Variation in the relationship between numbers of breeding pairs and woodland area for passerines in fragmented habitat

Species may differ in the relationship between the numbers of breeding pairs present and woodland area, because the proportion of a wood that forms suitable habitat will vary with woodland size. In this paper, we examine the pattern of variation in abundance with woodland area for eight breeding bird species, and also show how this pattern varied between years. During 1990-1997, we made annual censuses of 53–160 woods, of up to 10 ha in size, and fitted a power function to describe the relationships between numbers of breeding pairs and woodland area. Seven of the eight species, blackbird Turdus merula , dunnock Prunella modularis , wren Troglodytes troglodytes , great tit Parus major , chaffinch Fringilla coelebs , robin Erithacus rubecula and blue tit Parus caeruleus showed a pattern of proportionally higher numbers in smaller woods. Only long-tailed tit Aegithalos caudatus occurred in proportionally higher numbers in larger woods. Blackbird and dunnock showed a trend towards lower numbers in large woods during years with low regional population levels; for these species large woods may provide sub-optimal habitat. Great tit, blue tit, chaffinch and robin showed the opposite trend, towards lower numbers in small woods during years with low regional population levels; for these species small woods may provide sub-optimal habitat. Wren and long-tailed tit, which also showed large annual population fluctuations, showed no change in distribution with regional population level. In great tit and chaffinch, the distribution of pairs in any one year may have been influenced by site fidelity producing a lag in the response associated with regional population levels.     

Local extinctions and recolonisations of passerine bird populations in small woods

This paper considers, for eight species of woodland bird, the factors that influenced both local extinctions and recolonisations in 145 woods over 3 years. In all species, probability of local extinction was inversely related to population size; most local extinctions occurred in woods containing one to three breeding pairs. However, considerable variation in extinction probabilities occurred between species and between years. In addition, the suitability of habitat within a wood (more extinctions in less suitable woods) was important for wren Troglodytes troglodytes, song thrush Turdus philomelos and blue tit Parus caeruleus; also, the structure of the surrounding landscape was important for blue tit, great tit Parus major, and chaffinch Fringilla coelebs (more extinctions in localities with less woodland). In only two species was the probability of recolonisation related to any of the measured variables. Wrens were more likely to recolonise larger woods, whereas song thrushes were more likely to recolonise woods with a high habitat suitability rating and those which are more isolated from other woodland     

Influences of population size and woodland area on bird species distributions in small woods

Distributions of individual bird species in 151 small woods (size range 0.02–30 ha) were investigated in 3 consecutive years during which the abundance of certain species varied markedly. Relationships between the probabilities of certain bird species breeding and woodland area were described using incidence functions derived from logistic regression analysis. In general, for species which were largely dependent on woodland and seldom occurred in other habitats (such as hedgerows and gardens), the probability of breeding approached 100% only for woods of 10 ha and more, whereas species with less stringent habitat requirements occurred in the majority of woods, including those of 1 ha and less. The sensitivity of incidence functions to changes in regional abundance and the size distribution of the study woods was examined. For some species, distribution patterns could not be distinguished from those expected if pairs had been distributed in proportion to woodland area (random placement), but the majority did not conform to random placement in at least 1 of the 3 years. This nonconformity was consistent across all 3 years for some species, such as wren (Troglodytes troglodytes), despite substantial fluctuations in population sizes between years, while for others, such as robin (Erithacus rubecula), distribution patterns changed with changes in regional abundance. The results suggested that some species, such as wren and blackbird (Turdus merula), preferred small woods, while other species, such as chiffchaff (Phylloscopus collybita), preferred large woods. For several other species, including robin, great tit (Parus major), long-tailed tit (Aegithalos caudatus) and marsh tit (P. palustris), small woods appeared to be sub-optimal under at least some conditions.     

