The biannual primary moult of Willow Warblers Phylloscopus trochilus in Europe and Africa

The Willow Warbler Phylloscopus trochilus is one of the few bird species that undergoes two primary moults a year, a post-nuptial moult in the breeding area and a moult in the wintering area. Primary-moult data for Willow Warblers from Finland, Sweden, Britain, the Netherlands, Belgium. Guinea-Bissau, Uganda, Kenya, Malawi, Zambia, Zimbabwe, Botswana and South Africa are analysed. The parameters of primary moult (mean starting date, standard deviation of starting date, and duration) are estimated using the techniques of Underhill & Zucchini (T.988 Ibis 1 30: 358–372) and Underhill, Zucchini & Summers (1990 Ibis 132: 118-12 3). The scheduling of moult in relation to theother main components of the annual cycle, breeding and migration, is considered. The mean durations of post-nuptial moult for P. t. trochilus and P. t. acredula are 36.5 and 38.3 days, respectively; the start and termination of moult for P. t. trochilus are about 3.5 days later for each degree of latitude northwards, and the start and termination of moult for P. t. acredula, are about 10 days later than that of the most northerly populations of P. t. trochilus studied. Females start their postnuptial moult about 10 days later than males. Southward migration commences as soon as post-nuptial moult is complete. There is an increasing constraint on the timing of breeding and post-nuptial moult events at higher latitudes, leading to overlap between them. The duration of pre-nuptial moult is longer than that of post-nuptial moult, and is completed shortly prior to northward migration.     

Habitat selection, foraging niches and horizontal spacing of Willow Warbler Phylloscopus trochilus and Chiffchaff P. collybita in an area of sympatry

Habitat selection of the Willow Warbler and the Chiffchaff was studied along a succession of grey alder forest in Central Norway (63^oN) where both species breed. Both were found in all serai stages, but there was a slight tendency for the Willow Warbler to be found more often in the voungest stages than was the Chiffchaff.
Foraging niches and horizontal spacing were studied within one particular forest. Most of the records of both Willow Warbler and Chiffchaff came from the tree layer, and the overlap between the species in their foraging niches was considerable. However, the species were found to occupy almost mutually exclusive territories.     

Using stable isotopes to link breeding population trends to winter ecology in Willow Warblers,Phylloscopus trochilus

Capsule Stable isotope composition of feathers grown in Africa indicates that Willow Warblers, Phylloscopus trochilus, breeding in areas of Britain experiencing different population trends may vary in their distribution, timing of moult and/or use of prey resources during the non-breeding season.

Aims To compare stable isotope ratios of feathers of Willow Warblers breeding in parts of Britain with differing rates of population change.

Methods Feathers were collected from three regions with differing recent population trends; northern Scotland (population increased), western Wales (declined slightly) and eastern England (declined rapidly). Collection occurred at ringing sites run by British Trust for Ornithology (BTO) volunteers as part of the Constant Effort Sites (CES) scheme between April and July in 2008 and 2009.

Results Willow Warblers from northern Scotland had significantly higher δ ¹⁵N and δ ¹³C values than those from eastern England, although the regional mean values did not differ greatly. There was no significant difference in δ ¹⁵N and δ ¹³C values between the sexes, but δ ¹⁵N values were significantly greater in individuals caught later in the breeding season.

Conclusions During the premigratory moult period in Africa, Willow Warblers from different parts of Britain may vary in their distribution, timing of moult and/or use of prey resources.     

