Patterns of stable isotope signatures in willow warbler Phylloscopus trochilus feathers collected in Africa

We conducted stable isotope analyses of nitrogen and carbon on feathers obtained from willow warblers in Africa to find an explanation for a previously observed pattern of different δ¹⁵N and δ¹³C values across a migratory divide in central Scandinavia. A new data set confirms that north Scandinavian birds of the subspecies P. t. acredula have higher δ¹⁵N values than south Scandinavian birds of the subspecies P. t. trochilus . In Africa, we found significant differences for both δ¹⁵N and δ¹³C values among feathers collected from major geographical regions as well as between countries within regions. Isotope signatures of δ¹⁵N and δ¹³C in Scandinavian acredula matched well with those of willow warblers sampled in southern parts of Africa, but differed from samples obtained in East and West Africa. Isotope signatures in Scandinavian trochilus did not agree with the pattern in any of the three African regions (West, East or South). However, a more detailed analysis of the isotopic data in feathers from countries within West Africa, which is the wintering region of Swedish trochilus based on ringing recoveries, revealed a correspondence with samples from Liberia, the Ivory Coast and Nigeria.     

Characterization of a divergent chromosome region in the willow warbler Phylloscopus trochilus using avian genomic resources

Genome scans have made it possible to find outlier markers thought to have been influenced by divergent selection in almost any wild population. However, the lack of genomic information in nonmodel species often makes it difficult to associate these markers with certain genes or chromosome regions. Furthermore, the extent of linkage disequilibrium (LD) in the genome will determine the density of markers required to identify the genes under selection. In this study, we investigated a chromosome region in the willow warbler Phylloscopus trochilus surrounding a single marker previously identified in a genome scan. We first located the marker in the assembled genome of another species, the zebra finch Taeniopygia guttata, and amplified surrounding sequences in Fennoscandian willow warblers. Within an investigated chromosome region of 7.3 Mb as mapped to the zebra finch genome, we observed elevated genetic differentiation between a southern and a northern population across a 2.5-Mb interval comprising numerous coding genes. Within the southern and northern populations, higher values of LD were mostly found between SNPs within the same locus, but extended across distantly situated loci when the analyses were restricted to sampling sites showing intermediate allele frequencies of southern and northern alleles. Our study shows that cross-species genome information is a useful resource to obtain candidate sequences adjacent to outlier markers in nonmodel species.     

Weak geographical structure in sperm morphology across the range of two willow warbler Phylloscopus trochilus subspecies in Scandinavia

Sperm morphology is highly diversified among species and at higher taxonomic levels. In birds, there is also increasing evidence of geographical differentiation in sperm traits within species, especially in those with strong sperm competition. Geographical divergences in sperm traits might imply the formation of a reproductive barrier in a speciation process. Here we study sperm morphology variation of willow warblers Phylloscopus trochilus in a geographical context in Scandinavia, across the range of two subspecies that are differentiated in certain genetic markers, morphology and migratory routes. We describe geographical patterns in genotypes (two previously described single‐nucleotide polymorphism (SNP) markers and four polymorphic microsatellites); in wing length, tarsus length and body mass; and in sperm traits of 330 male willow warblers sampled at 33 localities across Norway (58°N–69°N). Birds were on average larger and longer‐winged in the north (spp. acredula) than in the south (spp. trochilus), and showed a sigmoid change in the SNP allele frequencies and body morphology around 65°N. We found no evidence of genetic structuring in the microsatellites. There was no geographical variation in sperm traits across Norway, except that sperm heads were on average longer in the south. Sperm head length was also associated with the two SNP markers, with longer sperm heads for the southern alleles, and midpiece length was weakly associated with one of the SNP markers. Similar among‐male variances in total sperm length among the 33 sampling sites indicate uniform levels of sperm competition across Norway. We conclude that sperm morphology remains a rather undifferentiated trait between the two willow warbler subspecies in Scandinavia, which is consistent with a pattern of a shallow genetic divergence. This indicates that sperm morphology is not a reproductive barrier maintaining the narrow hybrid zone.     

Point of no return – absence of returning birds in the otherwise philopatric willow warbler Phylloscopus trochilus

The return of individual birds to a specific area in successional years, i.e. philopatry, is a remarkable behavioural trait. Here we report on the remarkably reversed: the complete absence of returning individuals of a migratory passerine with otherwise pronounced philopatry. At a high latitude study site in Abisko (68°32?N, 18°80?E) in northern Sweden none of the banded adult willow warblers Phylloscopus trochilus returned to breed 2011–2014. This is in stark contrast to all other reports in the literature and also to our two southern study sites (at 56°56?N, 18°10?E and at 58°94?N, 17°14?E) where 18–38% of adults returned. We investigated this aberrant pattern found in Abisko by analysing three parameters known to influence philopatry; nest predation, breeding success and breeding density, and predicted that absence of philopatry should co‐occur with low breeding success, low breeding density and/or high nest predation. The results did not corroborate this, except that breeding density was lower at Abisko (49–71 pairs km^–2) than at the southern sites (106 pairs km^–2, 101 pairs km^–2). Instead, we suggest the hypothesis that the absence of philopatry is caused by an influx of southern, dispersal‐prone individuals deploying another breeding strategy and that this intra‐specific range expansion is enabled by milder climate and low population density.     

