-1993 when all breeding Great Reed Warblers were individually marked and >95% of the young fledged in the study area were ringed. Between-year breeding site fidelity of adults was relatively high both for males (55%) and females (51%), and it seemed not to be affected by previous breeding results. Natal site fidelity was also relatively high as 14% of the fledglings hatched in the study area returned to breed in subsequent years. Each year, however, about 50% of the breeders were hatched outside the study area (i. e. immigrants). The production of lifetime recruits was biased so that 17% of the males and 13% of the females produced more than 50% of the recruits in the population. There was no difference in lifetime reproductive success between native birds and immigrants, suggesting that immigrants contributed substantially to the local gene pool. Despite this, effects of non-incestuous inbreeding, measured as lowered hatchability of eggs among broods of genetically more similar pair members, were found in the study population. This may be a result of the dispersal pattern between local Great Reed Warbler populations in Sweden and also a consequence of the short history of these small populations.

Female Settling Pattern and Polygynous Breeding in a Great Reed Warbler Population in Aomori Prefecture, Northern Japan

Female Settling Pattern and Polygynous Breeding in a Great Reed Warbler Population in Aomori Prefecture, Northern Japan

Female settling pattern and polygynous breeding in a Great Reed Warbler population in Aomori Prefecture

Female settling pattern and polygynous breeding in a Great Reed Warbler population in Aomori Prefecture

Age and Other Factors Influencing Mating Status in German Great Reed Warblers (Acrocephalus arundinaceus)

We examined reproductive success of monogamous, primary and secondary females as well as factors influencing male mating status in a polygynous Great Reed Warbler population by analysing long term data. The analysis is based on 428 nests of which at least one parent was individually colour-ringed. The number of males studied over a 15 year period averaged 30/yr. of which 11.3% were polygynous and 13.9% unmated. Primary broods of polygynists started 10 days earlier than broods of secondary females. Average fledging success of secondary females was lower than that of monogamous females (relative success 0.79) but did not differ significantly from that of simultaneously monogamous females (relative success 0.85). The percentage of fledglings recruited to the breeding population overall was highest in primary broods, but did not differ significantly between monogamous and secondary broods. Mating status of males was influenced by age, with a higher proportion of older males becoming polygynous whereas correlations between age and mating status of females was not significant. We also investigated the relative importance of male and territory quality in female choice by analysing 3 territorial and 11 male characteristics (physical traits, age, repertoire size, aggressiveness, hormones) of 29 males (4 polygynous, 3 unmated, 22 monogamous) using a discriminant analysis. The best predictors for polygyny were a long reed edge against open water, lower aggressiveness, and a higher song repertoire size. We conclude (1) that it is not necessary to invoke female deception, which had been favoured in explaining polygyny in our population (2) that ecological conditions for polygyny are suboptimal in our study area but there seem to exist enough predictable differences in breeding situations to allow female choice and (3) that females base their mate choice on both male characteristics and territory quality, and that these factors are positively correlated.

Polygyny in the great reed warbler, Acrocephalus arundinaceus: a possible case of deception

Polygyny was studied for 1 year in a great reed warbler population breeding in southern Germany. Data on various parameters of male and territory characteristics were collected and subjected to multivariate statistical analysis in order to assess their relative importance in female choice. Differences in territory characteristics appeared to be more important (Tab. 1), but there was a strong correlation between male and territory characteristics (Fig. 2, 3). Although the evolutionary advantages of polygyny to males are obvious (Fig. 5), those to females are far from clear. Models based upon the polygyny threshold and sexy son hypotheses predict that females should gain evolutionary advantage in either the short or long-term. Our data did not confirm such predictions, and secondary females showed greatly reduced breeding success (Fig. 4). If females were able to assess the status of mated males, they should instead select unmated males. Great reed warblers are a migrant species with a short breeding cycle in which the male plays relatively little part. They defend large territories in reed beds where visibility is reduced. These factors may permit males to practice a form of deception, by moving and attracting a second female who has little chance to assess that he is already mated.