Microgeographic Variation, Habitat Effects and Individual Signature Cues in Calls of Chiffchaffs Phylloscopus collybita canarensis

Variation of signals is a widespread feature in animal communication. Signals usually vary among signallers and in some cases vary across geographic areas. Individual variation provides the basis for individual recognition and thus has important implications for social interactions. Microgeographic variation can reveal insights into patterns of dispersal and into timing and mechanism of acquisition or development of behavioral traits. In addition, it may reflect adaptations of signals to the transmission characteristics of local habitat types. In song birds these kinds of variation have been documented in particular for male song. Here, we examined individual signature cues, site-specific variation, and variation among habitat types in tonal contact calls of 26 chiffchaffs, Phylloscopus collybita canarensis , recorded on two Canary islands. Multiparametric analyses of calls and subsequent discriminant function analyses revealed clear individual differences as well as microgeographic variation in call structure. Call structures differed not only between islands but also among the different locations on Tenerife, indicating that individuals on the same island do not share the same call, as suggested earlier. Calls recorded in different types of habitat, however, did not differ in structure as predicted by the 'acoustic adaptation hypothesis'. The findings indicate that individual recognition may be possible on the basis of calls and they contribute to the understanding of development of behavioural trails in relation to life history patterns such as the timing and pattern of dispersal.     

Woodland structure,rather than tree identity,determines the breeding habitat of Willow Warblers Phylloscopus trochilus in the northwest of England

Capsule Woodland structure, rather than tree species, is the most important determinant of breeding habitat selection by Willow Warblers in North West England.

Aims To examine how habitat characteristics predict the occurrence of male Willow Warbler territories.

Methods Woodland structure (trunk density, trunk diameter, canopy cover and understory cover), tree species and food abundance were compared between woodland areas within and outside of male territories at a site in the UK.

Results Territories contained higher trunk numbers, had a narrow range of trunk diameters, and intermediate canopy cover. Food abundance did not differ with occupancy. Willow and alder were the most common trees within territories, in contrast to birch which has been found in previous studies. The habitat structure matches young woodlands, where birches often grow. However, at the study site the birches were large and mature, and therefore unsuitable. Moreover, woodland structure variables were better predictors of occupancy than any particular tree genera.

Conclusion The results indicate that vegetation structure, but not tree species or food availability, influence breeding habitat selection by Willow Warblers. The preferred structure is similar to coppice woodlands; therefore, the Willow Warbler decline may be linked to the loss of this traditional management across south England.     

The presence of non-territorial males in Willow Warbler Phylloscopus trochilus populations–a removal study

A total of six (unmarked) and 24 (individually colour-ringed) male Willow Warblers Phylloscopus trochilus were removed from their territories in two study areas: subalpine birch and lowland mixed forest, respectively. The removals were made during three separate periods throughout the breeding season: (1) pair-formation and egg-laying; (2) early incubation; and (3) middle and second half of the incubation period. During Period 1 all of the males removed ( n = 10) were quickly replaced by new males, while during Period 2 eight of the 11 males removed were replaced. The new males defended basically the same territorial borders as the males they replaced. During Period 3 only one of the nine males removed was replaced. Since the replacements occurred within a few hours of the removals, it is suggested that the new settlers were non-territorial males that were already present in the respective areas before the experimental removals were made. The mean wing-lengths of the two groups of males indicated that the initial occupants of the territories were 2 years old or older in both study areas, whereas the new settlers in the subalpine birch forest (mean Willow Warbler density: 55 territories/km²) were 1-year-old birds, and two years old, or older, in the mixed forest area (mean Willow Warbler density: 133 territories/km²).     

Differential migration of chiffchaffs Phylloscopus collybita and P. ibericus in Europe and Africa