Woodland area,species turnover and the conservation of bird assemblages in lowland England

A study over 4 years into the number of breeding bird species and species turnover (extinctions and colonisations) in relation to area was conducted in 35 woodlands, set in an intensively farmed landscape, in north-east Essex, UK. A total of 46 species was recorded. The number of species breeding increased with woodland area; the slope of the species–area relationship did not differ between years. Habitat diversity was the only other measured variable to influence species richness. Absolute species turnover was independent of woodland area but relative turnover declined with increase in woodland area. The numbers of territories of nine species were determined. For four summer visitors the number of woods occupied increased as the overall populations increased but, for the other species, changes in overall population size led to changes in numbers in occupied woods. Chaffinch Fringilla coelebs and Song Thrush Turdus philomelos were more associated with woodland edges, Nightingale Luscinia megarhynchos, Garden Warbler Sylvia borin, Chiffchaff Phylloscopus collybita and Willow Warbler P. trochilus with interiors. Several species showed an inverse relationship between population density and woodland area. Collections of small woods hold similar species richness to single large woods. While the acquisition of large woods for conservation purposes should be a priority, the extension of smaller woods to a size of about 10 ha would be highly beneficial to both the species richness and population stability of regional woodland bird assemblages.     

Breeding bird communities on Scottish farms

The Common Birds Census, begun by the British Trust for Ornithology has now been monitoring bird population levels on a nationwide basis since 1962. Separate indices were constructed for Scotland and the rest of Great Britain for 12 of the commonest species. Of the species examined blue tit (Parus caeruleus), wren (Troglodytes troglodytes), song thrush (Turdus philomelos), lapwing (Vanellus vanellus), chaffinch (Fringilla coelebs) and skylark (Alauda arvensis) have increased since 1963 and show very similar population trends to the rest of Great Britain. Whitethroat (Sylvia communis), willow warbler (Phylloscopus trochilus) and swallow (Hirundo rustica) have all declined nationally, the decline in the swallow appears to have begun earlier in Scotland. The blackbird (Turdus merula) and dunnock (Prunella modularis) showed similar upward trends until 1972 when the Scottish population started to decline; a much more noticeable decline in the Scottish robin (Erithacus rubecula) population began in 1966. The total density of territory-holding males has not increased but the biomass has and this is due to more, bigger birds but fewer passerines. The species diversity measured by the Shannon Wiener index (H.) has improved a little since 1971 and it is concluded that during the study period the farmland plots censused in Scotland have on average not shown evidence of deterioration as habitats for territory-holding birds.     

Bird species turnover and stochastic extinction in woodland fragments

Year-to-year turnover in bird species composition was recorded across, the whole size range (0 02-30 ha) of 146 woods studied The mean number of resident breeding species both lost and gained per wood between consecutive breeding seasons was 2 (range 0-8) No relationship was found between this absolute turnover rate and woodland area, or any other of 24 predictor variables (describing woodland structure, isolation, connectedness and surrounding land use) Extriction and colonisation rates (in terms of numbers of species lost and gained) were also unrelated to woodland area In all sizes of woods, the species most likely to show local extinctions and colonisations were those with small populations within those woods, but the identity of the species concerned changed as woodland area increased In the smallest woods, the majority of turnover involved common species, such as wren and dunnock, which occurred in only small numbers in these small woods As woodland area increased, these species attained sufficient numbers to usually avoid stochastic extinction The majority of turnover was then due to more specialist (and less numerous) woodland species, such as great-spotted woodpecker and marsh tit, which were usually lacking in small woods In Britain, much existing broadleaved woodland falls within the size range studied Thus the numbers of many bird species are liable to be small enough for yearly turnover in woodland bird communities to be appreciable, and for the long-term persistence of individual species in particular woods to depend on dispersal     

Host specificity in avian blood parasites: a study of Plasmodium and Haemoproteus mitochondrial DNA amplified from birds

