The Large-billed Reed Warbler Acrocephalus orinus revisited

The Large-billed Reed Warbler Acrocephalus orinus is known only from the type specimen, collected in Himachal Pradesh, India, in 1867. The specimen is poorly prepared, and it has been suggested that it could represent an isolated form of the Clamorous Reed Warbler A. stentoreus or an aberrant Blyth's Reed Warbler A. dumetorum. We tested the affinity of A. orinus by (1) re-examining the morphology of the type specimen and (2) amplifying and sequencing a portion of its mitochondrial cytochrome b gene. Both the morphological and the mitochondrial analyses showed the specimen to be similar to dumetorum, but distinct enough to qualify as a species of its own. Relative to dumetorum, it has a more rounded wing, longer bill, longer and more graduated tail with more pointed tail feathers, and larger claws. The divergence in mitochondrial DNA between orinus and dumetorum was 7.8%, well above the value expected between subspecies. A. orinus is smaller than any of the forms of A. stentoreus or the related Australian Reed Warbler A. australis. It has a somewhat longer first primary, more pointed tail feathers and paler, less robust feet and claws. DNA comparison places it in the clade of small unstreaked Acrocephalus warblers, and apart from the clade of large unstreaked warblers that contains stentoreus and australis

Predictors of natal dispersal in great reed warblers: results from small and large census areas

For most species, it is currently not known whether individual characteristics and population parameters affecting dispersal distances within the local population are also affecting dispersal of individuals emigrating longer distances. We evaluated this in the great reed warbler Acrocephalus arundinaceus by analysing data of natal dispersal distances within (1) the local hatching area in southern Central Sweden and (2) the species' whole breeding range in Sweden. The different scales gave conflicting results. Site of birth was the main predictor of natal dispersal distance in the local census area, whereas in the larger census area the dispersal distance differed significantly between hatching years. Previous studies have shown that the recruitment rate of great reed warblers within the local study area differs between hatching years and also that it declines during the course of the breeding season. It was not known, however, whether these relationships reflected variation in survival or dispersal. The results from the present study suggest that the differences in local recruitment rate between hatching years is partly caused by annual variation in emigration rate, whereas the decline in local recruitment rate with season mainly reflects mortality of late-hatched nestlings. Our study shows a twofold advantage of a large census area when studying dispersal. First, predictors of longer-distance dispersal will be revealed and, second, the variation in local recruitment rate can be separated into the two components, survival and dispersal

Range expansion and the possibility of an emerging contact zone between two subspecies of Chiffchaff Phylloscopus collybita ssp.

The Chiffchaff Phylloscopus collybita is represented in Sweden by two different subspecies; the northern well-established abietinus and the southern recently established collybita which has expanded its range northward during the past two decades. At present, an area approximately 500 km wide separates the two subspecies. In order to document differences between the northern and southern populations we compared morphology, vocalisation, habitat choice, and neutral genetic variation in mitochondrial (mt) DNA and at four microsatellite loci of 30 male Chiffchaffs from each subspecies. Our results show significant differences in several morphological traits and in song. Playback experiments revealed a significant difference in aggressive behaviour depending on which population-specific song that was played to the birds. Mitochondrial DNA was geographically structured with similar to 90% of the birds carrying a mt haplotype matching their sample population. No allelic differences at the microsatellite loci were found between populations. Our data demonstrate a substantial differentiation between the northern and southern populations despite gene flow, clearly separating them into the subspecies abietinus and collybita

The use of AFLP to find an informative SNP: genetic differences across a migratory divide in willow warblers

We used the amplified fragment length polymorphism (AFLP) method to obtain genetic markers distinguishing two subspecies of willow warblers Phylloscopus trochilus that have different migratory behaviours but are not differentiated in mitochondrial DNA or at several microsatellite loci. With the inverse-polymerase chain reaction (PCR) approach we converted a dominant AFLP-marker to a codominant single nucleotide polymorphism (SNP). Across Scandinavia we typed 621 birds at the SNP locus AFLP-WW1 and we found a sigmoid change in allele frequencies centred around 62 degrees latitude. North of the latitudinal cline was a west-east cline. Both clines are narrower than one would expect from dispersal distances in willow warblers, which suggests that these are maintained by selection. The latitudinal cline at the locus AFLP-WW1 is paralleled by changes in several other traits, all of which might be maintained by a single selective force. The most plausible selection factor that we have identified is selection against hybrids because of inferior migratory behaviour. The selective force maintaining the east-west cline is less obvious. We discuss alternatives to the selection scenario, involving colonization history and asymmetric gene flow