Differential migration is a widespread, but poorly understood, phenomenon in birds. In this paper, we present the first detailed field study of differential migration in the Old World warbler (Sylviidae) family. We studied two chiffchaff Phylloscopus [collybita] semispecies: the common chiffchaff P. [c.] collybita and the Iberian chiffchaff P. [c.] ibericus. Using data collected at several latitudes in Europe and Africa, we present convincing evidence for differential distance migration of sexes in chiffchaffs, with females moving further than males. Interestingly, while there was a pronounced gradient in the sex‐ratios in Europe and North Africa (with an increasing proportion of females with declining latitude), no clear pattern was found south of the Sahara, where sex‐ratios were more male‐biased than predicted by a simple latitude model. This suggests that, amongst the chiffchaffs wintering in West Africa, a large proportion is composed by Iberian birds, and provides support to previous suggestions that Iberian chiffchaffs are long distance migrants. Results from detailed studies in Senegal also show that chiffchaffs display differential timing of spring migration, with males leaving the winter quarters considerably earlier than females. The results are discussed in the framework of the three main (non‐mutually exclusive) hypotheses attempting to explain the latitudinal segregation of the sexes. Given the relative failure of standard comparative studies to discriminate between competing single‐factor hypotheses to explain differential migration, it is argued that the chiffchaff species complex might be particularly suited to study this issue using a new approach suggested by Cristol et al. (1999) : detailed (further) comparisons between closely related species (such as the common and the Iberian chiffchaffs) could help identifying the key factors to be incorporated into optimality models that can predict relative distance of migration of different sex or age classes.     

Willow Warbler Phylloscopus trochilus habitat in woods with different structure and management in southern England

Capsule Use of Light Detection and Ranging (LiDAR) data identified suitable Willow Warbler habitat based on mean vegetation height. This habitat model provided maps of distribution and occupation of suitable habitat.

Aims To identify habitat associations in woods with different vegetation structure and management systems during a period of low Willow Warbler populations.

Methods Locations of all Willow Warblers were mapped during the breeding season in three woods of contrasting management; recent low intervention, actively coppiced woodland and high forest with clear‐fells. Height profile models of each wood were derived from airborne LiDAR. The mean vegetation height at locations with Willow Warblers and a sample from the rest of the wood were used to produce models of optimum habitat and breadth of habitat occupied in each wood. The habitat model was then used to produce maps of suitable habitat.

Results The habitat models did not differ between woods, with highest probability of Willow Warbler occurrence in mean vegetation heights of 3.7–5.3 m. Habitat of heights 6–11 m appeared less suitable, being only partly occupied. Habitat maps showed that habitat of suitable height was only occupied when it occurred as large patches; smaller patches (mostly <0.5 ha) and edges along rides and fields were not used.

Conclusion The use of LiDAR derived measures of vegetation height identified areas of suitable habitat for Willow Warblers. Willow Warblers occupied areas of low mean vegetation height either as early successional or open canopy woodland in all woods. Height‐based habitat maps can identify areas of suitable habitat within larger expanses of heterogeneous woodland and are a potentially useful tool in assessing changes in extent of what are often temporary patches of habitat.     

Moult, flight muscle "hypertrophy" and premigratory lipid deposition of the juvenile Willow warbler, Phylloscopus trochilus

In England juvenile Willow warblers undergo premigratory lipid deposition in August. Prior to this there is a moult of the juvenile plumage which results in a reduction in both the glycogen content and the wet weight of the pectoralis muscles, the latter reflecting a decrease in the lipid-free dry weight. Muscle lipid levels are apparently unaffected by this moult. The decrease in the weight of the lipid-free dry fraction of the body extends also to components other than these muscles at this time. A correlation between the wet weight and the glycogen content of the pectoralis muscles suggests that cold stress may be the cause of the low glycogen levels found in the middle of the moult; part of this increased demand for thermogenesis may be due to lower body insulation brought about by feather loss during the moult. The muscle "hypertrophy" found in the premigratory period is simply a return to pre-moult weights.
The uniformity of total body weights during and after the moult indicates that at the end of this moult the water fraction of the body decreases in weight. The pectoralis muscles of juvenile Willow warblers are not fully grown on fledging; these unmoulted birds may also have higher body lipid levels than during the following moult.     