A fragment of the mitochondrial cytochrome b gene of avian malaria (genera Haemoproteus and Plasmodium) was amplified from blood samples of 12 species of passerine birds from the genera Acrocephalus, Phylloscopus and Parus. By sequencing 478 nucleotides of the obtained fragments, we found 17 different mitochondrial haplotypes of Haemoproteus or Plasmodium among the 12 bird species investigated. Only one out of the 17 haplotypes was found in more than one host species, this exception being a haplotype detected in both blue tits (Parus caeruleus) and great tits (Parus major). The phylogenetic tree which was constructed grouped the sequences into two clades, most probably representing Haemoproteus and Plasmodium, respectively. We found two to four different parasite mitochondrial DNA (mtDNA) haplotypes in four bird species. The phylogenetic tree obtained from the mtDNA of the parasites matched the phylogenetic tree of the bird hosts poorly. For example, the two tit species and the willow warbler (Phylloscopus trochilus) carried parasites differing by only 0.6% sequence divergence, suggesting that Haemoproteus shift both between species within the same genus and also between species in different families. Hence, host shifts seem to have occurred repeatedly in this parasite host system. We discuss this in terms of the possible evolutionary consequences for these bird species.     

Landscape composition models for breeding bird populations in lowland English farmland over a 20 year period

The aim of this paper was to assess which components of landscape structure in lowland English farmland could best predict patterns m bird community composition, total bird density and densities of 12 bird species As far as possible, predictors were sought that were consistent m their effects over a 20 yr period The data were derived from mapping censuses in 1966, 1970, 1974, 1978, 1982 and 1986 with yearly sample sizes ranging from 41 to 51 plots, of minimum size 50 ha Landscape composition was quantified in terms of 10 landscape variables representing broad structural (e g density of woodland) and crop attributes (e g proportion of area under cereals) plus altitude Gradients in bird community composition were defined as plot scores on axis 1 of a detrended correspondence analysis (AXIS1) This gradient consistently represented a gradient from plots rich in woodland bird species to ones rich in field-feeding and wetland species Multiple regression was used to build models containing 2-, 3- and 4-vanables Two approaches were used to combine data from the six years and models based on the single best subset in each year were found to be most consistent with respect to predictor selection Predictors were tested by building models based on each combination of five years and assessing how well the predictors explained variation in the bird variables for the sixth, independent year The predictors were relatively successful in explaining variations in densities of wren, robin, willow warbler, blue tit, great tit and chaffinch but especially AXIS1 which was explained in terms of woodland density, field size and altitude The predictors were unsuccessful in explaining total bird density, song thrush density and whitethroat density Structural attributes of landscape (especially density of hedgerow and woodland, but also rivers and ponds) were frequent predictors of species densities, but variables relating to farming system were not, with the exception of skylark and yellowhammer which were positively associated with extent of cereal crops Implications for landscape design and management, and the relative contribution of woodland and hedgerows to bird communities m farmland landscapes are discussed There was considerable variation between individual years which suggests that models of bird-habitat relationships based on single years may have limited general application     

Woodland birds in patchy landscapes: the evidence base for strategic networks

Habitat creation and management within wooded networks is a potentially effective strategy to reduce ecological isolation and the deleterious effects of fragmentation. However, questions remain over the relative advantages of different approaches, e.g. buffering patches vs. increasing connectivity. Potential effects of woodland fragmentation include reduction in regional woodland cover, reduced patch size, edge effects with loss of core habitat, and increased isolation with disruption of dispersal and metapopulation dynamics. We adopt an evidence-based approach to review how each of these affects woodland birds with an emphasis on studies from the UK and use this to identify management priorities for mitigation. There is evidence for both patch area and composition effects: larger woodlands support more woodland bird species, and woods located within sparsely wooded landscapes are less valuable to specialist woodland species. Bird assemblages show a nested pattern with respect to area, and thus species found in small woods also occur in large woods but not vice versa. However, small woods may be preferred by a few edge species, while small woods also have greater variability in bird species composition. Consideration of the metapopulation dynamics of specialist species with poor dispersal shows that creating or buffering large woodlands is more efficient than a greater total area of small fragments. Connectivity appears most useful for widespread generalist species with almost continuous populations. Woodland structure and quality are of overwhelming importance: as well as mature woodland, young growth, scrub and edges are also key components. There is an urgent need to examine the relationship between nest predation and landscape structure within UK woodlands.     

Does landscape structure affect resource tracking by avian frugivores in a fragmented Afrotropical forest?