Functional infertility among territorial males in two passerine species, the willow warbler Phylloscopus trochilus and the bluethroat Luscinia svecica

Information about male infertility in free-living birds is scarce, but anecdotal and circumstantial evidence suggests that it does occur regularly at a low frequency. In this paper we document three cases of azoospermia in two passerine species, the willow warbler Phylloscopus trochilus and the bluethroat Luscinia svecica at their breeding grounds in South Norway. In willow warblers, two males out of a sample of 50 territory holders had no sperm in their seminal glomera, the storage site of sperm ready for ejaculation. The two males also had very small testes. One out of 48 bluethroat males also had no sperm in the seminal glomera. This male had an extreme asymmetry of the testes, with the right testis being about twice as large as the left. He also failed to fertilize any eggs in his own nest, as well as in neighbouring nests, as revealed by microsatellite genotyping. Thus, the proportion of males without sperm seems to be at a magnitude of a few (2–4) percent in both species. These are among the first estimates of the frequency of azoospermia in wild birds. Our results indicate a significant risk for sexually monogamous females of laying unfertilized eggs, which could favour the evolution of extra-pair copulation as a fertility insurance strategy in females.     

Raised thermoregulatory costs at exposed song posts increase the energetic cost of singing for willow warblers Phylloscopus trochilus

Sexually selected displays, such as bird song, are expected to be costly. We examined a novel potential cost to bird song: whether a less favourable microclimate at exposed song posts would be predicted to raise metabolic rate. We measured the microclimate and height at which willow warblers Phylloscopus trochilus sang and foraged. Song posts were higher than foraging sites. The wind speed was 0.6±0.3 ms⁻¹ greater at song posts (mean±SD, N=12 birds). Song rate and song post selection were not influenced consistently by temperature or wind speed, but the birds sang from lower positions on one particularly windy day. This may have resulted from difficulty in holding on to exposed branches in windy conditions rather than a thermoregulatory constraint. The results suggest that the extra thermoregulatory costs at song posts would increase metabolic rate by an average of 10±4% and a maximum of 25±8% (N=12 birds) relative to birds singing at foraging sites. We estimated that metabolic rate would be 3–8% greater during singing than during quiet respiration because of heat and evaporative water loss in exhaled gases. The combined energy requirements for sound production, thermoregulation at exposed song posts and additional heat loss in exhaled air could increase the metabolic rate of willow warblers by an average of 14–23%, and a maximum of 42–63%, during singing. The energetic cost of singing may thus be much greater for birds in a cold, windy environment than for birds singing in laboratory conditions.     

Long‐term phenological shifts and intra‐specific differences in migratory change in the willow warbler Phylloscopus trochilus

Climate change can influence many aspects of avian phenology and especially migratory shifts and changes in breeding onset receive much research interest in this context. However, changes in these different life‐cycle events in birds are often investigated separately and by means of ringing records of mixed populations. In this long‐term study on the willow warbler Phylloscopus trochilus, we investigated timing of spring and autumn migration in conjunction with timing of breeding. We made distinction among individuals with regard to age, sex, juvenile origin and migratory phase. The data set comprised 22‐yr of ringing records and two temporally separated data sets of egg‐laying dates and arrival of the breeding population close to the ringing site. The results reveal an overall advancement consistent in most, but not all, phenological events. During spring migration, early and median passage of males and females became earlier by between 4.4 to 6.3 d and median egg‐laying dates became earlier by 5 d. Male arrival advanced more, which may lead to an increase in the degree of protandry in the future. Among breeding individuals, only female arrival advanced in timing. In autumn, adults and locally hatched juvenile females did not advanced median passage, but locally hatched juvenile males appeared 4.2 d earlier. Migrating juvenile males and females advanced passage both in early and median migratory phase by between 8.4 to 10.1 d. The dissimilarities in the response between birds of different age, sex and migratory phase emphasize that environmental change may elicit intra‐specific selection pressures. The overall consistency of the phenological change in spring, autumn and egg‐laying, coupled with the unchanged number of days between median spring and autumn migration in adults, indicate that the breeding area residence has advanced seasonally but remained temporally constant.     