Patterns of genetic,phenotypic, and acoustic variation across a chiffchaff (Phylloscopus collybita abietinus/tristis) hybrid zone

Characterizing patterns of evolution of genetic and phenotypic divergence between incipient species is essential to understand how evolution of reproductive isolation proceeds. Hybrid zones are excellent for studying such processes, as they provide opportunities to assess trait variation in individuals with mixed genetic background and to quantify gene flow across different genomic regions. Here, we combine plumage, song, mtDNA and whole‐genome sequence data and analyze variation across a sympatric zone between the European and the Siberian chiffchaff (Phylloscopus collybita abietinus/tristis) to study how gene exchange between the lineages affects trait variation. Our results show that chiffchaff within the sympatric region show more extensive trait variation than allopatric birds, with a large proportion of individuals exhibiting intermediate phenotypic characters. The genomic differentiation between the subspecies is lower in sympatry than in allopatry and sympatric birds have a mix of genetic ancestry indicating extensive ongoing and past gene flow. Patterns of phenotypic and genetic variation also vary between regions within the hybrid zone, potentially reflecting differences in population densities, age of secondary contact, or differences in mate recognition or mate preference. The genomic data support the presence of two distinct genetic clades corresponding to allopatric abietinus and tristis and that genetic admixture is the force underlying trait variation in the sympatric region—the previously described subspecies (“fulvescens”) from the region is therefore not likely a distinct taxon. In addition, we conclude that subspecies identification based on appearance is uncertain as an individual with an apparently distinct phenotype can have a considerable proportion of the genome composed of mixed alleles, or even a major part of the genome introgressed from the other subspecies. Our results provide insights into the dynamics of admixture across subspecies boundaries and have implications for understanding speciation processes and for the identification of specific chiffchaff individuals based on phenotypic characters.     

Die Dauer der Gro?fiedermauser beim Fitis(Phylloscopus trochilus) und beim Zilpzalp(Ph. collybita) unter verschiedenen photoperiodischen Bedingungen

Ohne Zusammenfassung     

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 ∼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 .     

Asymmetric size and shape variation in the Central European transect across the house mouse hybrid zone

We studied asymmetric variation of the mandible in the Central European portion of the hybrid zone between two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus. Within introgression classes, defined by the share of diagnostic allozymes, we quantified the directional and fluctuating component of asymmetric variation, as well as skewness and kurtosis of individual asymmetry distributions. Furthermore, in the same manner we re‐analysed asymmetric variation of the ventral side of the skull. According to the quadratic polynomial model, the mandible shape‐fluctuating asymmetry, but not size‐fluctuating asymmetry, was significantly decreased in the centre of the hybrid zone (with a minimum predicted for a hybrid index of 0.41). On the contrary, the skull shape‐fluctuating asymmetry non‐monotonically increased towards the musculus side of the hybrid zone (with a peak predicted for a hybrid index of 0.86). Thus, the impact of hybridization on fluctuating asymmetry is trait‐specific in this portion of the house mouse hybrid zone. The only general feature of asymmetric variation we observed was the shift towards the platykurtosis of asymmetry distributions in the centre of the hybrid zone. Taken together, we suggest genetic variability for right–left asymmetries to be generally increased, but the developmental instability of mandible shape to be decreased, by hybridization. We hypothesize the decrease of developmental instability to be caused by overdominant effects on developmental dynamics rather than by increased heterozygosity. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 13–27.     

Polygyny in Willow Warblers <Emphasis Type="Italic">Phylloscopus trochilus</Emphasis> (Sylviidae,Passeriformes) in Middle European Russia: Differences in songs of polygynous and monogynous males

The study was performed in May to July of 1995 in Middle Russia (Skadovskii Zvenigorod Biostation, MSU, Moscow oblast: 55°44′ N, 36°51′ E). Monoterritorial polygyny was observed in one of seven studied Willow Warbler males Phylloscopus trochilus. The distance between the nests of the first and second females was 90 m. In the nest of the first female, five nestlings hatched on June 10 and the flight of the fledglings occurred on June 23. In the nest of the second female, a 5-to 6-day-old nestling of the cuckoo Cuculus canorus was found on June 18. The male actively participated in feeding the young in the first nest and did not participate in the feeding of the young in the second nest. The songs of the bigamous male differ significantly from the songs of monogamous males in the middle and final parts of the breeding cycle. The song of the bigamous male in the idle and end of the breeding season is similar to that in the beginning of the breeding season. Unmated males in the middle of the breeding season also retain a more permanent structure of their songs in comparison with breeding males. This case of polygyny seems to be related to special characteristics of the males such as their singing activity and the structure of their songs.     