Frugivorous species heavily depend on patchy food resources and are believed to track these in space and time, thereby providing an important seed dispersal function that might be critical toward the regeneration of fruiting plants. However, isolation of suitable food patches due to habitat fragmentation or changes in landscape connectivity may hamper food tracking behaviour and adversely affect populations of both frugivores (through starvation) and food plants (through interruption of seed dispersal). We here test whether density fluctuations in four frugivorous Afrotropical bird species were larger and/or matched fluctuations in ripe fruit densities better in study plots embedded in large tracts of indigenous forest than in equally-sized plots embedded in cultivated lands. We compared these results with those of four non-frugivorous species (out-group) which were not expected to track fruit resources. Whereas densities of both frugivores and fruit crops strongly fluctuated in space and time, these fluctuations were not synchronised, nor did the level of synchrony differ in relation to matrix type. For some but not all bird species, lower densities and smaller temporal fluctuations in forest plots surrounded by cultivation may reflect decreased mobility. The observed fluctuations in bird densities most likely reflect exchange with the surrounding landscape matrix, suggesting that small pockets of fruiting trees in farmland may comprise critical food resources for frugivores inhabiting highly fragmented landscapes, apart from increasing connectivity for both bird and seed dispersal.     

Estimating breeding season abundance of golden-cheeked warblers in Texas,USA

Population abundance estimates using predictive models are important for describing habitat use and responses to population-level impacts, evaluating conservation status of a species, and for establishing monitoring programs. The golden-cheeked warbler (Setophaga chrysoparia) is a neotropical migratory bird that was listed as federally endangered in 1990 because of threats related to loss and fragmentation of its woodland habitat. Since listing, abundance estimates for the species have mainly relied on localized population studies on public lands and qualitative-based methods. Our goal was to estimate breeding population size of male warblers using a predictive model based on metrics for patches of woodland habitat throughout the species' breeding range. We first conducted occupancy surveys to determine range-wide distribution. We then conducted standard point-count surveys on a subset of the initial sampling locations to estimate density of males. Mean observed patch-specific density was 0.23 males/ha (95% CI = 0.197–0.252, n = 301). We modeled the relationship between patch-specific density of males and woodland patch characteristics (size and landscape composition) and predicted patch occupancy. The probability of patch occupancy, derived from a model that used patch size and landscape composition as predictor variables while addressing effects of spatial relatedness, best predicted patch-specific density. We predicted patch-specific densities as a function of occupancy probability and estimated abundance of male warblers across 63,616 woodland patches accounting for 1.678 million ha of potential warbler habitat. Using a Monte Carlo simulation, our approach yielded a range-wide male warbler population estimate of 263,339 (95% CI: 223,927–302,620). Our results provide the first abundance estimate using habitat and count data from a sampling design focused on range-wide inference. Managers can use the resulting model as a tool to support conservation planning and guide recovery efforts. © 2012 The Wildlife Society.     

Reactions of passerine birds to aposematic and non-aposematic firebugs (Pyrrhocoris apterus ; Heteroptera)

In spite of the existence of many experimental studies on the function of warning coloration in insects, little is known about the universality of reactions of different predators towards a particular warning signal. Reactions of nine passerine bird species, namely Parus major , Parus caeruleus , Aegithalos caudatus , Erithacus rubecula , Turdus merula , Sylvia atricapilla , Fringilla coelebs , Carduelis chloris and Emberiza citrinella , to the firebug Pyrrhocoris apterus wildtype (brachypterous adults) and its artificially obtained (painted) brown non-aposematic variant were compared. Most insectivorous birds (great tits, blue tits, robins and blackcaps) distinguished between aposematic and non-aposematic bugs, attacking the former less often. Partly granivorous buntings and finches did not distinguish between them, and attacked both variants equally. As all the birds were caught in the wild, the results can be interpreted in terms of the presence of a higher proportion of experienced individuals among insectivorous than among omnivorous species. Two insectivorous species differed from others. The heaviest blackbird attacked and killed aposematic as well as non-aposematic firebugs, and, in contrast, the lightest long-tailed tit avoided both variants. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78 , 517–525.     