Patterns of nest predation contribute to polygyny in the Great Reed Warbler

According to the polygyny threshold model, females are compensated for the cost of sharing a territory with other females by breeding in territories of higher quality than those of monogamously mated females. In the polygynous Great Reed Warbler (Acrocephalus arundinaceus), the variation in territory quality may be associated with nest site characteristics or food supply. In this study, we examined the probability of nest predation in Great Reed Warblers in relation to an indirect measure of territory quality (territory attractiveness rank as indexed by settlement order) and to several variables of nest site structure. Artificial nests with attached soft plasticine model eggs were placed in Great Reed Warbler territories of different attractiveness; Marks in the soft model eggs were compared to marks from possible predator species. Both inter- and intraspecific predators were found to prey upon eggs in artificial nests. Marsh Harriers (Circus aeruginosus) predominantly preyed upon artificial nests located in territories of low attractiveness, in low-density reed beds, and early in the season. Bitterns (Botaurus stellaris) and Water Rails (Rallus aquaticus) showed a preference for artificial nests located inside the reeds. When all types of predators of artificial nests were pooled, the probability of nest predation was lower in more attractive territories. This pattern was corroborated in an analysis of predation on real nests. In general, polygyny occurred in territories with a low rate of nest predation, whereas unmated males held territories where the risk of predation was high. The low rate of nest predation in attractive territories on average compensated secondary females for the cost associated with sharing a male (i.e., reduced male assistance when feeding nestlings). The polygyny threshold model has been around for several decades, however, only few studies have identified the factors that may compensate secondary females for the cost of sharing a mate. In the present study we were able to show that the patterns of nest predation can be an important territory quality parameter that compensate secondary females, and, hence, contribute to the occurrence of avian polygyny

Heritability of dispersal in the great reed warbler

Dispersal is commonly considered to be a condition-dependent behaviour with no or low heritability. Here, we show that dispersal in the great reed warbler (Acrocephalus arundinaceus ) has a high heritability. Analyses of capture-recapture data of male great reed warblers gathered from the species' whole Swedish breeding range revealed a remarkable offspring-parent resemblance in dispersal behaviour (philopatry vs. inter-population dispersal). Also, the degree of dispersal differed between cohorts, which shows that dispersal was partly conditionally dependent. The offspring to mid-parent estimate of heritability was 0.50. In a previous study of the same data set of male offspring, we did not detect associations between dispersal and several relevant environmental, parental and offspring condition factors. Thus, our results indicate that variation in dispersal partly has a genetic basis in great reed warblers

A new approach to study dispersal: immigration of novel alleles reveals female-biased dispersal in great reed warblers

We use the assignment technique and a new approach, the 'novel allele technique', to detect sex-biased dispersal in great reed warblers Acrocephalus arundinaceus . The data set consisted of immigrants and philopatric birds in a semi-isolated population in Sweden scored at 21 microsatellite loci. Fourteen cohorts were represented of which the four earliest were used to define a reference population. Female immigrants had lower assignment probability than males (i.e. were less likely to have been sampled in the reference population), and carried the majority of 'novel alleles' (i.e. alleles observed in the population for the first time). The difference in number of novel alleles between sexes was caused by a strong over-representation of females among the few individuals that carried several novel alleles, and there was a tendency for a corresponding female bias among individuals with low assignment probabilities. Immigrant males had similar or lower reproductive success than females. These results lead us to conclude that important interregional gene flow in great reed warblers depends on relatively few dispersing females, and that the novel allele technique may be a useful complement to the assignment technique when evaluating dispersal patterns from temporally structured data

Between-year variation of MHC allele frequencies in great reed warblers: selection or drift?

The major histocompatibility complex (MHC) genes are extremely polymorphic and this variation is assumed to be maintained by balancing selection. Cyclic interactions between pathogens and their hosts could generate such selection, and specific MHC alleles or heterozygosity at certain MHC loci have been shown to confer resistance against particular pathogens. Here we compare the temporal variation in allele frequencies of 23 MHC class I alleles with that of 23 neutral microsatellite markers in adult great reed warblers (a passerine bird) in nine successive cohorts. Overall, the MHC alleles showed a significantly higher variation in allele frequencies between cohorts than the microsatellite alleles, using a multi-variate genetic analysis (AMOVA). The frequency of two specific MHC alleles, A3e (P = 0.046) and B4b (P = 0.0018), varied more between cohorts than expected from random, whereas none of the microsatellite alleles showed fluctuations exceeding the expectation from stochastic variation. These results imply that the variation in MHC allele frequencies between cohorts is not a result of demographic events, but rather an effect of selection favouring different MHC alleles in different years