Stable isotopes examined across a migratory divide in Scandinavian willow warblers (Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula) reflect their African winter quarters

The C and N isotopes of feathers from two subspecies of willow warblers Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula) are isotopically distinct. Our analysis of 138 adult males from 14 sites distributed across Sweden shows that the mean delta15N and delta13C values of subspecies acredula (from latitudes above 63 degrees N) were significantly higher than the mean delta15N and delta13C values of subspecies trochilus (from latitudes below 61 degrees N). The analysed willow warbler feathers had been moulted in the winter quarters and the observed isotopic signatures should thus reflect the isotopic pattern of food assimilated in Africa. The isotopic data observed in Sweden match the cline in morphology, both showing abrupt changes around 62 degrees N. This result agrees with data from ringing recoveries indicating that the two subspecies occupy geographically and isotopically distinct wintering grounds in Africa. Our isotopic data suggest that analysis of stable isotopes of C and N is a promising method to track wintering quarters of European birds that migrate to Africa.     

Magnetic resonance Imaging versus chemical fat extraction in a small passerine, the willow warbler Phylloscopus trochilus: a fat-score based statistical comparison

The amount of fat of a bird is a fundamental metric commonly used by avian biologists in studies of migration, winter energy management and condition. The methods used for estimating fat content range from simply measuring body mass and subtracting the estimated lean mass, visual scoring of subcutaneous fat deposits, to destructive extraction using a Soxhlet apparatus. We used magnetic resonance imaging (MRI) for non-invasive estimation of fat content in willow warblers Phylloscopus trochilus . According to an indirect comparison, made using a standardized fat scoring system as a common measure, the MRI method gave very similar fat amount as Soxhlet extraction and we discuss advantages of using MRI over alternative methods. It was also shown that the commonly used fat scoring system yielded a nonlinear relationship between fat mass and fat score.     

Adaptive features of the ecology and annual cycle of the willow warbler (Phylloscopus trochilus L.) at the northern boundary of the Siberian part of the range

The ecology of the willow warbler in the north of Western Siberia is considered, and the adaptations that enable the spread of this species to the Subarctic are analyzed. It is established that one of the key factors that caused the change in the range of this species is the northward distribution of shrubs and, hence, the biomass of insects (available food items of these birds).     

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.     

Extra-pair paternity in the Skylark Alauda arvensis

We present the first quantitative data on the genetic breeding system of a lark (Alaudidae), the Skylark Alauda arvensis . Using a set of eight microsatellite loci isolated in a variety of passerine species, we genotyped 171 offspring from 52 broods of Skylark and detected 35 extra-pair offspring (20%), in 14 different broods (27%). All offspring matched their putative mother, so there was no evidence of intraspecific brood parasitism. Previous non-genetic studies had suggested that the species was predominantly socially monogamous, with only rare occurrences of social polygyny and polyandry, although some behaviours, such as mate guarding, did suggest the possibility of extra-pair copulations. The relatively high level of extra-pair paternity in this species is likely to affect the variation in male reproductive success because extra-pair paternity was non-randomly distributed amongst males, with those with shorter wings more likely to be cuckolded.     

Extra-pair paternity in waved albatrosses

We estimated the rate of extra‐pair fertilizations (EPFs) in waved albatrosses (Phoebastria irrorata) on Isla Española, Galápagos, Ecuador, using multilocus minisatellite DNA fingerprinting. Waved albatrosses are socially monogamous, long‐lived seabirds whose main population is on Española. Aggressive extra‐pair copulation (EPC) attempts have been observed in the breeding colony during the days preceding egg‐laying. Our genetic analyses of 16 families (single chicks and their attending parents) revealed evidence of EPFs in four families. In all cases males were the excluded parent. These data suggest that waved albatrosses have an unusually high rate of EPF relative to taxa with similar life histories. Future behavioural observations will determine the extent to which forced vs. unforced EPCs contribute to this high EPF rate.     

Polygyny in the Willow Warbler Phylloscopus trochilus in Swedish Lapland

The breeding sex ratio of Willow Warblers Phylloscopus trochilus in a subalpine birch forest was strongly female biased. Very few territorial males remained unmated and most became polygynous. Bigyny was the rule, but some males probably had three females. Male assistance at secondary nests varied and was probably dependent on temporal distance between the females' nests. There was no significant reduction in reproductive output for these females. Variation in the sex ratio of breeding birds can be explained, at least partly, by variation in the timing of male settlement.     