Viability of meta-populations of wetland birds in a fragmented landscape: testing the key-patch approach

The key patch approach assumes that metapopulations in fragmented landscapes are likely to be viable with at least one “key” sub-population that is sufficiently large to ensure re-colonization of surrounding minor habitat patches. It is based on a minimum viable number of breeding pairs and within-breeding season dispersal distance, linked to size of the animal and longevity. It was tested using census data of 15 wetland bird species (bearded tit, bluethroat, great reed warbler, sedge warbler, Savi’s warbler, grasshopper warbler, spotted crake, water rail, common snipe, common teal, garganey, little bittern, night heron, great bittern and marsh harrier) in 14 wetland complexes of variable size (3–55 km²) spread across the Netherlands (distances ranging 4–156 km). First, for each species it was assessed whether a wetland harbored a key subpopulation, which was the case for the sedge warbler (7 key subpopulations), grass-hopper warbler (2), water rail (2), bearded tit, bluethroat, Savi’s warbler, common teal, garganey, great bittern and marsh harrier (all one key subpopulation).Together with the adjacent sub-populations present within breeding season dispersal distance, 10 out of the 15 studied species formed viable meta-populations. This was compared with the trend in the census data of 13 species for 1990–2000 and was found to correspond significantly (likelihood ratio test, P = 0.003): species without a viable meta-population had declined (2 out of 4) or remained stable (2 out of 4), whereas viable meta-populations had increased (6 out of 9) or remained comparatively stable (2 out of 9). One wetland complex, the Oostvaardersplassen, stood out in that it haboured key sub-populations for 9 out of the 15 species studied. Variation in quantity of specific habitat (area or perimeter marshland, woodland or open water) in a wetland complex was of limited importance explaining abundance patterns, since all covaried strongly with total area among the wetland complexes, with the exception of water perimeter. Apparently, these wetlands on peat harbour largely similar landscapes. Indeed, population sizes of most birds covaried strongly and in a PCA two distinct clusters of species were identified that shared high numbers of breeding pairs in the same, larger, wetland complexes, the first (3 species) including the great reed warbler, and the second (9 species) the water rail.     

The extended Moran effect and large-scale synchronous fluctuations in the size of great tit and blue tit populations

1. Synchronous fluctuations of geographically separated populations are in general explained by the Moran effect, i.e. a common influence on the local population dynamics of environmental variables that are correlated in space. Empirical support for such a Moran effect has been difficult to provide, mainly due to problems separating out effects of local population dynamics, demographic stochasticity and dispersal that also influence the spatial scaling of population processes. Here we generalize the Moran effect by decomposing the spatial autocorrelation function for fluctuations in the size of great tit Parus major and blue tit Cyanistes caeruleus populations into components due to spatial correlations in the environmental noise, local differences in the strength of density regulation and the effects of demographic stochasticity. 2. Differences between localities in the strength of density dependence and nonlinearity in the density regulation had a small effect on population synchrony, whereas demographic stochasticity reduced the effects of the spatial correlation in environmental noise on the spatial correlations in population size by 21.7% and 23.3% in the great tit and blue tit, respectively. 3. Different environmental variables, such as beech mast and climate, induce a common environmental forcing on the dynamics of central European great and blue tit populations. This generates synchronous fluctuations in the size of populations located several hundred kilometres apart. 4. Although these environmental variables were autocorrelated over large areas, their contribution to the spatial synchrony in the population fluctuations differed, dependent on the spatial scaling of their effects on the local population dynamics. We also demonstrate that this effect can lead to the paradoxical result that a common environmental variable can induce spatial desynchronization of the population fluctuations. 5. This demonstrates that a proper understanding of the ecological consequences of environmental changes, especially those that occur simultaneously over large areas, will require information about the spatial scaling of their effects on local population dynamics.     