Estimation of the Genetic Diversity of Willow Warbler Populations of the Subspecies <Emphasis Type="Italic">Phylloscopus trochilus acredula</Emphasis> (L.) in Different Parts of Its Nesting Area in the European Part of Russia

The genetic diversity (GD) of the Phylloscopus trochilus acredula subspecies populations was studied in different parts of the breeding area in the European part of Russia using microsatellite analysis. It was detected that the level of GD sufficient for the preservation of population genetic structure of the species with optimal ratio of intra- and interpopulation components of the gene diversity and minimal deviations from panmixia is maintained in the studied populations. The maximal values of GD indices were registered in an ecologically suboptimum zone on the periphery of the breeding range.     

Regional variation in canopy transpiration of Central European beech forests

Forest hydrologists have hypothesised that canopy transpiration (E_c) of European temperate forests occurs at rather similar rates in stands with different tree species and hydrologic regimes. We tested this hypothesis by synchronously measuring xylem sap flow in four mature stands of Fagus sylvatica along a precipitation gradient with the aim (1) of exploring the regional variability of annual canopy transpiration (E_c(t)) in this species, and (2) of analysing the relationship between precipitation (P) and E_c(t). E_c(t) rates of 216, 225, 272 and 303 mm year^–1 corresponded to precipitation averages of 520, 710, 801 and 1,040 mm year^–1 in the four stands. We explored the regional variability of E_c(t) in Central European colline to sub-montane beech stands in two meta-analyses based on (1) existing sap flow data on beech (n=5 observations), or (2) all canopy transpiration data on beech obtained by different techniques (sap flow, micrometeorological or soil water budget approaches, n=25). With a coefficient of variation (CV) of 20%, the regional variability of E_c(t) (213–421 mm year^–1) was smaller than the variation in corresponding precipitation (550–1,480 mm year^–1). The mean E_c(t) for beech was 289 (±58) mm year^–1 (n=25). A humped-shaped relationship between E_c(t) and P, with a broad transpiration maximum in the precipitation range from ca. 700 to 1,000 mm year^–1, was found which may indicate soil moisture limitation of transpiration for P –1, and reduced transpiration by increased cloudiness or leaf wetness for P>1,000 mm year^–1. Thus, the precipitation level significantly influences canopy transpiration of humid temperate forests; however, the size of the P influence on E_c(t) and, in part, the direction of its effect differ from forests in semi-arid or arid climates. European beech has the capacity to maintain high E_c rates in both humid and partly dry summer climates (P<550 mm year^–1).     

Genetic and morphological variation in a common European cyprinid, Leuciscus cephalus within and across Central European drainages

Genetic variability and differentiation of the common European cyprinid Leusciscus cephalus was investigated within and across the drainages of the rivers Rhine, Elbe and Danube using starch gel electrophoresis and morphological characters. Eleven of 24 presumptive enzyme loci showed some allozymic variation. Compared to other freshwater fish, genetic variability within all sites was high ( H _e=0·074–0·113). This was explained by the wide geographical and ecological range of the chub. In contrast, genetic and morphological differentiation between sites and drainages was low (F_ST=0·15), which may be due to the high dispersal ability of the chub and translocations of spawn by waterfowl. The phylogeographic structure of Leuciscus cephalus in north-eastern Bavaria showed low congruence with Cottus gobio populations sampled in the same area.