Once bitten twice shy: long‐term behavioural changes caused by trapping experience in willow warblers Phylloscopus trochilus

In dangerous situations, animals learn from experience to anticipate risks during similar subsequent occasions. Mist netting may represent just such a dangerous occasion and may serve as a tool to get insight into whether and how memorizing and recognition of the circumstances that form the prelude to a recapture can alter the animal's behaviour after capture, as well as it may affect the subsequent recapture rates. We used a playback lure to study the capture latency in willow warblers Phylloscopus trochilus, both without any and with a previous capture experience (either in the same year or in a previous one). We found increased response latencies to the playback lure for experienced males, compared to naive males. Naive males responded faster in the presence of a mist net than without a mist net; while experienced males tended to increase their response latencies. Individuals with between‐year capture experience showed exactly the same response pattern as individuals with a capture experience within the same year. These findings suggest that the birds may recall the capture event even a year later. It is very likely that the playback lure, with or without the net's presence, contributed significantly to the apparent risk of detection. We believe our study reveals that long‐lasting modifications of behaviour induced by capture and handling deserve further attention, as they might have a serious impact on the value of behavioural and ecological data, which is either based upon observing captured individuals and/or upon the recapture rates.     

Extra-pair paternity and sexual dimorphism in birds

Differences in the strength of sexual selection between males and females can lead to sexual dimorphism. Extra-pair paternity (EPP) can increase the variance in male reproductive success and hence the opportunity for sexual selection. Previous research on birds suggests that EPP drives the evolution of dimorphism in plumage colour and in body size. Because EPP increases the intensity of sexual selection in males, it should lead to increased dimorphism in species with larger or more colourful males, but decreased dimorphism in species with larger or more colourful females. We explored the covariation between EPP and sexual dimorphism in wing length and plumage colouration in 401 bird species, while controlling for other, potentially confounding variables. Wing length dimorphism was associated positively with the frequency of EPP, but also with social polygamy, sex bias in parental behaviour and body size and negatively with migration distance. The frequency of EPP was the only predictor of plumage colour dimorphism. In support of our prediction, high EPP levels were associated with sexual dichromatism, positively in species in which males are more colourful and negatively in those in which females are more colourful. Contrary to our prediction, high EPP rates were associated with increased wing length dimorphism in species with both male- and female-biased dimorphism. The results support a role for EPP in the evolution of both size and plumage colour dimorphism. The two forms of dimorphism were weakly correlated and predicted by different reproductive, social and life-history traits, suggesting an independent evolution.     

Do willow warblers Phylloscopus trochilus of northern origin start their autumn migration at an earlier age than their southern conspecifics?

Data on hatching time in the willow warbler for different regions (S Germany, S Sweden and N Sweden) are compared with the median of the corresponding migration diagrams. By doing so it is possible to calculate the age, at which the young of the year start their autumn migration. Our calculated values for the above mentioned regions are 68, 62–66 and 61–62 d, respectively. In our opinion this difference is too small to justify the statement, that northern birds migrate at an earlier age than their southern conspecifics. An alternative explanation is that the German migration diagrams are influenced by the passage of northern migrants, resulting in a later median and an apparently higher age for southern birds.     

Acoustic differentiation reflects mitochondrial lineages in Blyth's leaf warbler and white-tailed leaf warbler complexes (Aves: Phylloscopus reguloides, Phylloscopus davisoni)

The present study evaluated the degree of differentiation between closely-related species of Blyth's leaf warbler complex ( Phylloscopus reguloides s.l. ) and white-tailed leaf warbler complex ( Phylloscopus davisoni s.l. ) by molecular (cytochrome b and 16S rRNA) and bioacoustic markers (sonagraphic analysis of songs). Molecular phylogenetic results corroborate the recently suggested splittings of the two former taxa into three and two species, respectively. By contrast to previous opinion, differentiation of territorial songs parallels the genetic diversification in both groups. In the P. reguloides group, all recently established species can be distinguished by a characteristic song pattern. Regularly structured songs composed of repeated two- or three-note syllables were found in all species of the group, whereas trill songs with single notes repeated are unique to Phylloscopus claudiae . The phylogenetic relationships of Phylloscopus occipitalis are ambiguous among different molecular trees, but song pattern clearly corroborates a close relationship to the P. reguloides group. In the white-tailed leaf warbler group, songs are irregularly structured and often descending in frequency with exception of P. davisoni and Phylloscopus ogilviegranti disturbans . Song pattern corroborates the genetic affiliation of Phylloscopus hainanus and Phylloscopus xanthoschistos to the molecular white-tailed leaf warbler clade. All target species correspond to distinct acoustic clusters resulting from discriminant analysis of several spectral and structural song parameters. Linear correlation between genetic and acoustic differentiation (pairwise p -distances versus difference of discriminant functions) is significant within both groups, except for one case, in which discriminant function 1 was correlated with a spectral parameter (minimum frequency) only.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 584–600.