Habitat-mediated predation risk and decision making of small birds at forest edges

We studied the effect of four components of predation risk, namely predation pressure, spatial variation in woody cover, visibility, and flock size, on the behaviour of four species of the Parus guild (crested tit, P. cristatus , willow tit, P. montanus , coal tit, P. ater , and goldcrest, Regulus regulus ) at the edge of patches of mature boreal forest. Birds used the exposed side of the edge (matrix) mainly during periods with low levels of predation pressure by pygmy owls ( Glaucidium passerinum ). Some species avoided edges under low light conditions. Birds in large groups were more prone to cross the edge, whereas group cohesion increased in risky situations, especially in the most vulnerable species (coal tit and goldcrest). The effects of these three components of predation risk were not general in that only some species responded to them, and in that intra-specific responses were not always consistent. In contrast, all behaviours examined in all four bird species (occurrence, matrix crossing, edge crossing, and group cohesion) appeared to be strongly affected by habitat-mediated predation risk. Mature boreal forests appeared to be qualitatively superior to any type of matrix in terms of protection against predators. Birds generally avoided open matrix, and seemed to move towards and across more developed stages in forest regeneration according to the ratio between food intake and predation risk attributable to a given matrix type. Open matrix (farmland and clearcuts) and very young plantations strongly restricted the rate of bird movement between old forest patches. Our results contrast with the widespread thought that birds have a great potential to use fragmented landscapes in a fine-grained manner. These limited movements across the landscape during winter might have important repercussions on the occupation of some forest patches by tits, their subsequent reproduction, and in turn their population dynamics.     

Species-specific foraging behavior of birds in a riparian forest

In riparian forest, insectivorous birds are largely dependent on aquatic preys. However, the contribution made by aquatic preys to bird diets varies considerably among bird species. In the present study, bird foraging behaviors were observed in order to examine the relationship between bird foraging method and the variation in the contribution of aquatic preys. The great tit, the black-faced bunting, and the wren are largely dependent on aquatic preys by capturing them on the ground. Sallyers, the brown flycatcher, the pale-legged willow warbler, and the narcissus flycatcher are also largely dependent on aquatic preys and capture them mostly during flight. The narcissus flycatcher frequently utilises aquatic invertebrates dwelling on the ground. The leaf and branch gleaner, the crowned willow warbler, does not depend on aquatic prey as much. Although both the pygmy woodpecker and nuthatch are branch and trunk gleaners, the nuthatch utilises aquatic preys frequently by capturing them on the ground as well as during flight, but the pygmy woodpecker does not depend on aquatic preys. The marsh tit also does not depend on aquatic preys but carefully searches terrestrial prey that hide in the vegetation. The differential dependence on aquatic preys among species can lead to the heterogeneous distribution of birds within a riparian forest, suggesting that the indirect effect of aquatic preys on a forest ecosystem via birds can vary within a forest–stream ecotone.     

Predicting bird phenology from space: satellite‐derived vegetation green‐up signal uncovers spatial variation in phenological synchrony between birds and their environment

Population‐level studies of how tit species (Parus spp.) track the changing phenology of their caterpillar food source have provided a model system allowing inference into how populations can adjust to changing climates, but are often limited because they implicitly assume all individuals experience similar environments. Ecologists are increasingly using satellite‐derived data to quantify aspects of animals' environments, but so far studies examining phenology have generally done so at large spatial scales. Considering the scale at which individuals experience their environment is likely to be key if we are to understand the ecological and evolutionary processes acting on reproductive phenology within populations. Here, we use time series of satellite images, with a resolution of 240 m, to quantify spatial variation in vegetation green‐up for a 385‐ha mixed‐deciduous woodland. Using data spanning 13 years, we demonstrate that annual population‐level measures of the timing of peak abundance of winter moth larvae (Operophtera brumata) and the timing of egg laying in great tits (Parus major) and blue tits (Cyanistes caeruleus) is related to satellite‐derived spring vegetation phenology. We go on to show that timing of local vegetation green‐up significantly explained individual differences in tit reproductive phenology within the population, and that the degree of synchrony between bird and vegetation phenology showed marked spatial variation across the woodland. Areas of high oak tree (Quercus robur) and hazel (Corylus avellana) density showed the strongest match between remote‐sensed vegetation phenology and reproductive phenology in both species. Marked within‐population variation in the extent to which phenology of different trophic levels match suggests that more attention should be given to small‐scale processes when exploring the causes and consequences of phenological matching. We discuss how use of remotely sensed data to study within‐population variation could broaden the scale and scope of studies exploring phenological synchrony between organisms and their environment.     

BIRDS ON TENERIFE

.1. A visit was paid to Tenerife in March 1948.
2. Of 16 species common to Tenerife and Britain, all showed detectable differences in song or call-notes. The differences were in some cases slight but in others very marked.
3. Various species occupy broader habitats on Tenerife than in Western Europe, and this can in most cases be correlated with the absence of a related species which fills part of the habitat in Western Europe. For instance, on Tenerife Parus cceruleus breeds in conifers as well as broad-leaved woods, and Phylloscopus collybita breeds in low scrub as well as woods.
4. Fringilla ccelebs is more restricted in habitat on Tenerife than in Western Europe. a fact which can be correlated with the presence of the related F. teydea.
5. The bird fauna of the Canary Islands is recommended as a profitable field for ai detailed ecological study.     

Movement Patterns of Frugivorous Birds Promote Functional Connectivity among Chaco Serrano Woodland Fragments in Argentina

Movement patterns of frugivorous birds may be altered in anthropogenically fragmented landscapes, with possible consequences for seed dispersal and plant recruitment. We studied the movement patterns and functional connectivity of six frugivorous bird species (Colaptes melanochloros, Thraupis bonariensis, Pitangus sulphuratus, Saltator aurantiirostris, Turdus amaurochalinus, and Elaenia spp.) in a fragmented Chaco‐woodland landscape in Argentina. We recorded the directions of bird movements (arrivals and departures) and whether their destination was oriented toward a specific neighboring fragment. We evaluated the movement rates, distance of interpatch movement, and functional connectivity within the landscape for the six bird species. We applied a novel approach, graph theory, to represent bird movement patterns in the landscape and the functional connections among fragments for each bird species. Bird movements were recorded at point‐count stations established along the edges of each fragment. The directions of arrival and departure movements from and to neighboring fragments revealed complex movement patterns. However, the destination of bird movements after leaving the focal fragments was usually concentrated on only a few neighboring fragments of different sizes. Pitangus sulphuratus and T. bonariensis showed larger movement rates and higher functional connectivity (number of graphs and functional area) than the other frugivorous species. The functional connectivity mediated by movement of frugivorous birds may promote seed dispersal of many bird‐dispersed plant species. As forest loss and fragmentation of Chaco subtropical forests increase, understanding the pivotal role of mobile links exerted by avian seed dispersers is vital to maintaining and conserving this unique ecosystem.     

Conflicting patterns of mitochondrial and nuclear DNA diversity in Phylloscopus warblers

Molecular variation is often used to infer the demographic history of species, but sometimes the complexity of species history can make such inference difficult. The willow warbler, Phylloscopus trochilus, shows substantially less geographical variation than the chiffchaff, Phylloscopus collybita, both in morphology and in mitochondrial DNA (mtDNA) divergence. We therefore predicted that the willow warbler should harbour less nuclear DNA diversity than the chiffchaff. We analysed sequence data obtained from multiple samples of willow warblers and chiffchaffs for the mtDNA cytochrome b gene and four nuclear genes. We confirmed that the mtDNA diversity among willow warblers is low (pi = 0.0021). Sequence data from three nuclear genes (CHD-Z, AFLP-WW1 and MC1R) not linked to the mitochondria demonstrated unexpectedly high nucleotide diversity (pi values of 0.0172, 0.0141 and 0.0038) in the willow warbler, on average higher than the nucleotide diversity for the chiffchaff (pi values of 0.0025, 0.0017 and 0.0139). In willow warblers, Tajima's D analyses showed that the mtDNA diversity, but not the nuclear DNA diversity, has been reduced relative to the neutral expectation of molecular evolution, suggesting the action of a selective sweep affecting the maternally inherited genes. The large nuclear diversity seen within willow warblers is not compatible with processes of neutral evolution occurring in a population with a constant population size, unless the long-term effective population size has been very large (N(e) > 10(6)). We suggest that the contrasting patterns of genetic diversity in the willow warbler may reflect a more complex evolutionary history, possibly including historical demographic fluctuations or historical male-biased introgression of nuclear genes from a differentiated population of Phylloscopus warblers.